What is 8051 Standard?
Microcontrollers’ producers have been struggling for a long time for attracting more and more choosy customers. Every couple of days a new chip with a higher operating frequency, more memory and more high-quality A/D converters comes on the market.Nevertheless, by analyzing their structure it is concluded that most of them have the same (or at least very similar) architecture known in the product catalogs as “8051 compatible”. What is all this about?
The whole story began in the far 80s when Intel launched its series of the microcontrollers labelled with MCS 051. Although, several circuits belonging to this series had quite modest features in comparison to the new ones, they took over the world very fast and became a standard for what nowadays is ment by a word microcontroller.
The reason for success and such a big popularity is a skillfully chosen configuration which satisfies needs of a great number of the users allowing at the same time stable expanding ( refers to the new types of the microcontrollers ). Besides, since a great deal of software has been developed in the meantime, it simply was not profitable to change anything in the microcontroller’s basic core. That is the reason for having a great number of various microcontrollers which actually are solely upgraded versions of the 8051 family. What is it what makes this microcontroller so special and universal so that almost all the world producers manufacture it today under different name ?
- 4 Kb program memory is not much at all.
- 128Kb RAM (including SFRs as well) satisfies basic needs, but it is not imposing amount.
- 4 ports having in total of 32 input/output lines are mostly enough to make connection to peripheral environment and are not luxury at all.
The whole configuration is obviously envisaged as such to satisfy the needs of most programmers who work on development of automation devices. One of advantages of this microcontroller is that nothing is missing and nothing is too much. In other words, it is created exactly in accordance to the average user‘s taste and needs. The other advantage is the way RAM is organized, the way Central Processor Unit (CPU) operates and ports which maximally use all recourses and enable further upgrading.
8051 Microcontroller's pins
Pins 1-8: Port 1 Each of these pins can be
configured as input or output.
Pin 9: RS
Logical one on this pin stops microcontroller’s operating and erases the
contents of most registers. By applying logical zero to this pin, the program
starts execution from the beginning. In other words, a positive voltage pulse
on this pin resets the microcontroller.Pins10-17: Port 3 Similar to port 1, each of these pins can serve as universal input or output . Besides, all of them have alternative functions:
Pin 10: RXD Serial asynchronous communication input or Serial synchronous communication output.
Pin 11: TXD Serial asynchronous communication output or Serial synchronous communication clock output.
Pin 12: INT0 Interrupt 0 input
Pin 13: INT1 Interrupt 1 input
Pin 14: T0 Counter 0 clock input
Pin 15: T1 Counter 1 clock input
Pin 16: WR Signal for writing to external (additional) RAM
Pin 17: RD Signal for reading from external RAM
Pin 18, 19: X2, X1 Internal oscillator input and output. A quartz crystal which determines operating frequency is usually connected to these pins. Instead of quartz crystal, the miniature ceramics resonators can be also used for frequency stabilization. Later versions of the microcontrollers operate at a frequency of 0 Hz up to over 50 Hz.
Pin 20: GND Ground
Pin 21-28: Port 2 If there is no intention to use external memory then these port pins are configured as universal inputs/outputs. In case external memory is used then the higher address byte, i.e. addresses A8-A15 will appear on this port. It is important to know that even memory with capacity of 64Kb is not used ( i.e. note all bits on port are used for memory addressing) the rest of bits are not available as inputs or outputs.
Pin 29: PSEN If external ROM is used for storing program then it has a logic-0 value every time the microcontroller reads a byte from memory.
Pin 30: ALE Prior to each reading from external memory, the microcontroller will set the lower address byte (A0-A7) on P0 and immediately after that activates the output ALE. Upon receiving signal from the ALE pin, the external register (74HCT373 or 74HCT375 circuit is usually embedded ) memorizes the state of P0 and uses it as an address for memory chip. In the second part of the microcontroller’s machine cycle, a signal on this pin stops being emitted and P0 is used now for data transmission (Data Bus). In this way, by means of only one additional (and cheap) integrated circuit, data multiplexing from the port is performed. This port at the same time used for data and address transmission.
Pin 31: EA By applying logic zero to this pin, P2 and P3 are used for data and address transmission with no regard to whether there is internal memory or not. That means that even there is a program written to the microcontroller, it will not be executed, the program written to external ROM will be used instead. Otherwise, by applying logic one to the EA pin, the microcontroller will use both memories, first internal and afterwards external (if it exists), up to end of address space.
Pin 32-39: Port 0 Similar to port 2, if external memory is not used, these pins can be used as universal inputs or outputs. Otherwise, P0 is configured as address output (A0-A7) when the ALE pin is at high level (1) and as data output (Data Bus), when logic zero (0) is applied to the ALE pin.
Pin 40: VCC Power supply +5V
Input/Output Ports (I/O Ports)
All 8051 microcontrollers have 4 I/O ports, each consisting of 8 bits which can be configured as inputs or outputs. This means that the user has on disposal in total of 32 input/output lines connecting the microcontroller to peripheral devices.A logic state on a pin determines whether it is configured as input or output: 0=output, 1=input. If a pin on the microcontroller needs to be configured as output, then a logic zero (0) should be applied to the appropriate bit on I/O port. In this way, a voltage level on the appropriate pin will be 0.
Similar to that, if a pin needs to be configured as input, then a logic one (1) should be applied to the appropriate port. In this way, as a side effect a voltage level on the appropriate pin will be 5V (as it is case with any TTL input). This may sound a bit confusing but everything becomes clear after studying a simplified electronic circuit connected to one I/O pin.
