Microcontroller
7
8051 Microcontroller
2.1) Introduction
The 8051 is an 8-bit Microcontroller originally developed by Intel in 1980. The
block diagram of 8051 shows all the features unique to Microcontroller.
Internal ROM & RAM
Input &output ports with programmable pins
Timer and Counter
Serial data Communication
The 8051 architecture consists of these specific features
8-bit CPU with Register A and B
16-bit program counter (PC) and data pointer (DPTR)
8 bit program status word (PSW)
Internal ROM or EPROM
Internal RAM 128 bytes
In this
4 Register banks, each containing 8 registers
16 bytes which may be addressed at bit level
8 bytes of general purpose data memory
•
32 input /output pins arranged as 4-8 bit ports P0-P3
•
Two 16 bit timer/counters (TO-T1)
•
Fully duplexed serial data receiver/transmitter (SBUF)
•
Control register TMOD, TCON, SCOM, PCON, IP and IE
•
Oscillator and Clock circuit
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Fig 2.1: Pin description of 8051
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Fig 2.2: Block diagram of 8051
2.2) Programming model
The programming model of 8051 is shown in fig 2.3:
It is a collection of 8 and 16 bit register and 8 bit memory locations. These
registers and memory locations can be made to operate using software instructions
that are incorporated as a part of the design. Each register with exception of program
counter, has an internal one byte address assigned to it. Some registers such as TCON,
SCON, IP, IE, A, B, PSW and ports are all bit addressable and are marked with *
mark. These types have provision for reading or writing the entire byte of data and
also each individual bits may be read or altered.
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Fig 2.3: Programming model of 8051
These operations can be done by software instructions that are generally able
to specify a register by its address or its symbolic name or both. Let us consider each
block separately.
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2.2.1) 8051 oscillator and Clocks
Pins XTAL1 and XLAT2 are provided for connecting resonant network to
form an oscillator. The methods used in these networks are:
Ceramic resonators
Quartz crystal and capacitor.
Ceramic resonators
The cost of this is less compared to quartz crystal resonators, but in ceramic
resonators frequency stability and accuracy decreases and makes it a poor choice, if
high-speed serial data communication with other systems, or critical timing is to be
done.
Quartz crystal and capacitor are employed as shown:
Fig 2.4: Oscillator circuit & timing
Choosing of crystal depends on maximum and minimum frequencies specified by
8051 manufacturers.
Minimum frequency implies that some internal memories are
dynamic and must always operate above a minimum
frequency or data will be lost.
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One more aspect to be considered is serial data
communication needs often dictate the frequency of the
oscillator because of the requirement that internal counter
must divide the basic clock rate to yield standard
communication bit per second rates(baud rate).
OPERATION
The smallest interval of time to accomplish any simple instruction or part of a
complex instruction is called machine cycle. Program instruction are may require one
two or 4 machine cycles to be executed by the Microcontroller automatically
beginning with the instruction located at ROM memory address 0000H at the time
Microcontroller is first set / reset.
Time of execution of an instruction is,
Tinst =((C*12d) / crystal frequency)
C = No. Of cycles required for executing a particular instruction.
Two ALE pulses are produced per machine cycle. These are primarily used as
a timing pulse for external memory access indicates when every instruction byte is
fetched. Two bytes of single instruction may thus be fetched and executed in one
machine cycle. Single byte instruction are not executed in half cycle, however single
byte instruction “throw away “ the second byte. The next instruction is then fetched in
the following cycle.
2.2.2) Program Counter and Data Pointer
PROGRAM COUNTER
Program instruction bytes are fetched from locations in memory that are
present at address given by the PC. Program may be present in on-chip ROM at
addresses 0000h to 0FFFh, external to the chip for addresses that exceed 0FFFh, or
totally external for all addresses from 0000h to FFFFh. The PC is automatically
incremented after every instruction byte is fetched. The PC is the only register that
does not have an internal address.
DPTR
This register is made up of two 8-bit registers, named DPH and DPL, which
are used to furnish memory addresses for internal and external code access and
external data access. The DPTR is under the control of program instructions and can
be specified by its 16-bit name, DPTR, or by each individual byte name, DPH and
DPL, which also have different assigned address to it.