Input/Output (I/O) pin
This is a simplified overview of what is connected to a pin inside the microcontroller. It concerns all pins except those included in P0 which do not have embedded pullup resistor.
Output pin
A logic zero (0) is applied to a bit in the P register. By turning output FE transistor on, the appropriate pin is directly connected to ground.
Input pin
A logic one (1) is applied to a bit in the P register. Output FE transistor is turned off. The appropriate pin remains connected to voltage power supply through a pull-up resistor of high resistance.
Port 0
It is specific to this port to have a double purpose. If external memory is
used then the lower address byte (addresses A0-A7) is applied on it. Otherwise,
all bits on this port are configured as inputs or outputs.Another characteristic is expressed when it is configured as output. Namely, unlike other ports consisting of pins with embedded pull-up resistor ( connected by its end to 5 V power supply ), this resistor is left out here. This, apparently little change has its consequences:
If any pin on this port is configured as input then it performs as if it “floats”. Such input has unlimited input resistance and has no voltage coming from “inside”.
When the pin is configured as output, it performs as “open drain”, meaning that by writing 0 to some port’s bit, the appropriate pin will be connected to ground (0V). By writing 1, the external output will keep on “floating”. In order to apply 1 (5V) on this output, an external pull-up resistor must be embedded.
Port 1
This is a true I/O port, because there are no role assigning as it is the
case with P0. Since it has embedded pull-up resistors it is completely
compatible with TTL circuits.
Port 2
Similar to P0, when using external memory, lines on this port occupy
addresses intended for external memory chip. This time it is the higher address
byte with addresses A8-A15. When there is no additional memory, this port can
be used as universal input-output port similar by its features to the port 1.
Port 3
Even though all pins on this port can be used as universal I/O port, they
also have an alternative function. Since each of these functions use inputs,
then the appropriate pins have to be configured like that. In other words,
prior to using some of reserve port functions, a logical one (1) must be
written to the appropriate bit in the P3 register. From hardware’s perspective
, this port is also similar to P0, with the difference that its outputs have a
pull-up resistor embedded.
Current limitations on pins
When configured as outputs ( logic zero (0) ), single port pins can
"receive" current of 10mA. If all 8 bits on a port are active, total
current must be limited to 15mA (port P0: 26mA). If all ports (32 bits) are
active, total maximal current must be limited to 71mA. When configured as
inputs (logic 1), embedded pull-up resistor provides very weak current, but
strong enough to activate up to 4 TTL inputs from LS series.8051 Microcontroller Memory Organisation
The microcontroller memory is divided into Program Memory and Data Memory. Program Memory (ROM) is used for permanent saving program being executed, while Data Memory (RAM) is used for temporarily storing and keeping intermediate results and variables. Depending on the model in use ( still referring to the whole 8051 microcontroller family) at most a few Kb of ROM and 128 or 256 bytes of RAM can be used. However…All 8051 microcontrollers have 16-bit addressing bus and can address 64 kb memory. It is neither a mistake nor a big ambition of engineers who were working on basic core development. It is a matter of very clever memory organization which makes these controllers a real “ programmers’ tidbit“ .
Program Memory
The oldest models of the 8051 microcontroller family did not have internal
program memory . It was added from outside as a separate chip. These models are
recognizable by their label beginning with 803 ( for ex. 8031 or 8032). All
later models have a few Kbytes ROM embedded, Even though it is enough for
writing most of the programs, there are situations when additional memory is
necessary. A typical example of it is the use of so called lookup tables. They
are used in cases when something is too complicated or when there is no time
for solving equations describing some process. The example of it can be totally
exotic (an estimate of self-guided rockets’ meeting point) or totally common(
measuring of temperature using non-linear thermo element or asynchronous motor
speed control). In those cases all needed estimates and approximates are
executed in advance and the final results are put in the tables ( similar to
logarithmic tables ).
How does the microcontroller handle external memory depends on the pin EA logic state:
EA=0 In this case, internal program memory is completely ignored, only a program stored in external memory is to be executed.
EA=1 In this case, a program from builtin ROM is to be executed first ( to the last location). Afterwards, the execution is continued by reading additional memory.
in both cases, P0 and P2 are not available to the user because they are used for data nd address transmission. Besides, the pins ALE and PSEN are used too.
Data Memory
As already mentioned, Data Memory is used for temporarily storing and
keeping data and intermediate results created and used during microcontroller’s
operating. Besides, this microcontroller family includes many other registers
such as: hardware counters and timers, input/output ports, serial data buffers
etc. The previous versions have the total memory size of 256 locations, while
for later models this number is incremented by additional 128 available
registers. In both cases, these first 256 memory locations (addresses 0-FFh)
are the base of the memory. Common to all types of the 8051 microcontrollers.
Locations available to the user occupy memory space with addresses from 0 to
7Fh. First 128 registers and this part of RAM is divided in several blocks.The first block consists of 4 banks each including 8 registers designated as R0 to R7. Prior to access them, a bank containing that register must be selected. Next memory block ( in the range of 20h to 2Fh) is bit- addressable, which means that each bit being there has its own address from 0 to 7Fh. Since there are 16 such registers, this block contains in total of 128 bits with separate addresses (The 0th bit of the 20h byte has the bit address 0 and the 7th bit of th 2Fh byte has the bit address 7Fh). The third group of registers occupy addresses 2Fh-7Fh ( in total of 80 locations) and does not have any special purpose or feature.
Additional Memory Block of Data Memory
In order to satisfy the programmers’ permanent hunger for Data Memory,
producers have embedded an additional memory block of 128 locations into the
latest versions of the 8051 microcontrollers. Naturally, it’s not so simple…The
problem is that electronics performing addressing has 1 byte (8 bits) on
disposal and due to that it can reach only the first 256 locations. In order to
keep already existing 8-bit architecture and compatibility with other existing
models a little trick has been used.Using trick in this case means that additional memory block shares the same addresses with existing locations intended for the SFRs (80h- FFh). In order to differentiate between these two physically separated memory spaces, different ways of addressing are used. A direct addressing is used for all locations in the SFRs, while the locations from additional RAM are accessible using indirect addressing.
How to extend memory?
In case on-chip memory is not enough, it is possible to add two external
memory chips with capacity of 64Kb each. I/O ports P2 and P3 are used for their
addressing and data transmission.
From the users’ perspective, everything functions quite simple if properly connected because the most operations are performed by the microcontroller itself. The 8051 microcontroller has two separate reading signals RD#(P3.7) and PSEN#. The first one is activated byte from external data memory (RAM) should be read, while another one is activated to read byte from external program memory (ROM). These both signals are active at logical zero (0) level. A typical example of such memory extension using special chips for RAM and ROM, is shown on the previous picture. It is called Hardward architecture.
Even though the additional memory is rarely used with the latest versions of the microcontrollers, it will be described here in short what happens when memory chips are connected according to the previous schematic. It is important to know that the whole process is performed automatically, i.e. with no intervention in the program.
- When the program during execution encounters the instruction which resides in exter nal memory (ROM), the microcontroller will activate its control output ALE and set the first 8 bits of address (A0-A7) on P0. In this way, IC circuit 74HCT573 which "lets in" the first 8 bits to memory address pins is activated.
- A signal on the pin ALE closes the IC circuit 74HCT573 and immediately afterwards 8 higher bits of address (A8-A15) appear on the port. In this way, a desired location in addtional program memory is completely addressed. The only thing left over is to read its content.
- Pins on P0 are configured as inputs, the pin PSEN is activated and the microcon troller reads content from memory chip. The same connections are used both for data and lower address byte.
Addressing
While operating, processor processes data according to the program
instructions. Each instruction consists of two parts. One part describes what
should be done and another part indicates what to use to do it. This later part
can be data (binary number) or address where the data is stored. All 8051
microcontrollers use two ways of addressing depending on which part of memory
should be accessed:
Direct Addressing
On direct addressing, a value is obtained from a memory location while the
address of that location is specified in instruction. Only after that, the
instruction can process data (howdepends on the type of instruction: addition,
subtraction, copy…). Obviously, a number being changed during operating a
variable can reside at that specified address. For example:Since the address is only one byte in size ( the greatest number is 255), this is how only the first 255 locations in RAM can be accessed in this case the first half of the basic RAM is intended to be used freely, while another half is reserved for the SFRs.
MOV A,33h; Means: move a number from address 33 hex. to accumulator
Indirect Addressing
On indirect addressing, a register which contains address of another
register is specified in the instruction. A value used in operating process
resides in that another register. For example:Only RAM locations available for use are accessed by indirect addressing (never in the SFRs). For all latest versions of the microcontrollers with additional memory block ( those 128 locations in Data Memory), this is the only way of accessing them. Simply, when during operating, the instruction including “@” sign is encountered and if the specified address is higher than 128 ( 7F hex.), the processor knows that indirect addressing is used and jumps over memory space reserved for the SFRs.
MOV A,@R0; Means: Store the value from the register whose address is in the R0 register
into accumulatorOn indirect addressing, the registers R0, R1 or Stack Pointer are used for specifying 8-bit addresses. Since only 8 bits are avilable, it is possible to access only registers of internal RAM in this way (128 locations in former or 256 locations in latest versions of the microcontrollers). If memory extension in form of additional memory chip is used then the 16-bit DPTR Register (consisting of the registers DPTRL and DPTRH) is used for specifying addresses. In this way it is possible to access any location in the range of 64K.
SFRs (Special Function Registers)
SFRs are a kind of control table used for running and monitoring microcontroller’s operating. Each of these registers, even each bit they include, has its name, address in the scope of RAM and clearly defined purpose ( for example: timer control, interrupt, serial connection etc.). Even though there are 128 free memory locations intended for their storage, the basic core, shared by all types of 8051 controllers, has only 21 such registers. Rest of locations are intensionally left free in order to enable the producers to further improved models keeping at the same time compatibility with the previous versions. It also enables the use of programs written a long time ago for the microcontrollers which are out of production now.
This is a general-purpose register which serves for storing intermediate results during operating. A number (an operand) should be added to the accumulator prior to execute an instruction upon it. Once an arithmetical operation is preformed by the ALU, the result is placed into the accumulator. If a data should be transferred from one register to another, it must go through accumulator. For such universal purpose, this is the most commonly used register that none microcontroller can be imagined without (more than a half 8051 microcontroller's instructions used use the accumulator in some way).
B Register
B register is used during multiply and divide operations which can be
performed only upon numbers stored in the A and B registers. All other
instructions in the program can use this register as a spare accumulator (A).