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Microcontroller
A and B CPU Registers:
The A (accumulator) register is the most versatile of the two CPU registers
and is used for many operations, including addition, subtraction, integer
multiplication and division, and Boolean bit manipulations. The A register is also
used for all data transfers between the 8051 and any external memory. The B register
is used with the A register for multiplication and division operations
PROGRAM STATUS WORD:
[ BIT ADDRESSABLE SPECIAL FUNCTION REGISTER ]
7
CY
6
AC
5
4
FO
BIT
7
6
5
4
3
2
1
0
RS1
3
RS0
SYMBOL
CY
AC
FO
RS1
RS0
RS1
RS0
0
0
0
1
1
0
1
1
OV
------P
2
OV
1
-------
0
P
FUNTION
carry flag
Auxiliary Carry flag
User flag 0
Register bank select bit 1
Register bank select bit 2
bank selected
Register bank 0
Register bank 1
Register bank 2
Register bank 3
overflow flag
reserved for future use
parity flag
It is bit addressable as PSW.0 through PSW.1
Memories play an important role in computers and as well as in many
application kits.
These memories are basically classified as ROM and RAM.
ROM is used mainly to store program code bytes
RAM is used to store variable data that can be altered as the program runs.
In Microcontroller there are 2 types of architecture used to access memory
o Von Neumann architecture
o Harvard architecture
8051 has Harvard architecture, which uses same address in different
memories, for code and data.
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2.2.3) Internal ram
There is 128-byte internal RAM, these are organized into three distinct areas:
•
Thirty-two bytes from address 00h to 1Fh that make up 32 working
registers organized as four banks of eight registers each. These banks
are numbered 0-3 and are made up of eight registers named R0 to R7.
Each register can be addressed by name when its bank is selected or
by its RAM address. Bits RS0 and RS1 in the PSW determine which
bank of registers is currently in use at any time when the program is
running. Register banks not selected can be used as general purpose
RAM.
•
A bit-addressable area of 16 bytes occupies RAM byte addresses 20h
to 2Fh, forming a total of 128 addressable bits. An address bit may be
specified by its bit address of 00h to 7Fh, or 8 bits may form any byte
address from 20h to 2Fh.
•
A general purpose RAM area above the bit area, from 30h to 7Fh,
addressable as bytes.
Fig 2.5: Internal memory model
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2.2.4) The Stack and the Stack Pointer
.
The 8-bit Stack pointer (SP) register is used by the 8051 to hold an internal
RAM address that is called the top of the stack. The address held in the SP register is
the location in internal RAM where the last byte of data was stored by a stack
operation.
When data is to be placed on the stack, the SP increments before storing the
data on the stack. When the byte is read from the stack, and then the SP decrements to
point to the next available byte of stored data. So normally stack is placed high in
internal RAM, by an appropriate choice of the number placed in SP register to avoid
conflicts with the register, bit and scratch pad internal RAM areas.
2.2.5) INTERNAL ROM
The internal ROM has address from 0000h to FFFFh. The PC is ordinarily
used to address program code bytes from address 0000h-FFFFh,program addresses
higher than 0FFFh which exceed the internal ROM capacity will cause the 8051 to
automatically address fetch code bytes from an external memory, address 0000h to
FFFFh by connecting external access pin (EA=31) to ground.
The detailed explanation about external memory is given in chapter 3.
2.2.6) SPECIAL FUNCTION REGISTERS
In 8051 operation that do not use internal 128 bytes RAM address from 00h77h are done by a group of specific internal register each called a Special Function
Register (SRF), which are address from 80h-ffh. Some of these are also bit
addressable.
The SFR registers are displayed in the following table.
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Microcontroller
A
B
DPH
DPL
IE
IP
P0
P1
P2
P3
PCON
PSW
SCON
SBUF
SP
TMOD
TCON
TL0
TH0
TL1
Accumulator
Arithmetic
Addressing
External memory
Addressing
External memory
Interrupt enable
Control
Interrupt priority
I/O port latch
I/O port latch
I/O port latch
I/O port latch
Power Control
Program Status
word
Serial port Control
Serial port data
buffer
Stack pointer
Timer or mode
control
Timer/Counter
Control
Timer0 Low Byte
Timer0 High Byte
Timer1 Low Byte
Timer1High Byte
16
0E0
0F0
83
82
0A8
0B8
80
90
A0
0B0
87
0D0
98
99
81
89
88
8A
8C
8B
8D
TH1
2.2.7) Counters & Timers
Consist of two 16 bit Counters named T0 and T1 are provided for general use
of the programmer. Each may be programmed to count internal clock pulses, acting as
a timer or programmed to count external pulse as a counter. The Counter are divided
into two 8 byte register called the timer low (TL0,TL1) and high (TH,TH1) bytes. All
counter action is controlled by bit state in the timer mode control register TMOD, the
timer/counter control register (TCON) and certain program instruction.