R Registers (R0-R7)
The following example best illustrates the useful purpose of these registers. Suppose that mathematical operations on numbers previously stored in the R registers should be performed: (R1+R2) - (R3+R4). Obviously, a register for temporary storing results of addition is needed. Everything is quite simple and the program is as follows :
MOV A,R3; Means: move number from R3 into accumulator
ADD A,R4; Means: add number from R4 to accumulator (result remains in accumulator)
MOV R5,A; Means: temporarily move the result from accumulator into R5
MOV A,R1; Means: move number from R1 into accumulator
ADD A,R2; Means: add number from R2 to accumulator
SUBB A,R5; Means: subtract number from R5 ( there are R3+R4 )PSW Register (Program Status Word)
P - Parity bit. If a number in accumulator is even then this bit will be automatically set (1), otherwise it will be cleared (0). It is mainly used during data transmission and receiving via serial communication.
- Bit 1. This bit is intended for the future versions of the microcontrollers, so it is not supposed to be here.
OV Overflow occurs when the result of arithmetical operation is greater than 255 (deci mal), so that it can not be stored in one register. In that case, this bit will be set (1). If there is no overflow, this bit will be cleared (0).
RS0, RS1 - Register bank select bits. These two bits are used to select one of the four register banks in RAM. By writing zeroes and ones to these bits, a group of registers R0-R7 is stored in one of four banks in RAM.
F0 - Flag 0. This is a general-purpose bit available to the user.
AC - Auxiliary Carry Flag is used for BCD operations only.
CY - Carry Flag is the (ninth) auxiliary bit used for all arithmetical operations and shift instructions.
DPTR Register (Data Pointer)
These registers are not true ones because they do not physically exist. They
consist of two separate registers: DPH (Data Pointer High) and (Data Pointer
Low). Their 16 bits are used for external memory addressing. They may be
handled as a 16-bit register or as two independet 8-bit registers.Besides, the
DPTR Register is usually used for storing data and intermediate results which
have nothing to do with memory locations.
SP Register (Stack Pointer)
P0, P1, P2, P3 - Input/Output Registers
In case that external memory and serial communication system are not in use then, 4 ports with in total of 32 input-output lines are available to the user for connection to peripheral environment. Each bit inside these ports coresponds to the appropriate pin on the microcontroller. This means that logic state written to these ports appears as a voltage on the pin ( 0 or 5 V). Naturally, while reading, the opposite occurs – voltage on some input pins is reflected in the appropriate port bit.
The state of a port bit, besides being reflected in the pin, determines at the same time whether it will be configured as input or output. If a bit is cleared (0), the pin will be configured as output. In the same manner, if a bit is set to 1 the pin will be configured as input. After reset, as well as when turning the microcontroller on , all bits on these ports are set to one (1). This means that the appropriate pins will be configured as inputs.
Counters and Timers
As explained in the previous chapter, the main oscillator of the microcontroller uses quartz crystal for its operating. As the frequency of this oscillator is precisely defined and very stable, these pulses are the most suitable for time measuring (such oscillators are used in quartz clocks as well). In order to measure time between two events it is only needed to count up pulses from this oscillator. That is exactly what the timer is doing. Namely, if the timer is properly programmed, the value written to the timer register will be incremented or decremented after each coming pulse, i.e. once per each machine cycle cycle. Taking into account that one instruction lasts 12 quartz oscillator periods (one machine cycle), by embedding quartz with oscillator frequency of 12MHz, a number in the timer register will be changed million times per second, i.e. each microsecond.The 8051 microcontrollers have 2 timer counters called T0 and T1. As their names tell, their main purpose is to measure time and count external events. Besides, they can be used for generating clock pulses used in serial communication, i.e. Baud Rate.
Timer T0
As it is shown in the picture below, this timer consists of two registers –
TH0 and TL0. The numbers these registers include represent a lower and a higher
byte of one 16-digit binary number.
This means that if the content of the timer 0 is equal to 0 (T0=0) then both registers it includes will include 0. If the same timer contains for example number 1000 (decimal) then the register TH0 (higher byte) will contain number 3, while TL0 (lower byte) will contain decimal number 232.
Formula used to calculate values in registers is very simple:
TH0 × 256 + TL0 = T
Matching the previous example it would be as follows :
3 × 256 + 232 = 1000
Since the timers are virtually 16-bit registers, the greatest value that could be written to them is 65 535. In case of exceeding this value, the timer will be automatically reset and afterwords that counting starts from 0. It is called overflow. Two registers TMOD and TCON are closely connected to this timer and control how it operates.
TMOD Register (Timer Mode)
This register selects mode of the timers T0 and T1. As illustrated in the
following picture, the lower 4 bits (bit0 - bit3) refer to the timer 0, while
the higher 4 bits (bit4 - bit7) refer to the timer 1. There are in total of 4
modes and each of them is described here in this book.
Bits of this register have the following purpose:
- GATE1 starts and stops Timer 1 by means of a signal provided to the pin INT1 (P3.3):
- 1 - Timer 1 operates only if the bit INT1 is set
- 0 - Timer 1 operates regardless of the state of the bit INT 1
- C/T1 selects which pulses are to be counted up by the timer/counter 1:
- 1 - Timer counts pulses provided to the pin T1 (P3.5)
- 0 - Timer counts pulses from internal oscillator
- T1M1,T1M0 These two bits selects the Timer 1 operating mode.
- GATE0 starts and stops Timer 1, using a signal provided to the pin INT0 (P3.2):
- 1 - Timer 0 operates only if the bit INT0 is set
- 0 - Timer 0 operates regardless of the state of the bit INT0
- C/T0 selects which pulses are to be counted up by the timer/counter 0:
- 1 - Timer counts pulses provided to the pin T0(P3.4)
- 0 - Timer counts pulses from internal oscillator
- T0M1,T0M0 These two bits select the Timer 0 operating mode.
Timer 0 in mode 0 (13-bit timer)
This is one of the rarities being kept only for compatibility with the
previuos versions of the microcontrollers. When using this mode, the higher
byte TH0 and only the first 5 bits of the lower byte TL0 are in use. Being
configured in this way, the Timer 0 uses only 13 of all 16 bits. How does it
operate? With each new pulse coming, the state of the lower register (that one
with 5 bits) is changed. After 32 pulses received it becomes full and
automatically is reset, while the higher byte TH0 is incremented by 1. This
action will be repeated until registers count up 8192 pulses. After that, both
registers are reset and counting starts from 0.
Timer 0 in mode 1 (16-bit timer)
All bits from the registers TH0 and TL0 are used in this mode. That is why
for this mode is being more commonly used. Counting is performed in the same
way as in mode 0, with difference that the timer counts up to 65 536, i.e. as
far as the use of 16 bits allows.
Timer 0 in mode 2 (Auto-Reload Timer)
What does auto-reload mean? Simply, it means that such timer uses only one
8-bit register for counting, but it never counts from 0 but from an arbitrary
chosen value (0- 255) saved in another register.The advantages of this way of counting are described in the following example: suppose that for any reason it is continuously needed to count up 55 pulses at a time from the clock generator.
When using mode 1 or mode 0, It is needed to write number 200 to the timer registers and check constantly afterwards whether overflow occured, i.e. whether the value 255 is reached by counting . When it has occurred, it is needed to rewrite number 200 and repeat the whole procedure. The microcontroller performs the same procedure in mode 2 automatically. Namely, in this mode it is only register TL0 operating as a timer ( normally 8-bit), while the value from which counting should start is saved in the TH0 register. Referring to the previous example, in order to register each 55th pulse, it is needed to write the number 200 to the register and configure the timer to operate in mode 2.
Timer 0 in Mode 3 (Split
By configuring Timer 0 to operate in Mode 3, the 16-bit counter consisting
of two registers TH0 and TL0 is split into two independent 8-bit timers. In
addition, all control bits which belonged to the initial Timer 1 (consisting of
the registers TH1 and TL1), now control newly created Timer 1. This means that
even though the initial Timer 1 still can be configured to operate in any mode
( mode 1, 2 or 3 ), it is no longer able to stop, simply because there is no bit
to do that. Therefore, in this mode, it will uninterruptedly “operate in the
background “.
The only application of this mode is in case two independent 'quick' timers are used and the initial Timer 1 whose operating is out of control is used as baud rate generator.
TCON - Timer Control Register
This is also one of the registers whose bits directly control timer
operating.Only 4 of all 8 bits this register has are used for timer control, while others are used for interrupt control which will be discussed later.
- TF1 This bit is automatically set with the Timer 1 overflow
- TR1 This bit turns the Timer 1 on
- 1 - Timer 1 is turned on
- 0 - Timer 1 is turned off
- TF0 This bit is automatically set with the Timer 0 overflow.
- TR0 This bit turns the timer 0 on
- 1 - Timer 0 is turned on
- 0 - Timer 0 is turned off
How to start Timer 0 ?
Normally, first this timer and afterwards its mode should be selected. Bits
which control that are resided in the register TMOD:
This means that timer 0 operates in mode 1 and counts pulses from internal source whose frequency is equal to 1/12 the quartz frequency.
In order to enable the timer, turn it on:
Immediately upon the bit TR0 is set, the timer starts operating. Assuming that a quartz crystal with frequency of 12MHz is embedded, a number it contains will be incremented every microsecond. By counting up to 65.536 microseconds, the both registers that timer consists of will be set. The microcontroller automatically reset them and the timer keeps on repeating counting from the beginning as far as the bit’s value is logic one (1).
How to 'read' a timer ?
Depending on the timer’s application, it is needed to read a number in the
timer registers or to register a moment they have been reset.- Everything is extremely simple when it is needed to read a value of the timer which uses only one register for counting (mode 2 or Mode 3) . It is sufficient to read its state at any moment and it is it!
- It is a bit complicated to read a timer’s value when it operates in mode 2. Assuming that the state of the lower byte is read first (TL0) and the state of the higher byte (TH0) afterwards, the result is:
TH0 = 15 TL0 = 255
Everything seems to be in order at first sight, but the current state of register at the moment of reading was:
TH0 = 14 TL0 = 255
In case of negligence, this error in counting ( 255 pulses ) may occur for not so obvious but quite logical reason. Reading the lower byte is correct ( 255 ), but at the same time the program counter “ was taking “ a new instruction for the TH0 state reading, an overflow occurred and both registers have changed their contents ( TH0: 14→15, TL0: 255→0). The problem has simple solution: the state of the higher byte should be read first, then the state of the lower byte and once again the state of the higher byte. If the number stored in the higher byte is not the same both times it has been read then this sequence should be repeated ( this is a mini- loop consisting of only 3 instructions in a program).
There is another solution too. It is sufficient to simply turn timer off while reading ( the bit TR0 in the register TCON should be 0), and turn it on after that.
Detecting Timer 0 Overflow
Usually, there is no need to continuously read timer registers’ contents. It
is sufficient to register the moment they are reset, i.e. when counting starts
from 0. It is called overflow. When this has occurred, the bit TF0 from the
register TCON will be automatically set. The microcontroller is waiting for
that moment in a way that program will constantly check the state of this bit.
Furthermore, an interrupt to stop the main program execution can be enabled.