TMOD - controls 2 timers with each 4 bit controlling one Timer.
TCON - has control bits and flags for timer in upper nibble and control bit and flags
for external interrupts in the lower nibble.
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TCON
TF1
TR1
TF0
TR0
IE1
IT1
IE0
IT0
TCON is Bit addressable
Bit
7
SYMBOL
FUNTION
TF1 Timer 1 overflow flag. Set when timer rolls
from all 1s to 0Cleared when processor vectors
execute interrupt service routine located at
program address 001Bh.
6.
TR1 Timer 1 run control bit. Set to 1 by program to
enable Timer to count; Cleared to 0 by
program to halt timer Does not reset timer.
5
TF0
4
TR0 Timer 0 run control bit. Set to 1 by program to
enable timer to count; Cleared to 0 by program
to halt timer. Does not reset timer.
3
IE1
External interrupts 1Edge flag. Set to 1 when a
high to low edge signal is received on port3
pin3.3 (INT1). Cleared when processor vector
to interrupt service routine located at program
address 0013h. Not related to timer operator
program in modes 2 and 3.Stop bit in mode
1.Not used in mode 0.
2
IT1
External interrupt 1 signal type control bit. Set
to 1 by program to enable external interrupt 1
to be triggered by a falling edge signal. Set to 0
by program to enable a low level signal on
external interrupt 1 to generate an interrupt.
1
IE0
External interrupt 0. Edge Set to 1 when a high
to low edge signal is received on port 3 pin
3.2(INT0). Cleared when processor. Vector to
interrupt service routine located at program
address 0003h.Not related to timer operations
bit in mode 1.Not used in mode 0.
Timer 0 overflow flag. Set when timer rolls
from all 1s to 0s.Cleared when processor
vectors to execute interrupt service routine
located at program address 000Bh.
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Microcontroller
0
IT0
18
External interrupt 0 signal type control bit. Set
to 1 by program to enable external interrupt 0
to be triggered by a falling edge signal. Set to 0
by program to enable a low level signal on
external interrupt 1 to generate an interrupt.
M1
0
0
1
1
M0
0
1
0
1
MODE
0
1
2
3
2.2.8) Timer counter interrupts
The counters can be used to interrupt the program as follows. When the
program wishes to count a certain number of internal pulses or external events a
number is placed in one of the counters. The number represents the maximum count
less the desired count + 1. The counter increment from the initial number to maximum
and rolls over to zero and final pulse and also set a timer. The flag conditions may be
tested by an instruction to tell the program that the count has been accomplished or
the flag may be used to interrupt the program.
Timer:
If the counter to be used as a timer the following conditions should be applied
for basic timer control logic shown below;
fig 2.6: Timer/counter control logic
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Fig 2.7 Timer 1 and Timer 0 modes of operation
C/Ť bit in TMOD register must be set to zero
Bit TRX in TCON register must be set to one and gate bit in TMOD
register must be zero or external pin INT0 or INT1 must be one.
Timer mode of operation
There are four modes of operation specific mode is selected using bit M1 & M0 in
TMOD register
TIMER mode 0
TMOD register set to 00B, resulting in initializing THX register as 8-bit counter and
TLX as a five bit counter. The pulse input is divided by 32D in TL so that TH counts
the original oscillator frequency reduced by a total of 384D.
Timer mode 1
Differs by Mode 0 as TLX is configured as a full 8-bit counter. When mode bit are
set to 01B in TMOD.
Timer mode 2
TMOD register is set to 10B resulting to use only the TLX counter as a 8-bit counter.
THX is used to hold a value that is loaded into TLX every time, TLX overflow from
FFH to 00H. The timer flag is also set when TLX overflows.
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Microcontroller
Timer mode 3
In this both the Timers 0 and 1cannot set to mode 3., because placing timer1 in
MODE 3 causes it to stop counting. The control bit TRI and timer 1 flag TFI are used
by timer 0. Timer 0 in mode 3 becomes two completely separate 8-bit counters. TL0
is controlled by the gate arrangement and sets timer flag TF0 whenever it overflows
from FFh to 00h. TH0 receives the timer clock under the control of TRI only sets the
TFI flag when it overflows.