Assuming that it is needed to provide a program pause ( time the program
appeared to be stopped) in duration of for example 0.05 seconds ( 50 000
machine cycles ):First, it is needed to calculate a number that should be written to the timer registers:
This number should be written to the timer registers TH0 and TL0:
Once the timer is started it will continue counting from the written number. Program instruction checks if the bit TF0 is set, which happens at the moment of overflow, i.e. after exactly 50.000 machine cycles and 0.05 seconds respectively.
How to measure pulses?
How to count up pulses?
This time, the answer lies in the register TCON, and bit C/T0 respectively.
Similar to the previous example, this bit brings into an external signal. If
the bit is cleared everything occurs in the same way as in the previous
examples and the timer counts pulses from oscillator of defined frequency, i.e.
measures the time that went by. If the bit is set, the timer input is provided
with pulses from the pin P3.4 (T0). Since these pulses do not have some
definite time or order, it is not possible to measure time by counting them.
For that reason, this timer is turned into the counter. The highest frequency
that could be measured by such a counter is 1/24 frequency of used
quartz-crystal.
Timer 1
Referring to its characteristics, this timer is “ a twin brother “ to the Timer
0. This means that they have the same purpose, their operating is controlled by
the same registers TMOD and TCON and both of them can operate in one of 4
different modes.
UART (Universal Asynchronous Receiver and Transmitter)
One of the features that makes this microcontroller so powerful is an integrated UART, better known as a serial port. It is a duplex port, which means that it can transmit and receive data simultaneously. Without it, serial data sending and receiving would be endlessly complicated part of the program where the pin state continuously is being changed and checked according to strictly determined rhythm. Naturally, it does not happen here because the UART resolves it in a very elegant manner. All the programmer needs to do is to simply select serial port mode and baud rate. When the programmer is such configured, serial data sending is done by writing to the register SBUF while data receiving is done by reading the same register. The microcontroller takes care of all issues necessary for not making any error during data exchange.
Serial port should be configured prior to being used. That determines how many bits one serial “word” contains, what the baud rate is and what the pulse source for synchronization is. All bits controlling this are stored in the SFR Register SCON (Serial Control).
SCON Register (
- SM0 - bit selects mode
- SM1 - bit selects mode
- SM2 - bit is used in case that several microcontrollers share the same interface. In normal circumstances this bit must be cleared in order to enable connection to function normally.
- REN - bit enables data receiving via serial communication and must be set in order to enable it.
- TB8 - Since all registers in microcontroller are 8-bit registers, this bit solves the problem of sending the 9th bit in modes 2 and 3. Simply, bits content is sent as the 9th bit.
- RB8 - bit has the same purpose as the bit TB8 but this time on the receiver side. This means that on receiving data in 9-bit format , the value of the last ( ninth) appears on its location.
- TI - bit is automatically set at the moment the last bit of one byte is sent when the USART operates as a transmitter. In that way processor “knows” that the line is available for sending a new byte. Bit must be clear from within the program!
- RI - bit is automatically set once one byte has been received. Everything functions in the similar way as in the previous case but on the receive side. This is line a “doorbell” which announces that a byte has been received via serial communication. It should be read quickly prior to a new data takes its place. This bit must also be also cleared from within the program!
Mode 0:
In mode 0, the data are transferred through the RXD pin, while clock pulses appear on the TXD pin. The bout rate is fixed at 1/12 the quartz oscillator frequency. On transmit, the least significant bit (LSB bit) is being sent/received first. (received).
TRANSMIT - Data transmission in form of pulse train automatically starts on the pin RXD at the moment the data has been written to the SBUF register.In fact, this process starts after any instruction being performed on this register. Upon all 8 bits have been sent, the bit TI in the SCON register is automatically set.
RECEIVE - Starts data receiving through the pin RXD once two necessary conditions are met: bit REN=1 and RI=0 (both bits reside in the SCON register). Upon 8 bits have been received, the bit RI (register SCON) is automatically set, which indicates that one byte is received.
Since, there are no START and STOP bits or any other bit except data from the SBUF register, this mode is mainly used on shorter distance where the noise level is minimal and where operating rate is important. A typical example for this is I/O port extension by adding cheap IC circuit ( shift registers 74HC595, 74HC597 and similar).
Mode 1:
TRANSMIT - A sequence for data transmission via serial communication is automatically started upon the data has been written to the SBUF register. End of 1 byte transmission is indicated by setting the TI bit in the SCON register.
RECEIVE - Receiving starts as soon as the START bit (logic zero (0)) appears on the pin RXD. The condition is that bit REN=1and bit RI=0. Both of them are stored in the SCON register. The RI bit is automatically set upon receiving has been completed.
The Baud rate in this mode is determined by the timer 1 overflow time.
Mode 2
TRANSMIT - A sequence for data transmission via serial communication is automatically started upon the data has been written to the SBUF register. End of 1 byte transmission is indicated by setting the TI bit in the SCON register
Mode 3
Mode 3 is the same as Mode 2 except the baud rate. In Mode 3 is variable and
can be selected.
Baud Rate
Baud Rate is defined as a number of send/received bits per second. In case
the UART is used, baud rate depends on: selected mode, oscillator frequency and
in some cases on the state of the bit SMOD stored in the SCON register. All
necessary formulas are specified in the table :
Timer 1 is usually used as a baud rate generator because it is easy to adjust various baud rate by the means of this timer. The whole procedure is simple:
- First, Timer 1 overflow interrupt should be disabled
- Timer T1 should be set in auto-reload mode
- Depending on necessary baud rate, in order to obtain some of the standard values one of the numbers from the table should be selected. That number should be written to the TH1 register. That's all.