While timer 0 is in mode 3 with one important exception. No interrupts will be
generated by the timer 1 while timer 0 is using the TFI overflow flag.
Counting
The only difference between counting and timing is the source of the clock
pulses to the counters. When used as a timer the clock pulses are sourced from the
oscillator through the divide-by-12d circuit. When used as a counter pin T0 supplies
pulses to counter 0 and pin T1 to counter 1.The C/T bit in TMOD must be set to 1 to
enable pulses from the TX pin to reach the control circuit .
The inputs pulses on TX sampled during P2 of state 5 every machine cycle. A
change on the input from high to low between samples will increment the counter.
Each high and low state of the input pulses must thus be held constant for at least one
machine cycle to ensure reliable counting. Since this takes 24 pulses the maximum
input frequency that can be accurately counted is the oscillator frequency divided by
24. For our 6 megahertz crystal the calculation yields a maximum external frequency
of 250 kilohertz.
2.2.9) Serial data input/output
Computers must be able to communicate with other computers in modern
multiprocessor distributed system. One cost-effective way to communicate is to send
and receive data bits serially. The 8051 has a serial data communication circuit that
uses register SBUF to hold data. Register SCON controls data communication register
PCON controls data rates and pins RXD and TXD connect to the serial data network.
SCON
SM0
SM1
SM2
REN
TB8
RB8
TI
RI
PCON
SMOD
--------
--------
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---------
GF1
GF0
PD
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2.2.9) INPUT/OUTPUT PINS, PORTS AND CIRCUITS
The dip has 40 pins, and the success of the design in the market place was
determined by the flexibility built into the use of these pins. For this reason,24 of the
pins may each be used for one of two entirely different functions yielding a total pin
configuration of 64.The Function a pin performs at any given instant depends first on
what is physically connected to it and then on what software commands are used to
"program" the pin both of these factors are under the complete control of the 8051
programmer and circuit designer.
Fig 2.8: Port Pin circuits
PORT 0
Port 0 pins may serve as inputs, outputs or when used together as a bidirectional low-order address and data bus for external memory. For example when a
pin is to be used as an input a1 must be written to the corresponding port 2 latch by
the program, thus turning both of the output transistors off, which in turn causes the
pin to "float" in a high-impedance state and the pin essentially connected to the input
buffer.
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When used as an output, the pin latches that are programmed to a 0 will turn
on the lower FET grounding the pin. All latches that are programmed to a 1 still float
thus external pull up resistors will be needed to supply a logic high when using port 0
as an output.
PORT 1
Port 1 pins have no dual functions therefore the output latch is connected
directly to the gate of the lower FET, which has an FET circuit labeled internal FET
pull up as an active pull up load. Used as an input, a 1 is written to the latch, turning
the lower FET off, the pin and the input to the pin buffer are pulled high by the FET
load. An external circuit can overcome the high-impedance pull up and drive the pin
low to input a 0 or leave the input high for a 1. If used as an output the latches
containing a 1 can drive the input of an external circuit high through the pull-up.
If a 0 is written to the latch the lower FET is on, the pull-up is off and the pin can
drive the input of the external circuit low.
To aid in speeding up switches times when the pin is used as an output the
internal FET pull-up has another FET in parallel with it. The second FET is turned on
for two oscillator time periods during a low-to-high transition on the pin, as shown in
fig 2.7. This arrangement provides a low impedance path to the positive voltage
supply to help reduce rise in times in charging any parasitic capacitance in the
external circuitry.
PORT 2
Port 2 may be used as an input/output port similar in operation to port 1 the
alternate use of port 2 is to supply a high-order address byte in conjunction with the
port 0 low-order byte to address external memory. Port 2 pins are momentarily
changed by the address control signals when supplying the high byte of 16-bit
address. Port 2 latches remain stable when external memory is addressed as they do
not have to be turned around for data input in the case for port 0.
PORT 3
Port 3 is an input/output port similar to port 1. The input and output functions
can be programmed under the control of the P3 latches or under the control of various
other specific function registers. The port 3 alternate uses are shown in the following
table:
Unlike ports 0 and 2, which can have external addressing functions and change all
eight port bits when in alternate use, each pin of port 3 may be individually
programmed to be used either as I/O or as one of the alternate function.