Multiprocessor Communication
As described in the previous text, modes 2 and 3 enable the additional 9th
data bit to be part of message. It can be used for checking data via parity
bit. Another useful application of this bit is in communication between two
microcontrollers, i.e. multiprocessor communication. This feature is enabled by
setting the SM2 bit in the SCON register. The consequence is the following:
when the STOP bit is ready, indicating end of message, the serial port
interrupt will be requested only in case the bit RB8 = 1 (the 9th bit).The whole procedure will be performed as follows:
Suppose that there are several connected microcontrollers having to exchange data. That means that each of them must have its address. The point is that each address sent via serial communication has the 9th bit set (1), while data has it cleared (0). If the microcontroller A should send data to the microcontroller C then it at will place first send address of C and the 9th bit set to 1. That will generate interrupt and all microcontrollers will check whether they are called.
Of course, only one of them will recognize this address and immediately clear the bit SM2 in the SCON register. All following data will be normally received by that microcontroller and ignored by other microcontrollers.
8051 Microcontroller Interrupts
There are five interrupt sources for the 8051, which means that they can recognize 5 different event that can interrupt regular program execution. Each interrupt can be enabled or disabled by setting bits in the IE register. Also, as seen from the picture below the whole interrupt system can be disabled by clearing bit EA from the same register.Now, one detail should be explained which is not completely obvious but refers to external interrupts- INT0 and INT1. Namely, if the bits IT0 and IT1 stored in the TCON register are set, program interrupt will occur on changing logic state from 1 to 0, (only at the moment). If these bits are cleared, the same signal will generate interrupt request and it will be continuously executed as far as the pins are held low.
- EA - bit enables or disables all other interrupt sources (globally)
- 0 - (when cleared) any interrupt request is ignored (even if it is enabled)
- 1 - (when set to 1) enables all interrupts requests which are individually enabled
- ES - bit enables or disables serial communication interrupt (UART)
- 0 - UART System can not generate interrupt
- 1 - UART System enables interrupt
- ET1 - bit enables or disables Timer 1 interrupt
- 0 - Timer 1 can not generate interrupt
- 1 - Timer 1 enables interrupt
- EX1 - bit enables or disables INT 0 pin external interrupt
- 0 - change of the pin INT0 logic state can not generate interrupt
- 1 - enables external interrupt at the moment of changing the pin INT0 state
- ET0 - bit enables or disables timer 0 interrupt
- 0 - Timer 0 can not generate interrupt
- 1 - enables timer 0 interrupt
- EX0 - bit enables or disables INT1 pin external interrupt
- 0 - change of the INT1 pin logic state can not cause interrupt
- 1 - enables external interrupt at the moment of changing the pin INT1 state
Interrupt Priorities
It is not possible to predict when an interrupt will be required. For that reason,
if several interrupts are enabled. It can easily occur that while one of them
is in progress, another one is requested. In such situation, there is a
priority list making the microcontroller know whether to continue operating or
meet a new interrupt request.
The priority list cosists of 3 levels:
- Reset! The apsolute master of the situation. If an request for Reset omits, everything is stopped and the microcontroller starts operating from the beginning.
- Interrupt priority 1 can be stopped by Reset only.
- Interrupt priority 0 can be stopped by both Reset and interrupt priority 1.
- Once an interrupt service begins. It cannot be interrupted by another inter rupt at the same or lower priority level, but only by a higher priority interrupt.
- If two interrupt requests, at different priority levels, arrive at the same time then the higher priority interrupt is serviced first.
- If the both interrupt requests, at the same priority level, occur one after another , the one who came later has to wait until routine being in progress ends.
- If two interrupts of equal priority requests arrive at the same time then the interrupt to be serviced is selected according to the following priority list :
- External interrupt INT0
- Timer 0 interrupt
- External Interrupt INT1
- Timer 1 interrupt
- Serial Communication Interrupt
IP Register (Interrupt Priority)
The IP register bits specify the priority level of each interrupt (high or
low priority).
- PS -
- Priority 0
- Priority 1
- PT1 - Timer 1 interrupt priority
- Priority 0
- Priority 1
- PX1 - External Interrupt INT1 priority
- Priority 0
- Priority 1
- PT0 - Timer 0 Interrupt Priority
- Priority 0
- Priority 1
- PX0 - External Interrupt INT0 Priority
- Priority 0
- Priority 1
Handling Interrupt
Once some of interrupt requests arrives, everything occurs according to the
following order- Instruction in progress is ended
- The address of the next instruction to execute is pushed on the stack
- Depending on which interrupt is requested, one of 5 vectors (addresses) is written to the program counter in accordance to the following table:
4.
The appropriate subroutines processing interrupts
should be located at these addresses. Instead of them, there are usually jump
instructions indicating the location where the subroutines reside.
5. When interrupt routine
is executed, the address of the next instruction to execute is poped from
the stack to the program counter and interrupted program continues
operating from where it left off.
Reset
occurs when the RS pin is supplied with a positive pulse in duration of at
least 2 machine cycles ( 24 clock cycles of crystal oscillator). After that,
the microcontroller generates internal reset signal during which all SFRs,
excluding SBUF registers, Stack Pointer and ports are reset ( the state of the
first two ports is indefinite while FF value is being written to the ports
configuring all pins as inputs). Depending on device purpose and environment it
is in, on power-on reset it is usually push button or circuit or both connected
to the RS pin. One of the most simple circuit providing secure reset at the
moment of turning power on is shown on the picture.