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PIN
P3.0-RXD
P3.1-TXD
P3.2-INTO
P3.3- INT1
P3.4-TO
P3.5-T1
P3.6 –WR
P3.7-RD
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ALTERNATE USE
SFR
Serial Data Input
SBUF
Serial Data Output
SBUF
____
External Interrupt 0
TCON.1
____
External Interrupt 1
TCON.3
External Timer 0Input
TMOD
External Timer 1Input
TMOD
___
External memory write pulse ----___
External memory read pulse ------
2.2.10) INTERRUPTS
A computer program has 2 ways to determine the conditions that exist in
internal and external circuits. One method uses software instruction that jumps to
subroutine on the states of flags and port pins. The second method responds to
hardware signals called interrupts that force the program to call a subroutine.
Software techniques use up processor time that could be devoted to other tasks;
interrupts take processor time only when action by the program is needed.
Any interrupt can cause the 8051, to perform a hardware call to an interrupt
handling subroutine that is located at a predetermined absolute address in program
memory.
Five interrupts are provided in 8051.Three of these are generated automatically
by internal operation, Timer flag0, Timer flag 1 and then serial port interrupt (RI or
TI).Two interrupt are triggered by external signals provided by circuitry that is
connected to pins INT0 andINT1.
All interrupt functions are under the control of the program. The program is
able to alter control bits in the interrupt enable register (IE)the interrupt priority
register(IP) and Timer control register (TCON).The program can block all or any
combination of the interrupts from acting on the program by suitable setting or
clearing bits in these registers. The IE and IP registers are shown below:
IE
7
EA
6
------
5
ET2
4
ES
3
ET1
2
EX1
1
ET0
0
EX0
3
PT1
2
PX1
1
PT0
0
PX0
IP
7
-----
6
-----
5
PT2
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THE INTERRUPT ENABLE(IE) SPECIAL FUNCTION REGISTER
Bit
SYMBOL
FUNTION
7
EA
Enable interrupt bit. Cleared to 0 by
program to disable all interrupts;
Set to 1 to permit indiviual interrupts
to be enable bits.
6
---
Not implemented
5
ET2
Reserved for future use.
4
ES
Enable serial port interrupt.Set to 1
by program to enable serial port
interrupt. Cleared to 0 to disable
serial port interrupt.
3
ET1
Enable timer 1 overflow interrupt.Set to
1 by program to enable timer 1 overflow
interrupt; cleared to 0 to disable timer
1 overflow interrupt.
2
EX1
Enable external interrupt 1.Set to 1 by
program to enable INT1 low interrupt;
cleared to disanable interrupt
1
ET0
Enable timer 0 overflow interrupt. Set
to 1 by program to enable timer 0
overflow interrupt; cleared to 0 to
disable timer 0 overflow interrupt.
0
EX0
Enable external interrupt 0.Set to 1
by program to enable INT0 low
interrupt; cleared to disable
interrupt.
THE INTERRUPT PRIORITY (IP) SPECIAL FUNCTION REGISTER
Bit
SYMBOL
FUNTION
7
---
Not implemented
6
---
Not implemented
5
PT2
Reserved for future use.
4
PS
3
PT1
2
PX1
1
PT0
Priority of serial port
interrupt.Set/cleared by program
Priority of timer 1 overflow
interrupt 1. Set/cleared by program.
Priority of external interrupt.
Set/cleared by program
Priority of timer 0 overflow
interrupt. Set/cleared by program.
0
PX0
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Priority of external interrupt 0.
Set/cleared by program
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After the interrupt has been handled by the interrupt subroutine, which is
placed by the programmer at the interrupt location in program memory the interrupt
program must resume operation at the instruction where the interrupt took place.
2.2.11) RESET INTERRUPT
This is ultimate interrupt because the program may not block the action of
voltage on RST pin.This type of interrupt is often called nonmaskable because no
combination of bits can stop or mask the register action. After the RST pin is bought
high level interrupt is enabled when made low internal registers will have the values
shown:
Register
Pc
DPTR
A
B
SP
PSW
P0-3
IP
IE
TCON
TMOD
THO
TL0
TH1
TL1
SCON
SBUF
PCON
value
0000
0000
0000
00
07
FF
XXX0000b
0XX0000b
00
00
00
00
00
00
00
XX
0XXXXXXb
2.3) Summary
This chapter gives a detailed description of the 8051 Microcontroller. The next
chapter gives the design of Microcontroller systems.
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