Everything functions rather simply: upon the power is on, electrical condenser is being charged for several milliseconds through resistor connected to the ground and during this process the pin voltage supply is on. When the condenser is charged, power supply voltage is stable and the pin keeps being connected to the ground providing normal operating in that way. If later on, during the operation, manual reset button is pushed, the condenser is being temporarily discharged and the microcontroller is being reset. Upon the button release, the whole process is repeated…
Interrupt system applied on the 8051 microcontrollers practically stops operating and enables instructions to be executed one at a time by pushing button. Two interrupt features enable that:
What is going on? Once the pin P3.2 is set to “0” (for example, by pushing
button), the microcontroller will interrupt program execution jump to the
address 03hex, will be executed a mini-interrupt routine consisting of 3
instructions is located at that address.
The first instruction is being executed until the push button is pressed ( logic one (1) on the pin P3.2). The second instruction is being executed until the push button is released. Immediately after that, the instruction RETI is executed and processor continues executing the main program. After each executed instruction, the interrupt INT0 is generated and the whole procedure is repeated ( push button is still pressed). Button Press = One Instruction.
In order to terminate the Idle mode and make the microcontroller operate normally, it is necessary to enable and execute any interrupt or reset.Then, the IDL bit is automatically cleared and the program continues executing from instruction following that instruction which has set the IDL bit. It is recommended that three first following one which set NOP instructions. They do not perform any operation but keep the microcontroller from undesired changes on the I/O ports.
During Power Down mode, the state of all SFR registers and I/O ports remains unchanged, and after the microcontroller is put get into the normal mode, the content of the SFR register is lost, but the content of internal RAM is saved. Reset signal must be long enough approximately 10mS in order to stabilize quartz oscillator operating.
PCON register
The purpose of the Register PCON bits :
Everything functions rather simply: upon the power is on, electrical condenser is being charged for several milliseconds through resistor connected to the ground and during this process the pin voltage supply is on. When the condenser is charged, power supply voltage is stable and the pin keeps being connected to the ground providing normal operating in that way. If later on, during the operation, manual reset button is pushed, the condenser is being temporarily discharged and the microcontroller is being reset. Upon the button release, the whole process is repeated…
Through the program- step by step...
The microcontrollers normally operate at very high speed. The use of 12 Mhz
quartz crystal enables 1.000.000 instructions per second to be executed! In
principle, there is no need for higher operating rate. In case it is needed, it
is easy to built-in crystal for high frequency. The problem comes up when it is
necessary to slow down. For example, when during testing in real operating
environment, several instructions should be executed step by step in order to
check for logic state of I/O pins.Interrupt system applied on the 8051 microcontrollers practically stops operating and enables instructions to be executed one at a time by pushing button. Two interrupt features enable that:
- Interrupt request is ignored if an interrupt of the same priority level is being in progress.
- Upon interrupt routine has been executed, a new interrupt is not executed until at least one instruction from the main program is executed.
- External interrupt sensitive to the signal level should be enabled (for example INT0).
- Three following instructions should be entered into the program (start from address 03hex.):
The first instruction is being executed until the push button is pressed ( logic one (1) on the pin P3.2). The second instruction is being executed until the push button is released. Immediately after that, the instruction RETI is executed and processor continues executing the main program. After each executed instruction, the interrupt INT0 is generated and the whole procedure is repeated ( push button is still pressed). Button Press = One Instruction.
8051 Microcontroller Power Consumption Control
Conditionally said microcontroller is the most part of its “lifetime” is inactive for some external signal in order to takes its role in a show. It can make a great problem in case batteries are used for power supply. In extremely cases, the only solution is to put the whole electronics to sleep in order to reduce consumption to the minimum. A typical example of this is remote TV controller: it can be out of use for months but when used again it takes less than a second to send a command to TV receiver. While normally operating, the AT89S53 uses current of approximately 25mA, which shows that it is not too sparing microcontroller. Anyway, it doesn’t have to be always like this, it can easily switch the operation mode in order to reduce its total consumption to approximately 40uA. Actually, there are two power-saving modes of operation: Idle and Power Down.
Idle mode
Immediately upon instruction which sets the bit IDL in the PCON register,
the microcontroller turns off the greatest power consumer- CPU unit while peripheral
units serial port, timers and interrupt system continue operating normally
consuming 6.5mA. In Idle mode, the state of all registers and I/O ports is
remains unchanged.In order to terminate the Idle mode and make the microcontroller operate normally, it is necessary to enable and execute any interrupt or reset.Then, the IDL bit is automatically cleared and the program continues executing from instruction following that instruction which has set the IDL bit. It is recommended that three first following one which set NOP instructions. They do not perform any operation but keep the microcontroller from undesired changes on the I/O ports.
Power Down mode
When the bit PD in the register PCON is set from within the program, the
microcontroller is set to Powerdown mode. It and turns off its internal
oscillator reducing drastically consumption in that way. In power- down mode
the microcontroller can operate using only 2V power supply while the total
power consumption is less than 40uA. The only way to get the microcontroller
back to normal mode is reset.During Power Down mode, the state of all SFR registers and I/O ports remains unchanged, and after the microcontroller is put get into the normal mode, the content of the SFR register is lost, but the content of internal RAM is saved. Reset signal must be long enough approximately 10mS in order to stabilize quartz oscillator operating.
PCON register
The purpose of the Register PCON bits :
- SMOD By setting this bit baud rate is doubled.
- GF1 General-purpose bit (available for use).
- GF1 General-purpose bit (available for use).
- GF0 General-purpose bit (available for use).
- PD By setting this bit the microcontroller is set into Power Down mode.
- IDL By setting this bit the microcontroller is set into Idle mode.
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