8051 microcontroller notes continuous

The 8051 Microcontroller

Prepared By,
R-THANDAIAH PRABU M.E.,
Lecturer - ECE
thandaiah@gmail.com

8051 and PC
• The 8051 module connects to PC by using
RS232.
• RS232 is a protocol which supports half-
duplex, synchronous/asynchronous, serial
communication.
• We discuss these terms in following sections.
RS232
PC 8051

COM 1 port MAX232 UART

Data communication classification

Simplex vs. Duplex
Transmission
• Simplex transmission: the data can sent
in one direction.
– Example: the computer only sends data to
the printer.
Transmitter Receiver

• Duplex transmission: the data can be
transmitted and receive
Transmitter Transmitter
Receiver Receiver

Half vs. Full Duplex
• Half duplex: if the data is transmitted
one way at a time.

Receiver Transmitter
• Full duplex: if the data can go both
ways at the same time.
– Two wire conductors for the data lines.

Receiver Transmitter

Serial vs Parallel Data Transfer

Serial Transfer Parallel Transfer
D0 D0-D7
Sender Receiver Sender Receiver

Other control lines

Other control lines

Serial Communication
• How to transfer data?
– Sender:
• The byte of data must be converted to serial bits using
a parallel-in-serial-out shift register.
• The bit is transmitted over a single data line.
– Receiver
• The receiver must be a serial-in-parallel-out shift
register to receive the serial data and pack them into a
byte.
11101000001011
register register
8 1 8-bit
parallel-in ‘A’ serial-in character
serial-out parallel-out

Asynchronous vs. Synchronous
• Serial communication uses two
methods:
– In synchronous communication, data is
sent in blocks of bytes.
byte byte byte byte 01011111 01010101

sender preamble receiver

– In asynchronous communication, data is
sent in bytes.
byte byte byte
sender stop bit start bit receiver

UART & USART
• It is possible to write software to use both
methods, but the programs can be tedious
and long.
• Special IC chips are made for serial
communication:
– USART (universal synchronous-asynchronous receiver-
transmitter)
– UART (universal asynchronous receiver-transmitter)
• The 8051 chip has a built-in UART.
– Half-duplex
– Asynchronous mode only

Framing (1/3)
• How to detect that a character is sent via the
line in the asynchronous mode?
– Answer: Data framing!
• Each character is placed in between start and
stop bits. This is called framing.

Time (D0 first)

stop start
mark
bit
0 1 0 0 0 0 0 1 mark
bit

goes out last D7 D0 goes out first

Framing (2/3)
• The LSB is sent out first.
• The start bit is 0 (low) and always one bit.
• The stop bits is 1 (high).
• The stop bit can be one (if 8 bits used in ASCII) or
two bits (if 7 bits used in ASCII).
– In asynchronous serial communication,
peripheral chips and modems can be programmed
for data that is 7 or 8 bits.
• When there is no transfer, the signal is 1 (high),
which is referred to as mask.

Framing (3/3)
• We have a total of 10 bits for each character:
– 8-bits for the ASCII code
– 2-bits for the start and stop bits

• In some systems in order to maintain data
integrity, the parity bit is included in the data
frame.
– In an odd-parity bit system the total number of
bits, including the parity bit, is odd.
– UART chips allow programming of the parity bit
for odd-, even-, and no-parity options.

TxD and RxD pins in the 8051
• In 8051, the data is received from or
transmitted to
– RxD: received data (Pin 10, P3.0)
– TxD: transmitted data (Pin 11, P3.1)
• TxD and RxD of the 8051 are TTL
compatible.
• The 8051 requires a line driver to make
them RS232 compatible.

Baud Rates in the 8051
• The 8051 transfers and receives data serially at
many different baud rates by using UART.
• UART divides the machine cycle frequency by 32
and sends it to Timer 1 to set the baud rate.
• Signal change for each roll over of timer 1

11.0592 MHz
Machine cycle 28800 Hz
XTAL frequency ÷ 32
÷ 12 Timer 1
oscillator 921.6 kHz By UART To timer 1
To set the
Baud rate

Instructions Set
• 255 instructions, mnemonic code
• 1-3 byte instructions
• Opcode (operation code) +1,2 bytes
address, data
• 1-byte instructions – 139
2-byte instructions – 92
3-byte instructions – 24

• Coding format
– Opcode (operation code) + operands
• Operands – addressing modes
– Registers
– Direct : address
– Indirect
– Immediate : constant
– Relative : jump, branch, call (change PC)
– Long : call
– Indexed : array (base+offset)

8051 Addressing modes. (a) Register addressing (b) Direct addressing (c ) Indirect addressing (d) Immediate
addressing (e) Relative addressing (f) Absolute addressing (g) Long addressing (h) Indexed addressing

Coding format
- Addressing modes

8051 Registers
D7 D6 D5 D4 D3 D2 D1 D0
A
B
R0
8 bit Registers
R1
R2 DPTR DPH DPL
R3
R4 PC PC (Program counter)
R5
R6 8051 16 bit Registers
R7

8 bit Registers of the 8051

Five Addressing Modes
• Immediate
• Register
• Direct
• Register indirect
• Indexed

Immediate Addressing Mode
MOV A,#25H ;load 25H into A
MOV R4,#62 ;load the decimal value 62 into R4
MOV B,#40H ;load 40H into B
MOV DPTR,#4521H ;DPTR=4521H

MOV DPTR,#2550H ;is the same as:
MOV DPL,#50H
MOV DPH,#25H

Register Addressing Mode
MOV A,R0 ;copy the contents of R0 into A
MOV R2,A ;copy the contents of A into R2
ADD A,R5 ;add the contents of R5 to contents of A
ADD A,R7 ;add the contents of R7 to contents of A
MOV R6,A ;save accumulator in R6

MOV DPTR,#25F5H
MOV R7,DPL
MOV R6,DPH

Direct Addressing Mode
• RAM addresses 00 to 7FH
MOV R0,40H ;save content of RAM location 40H in R0
MOV 56H,A ;save content of A in RAM location 56H
MOV R4,7FH ;move contents of RAM location 7FH to R4

MOV A,4 ;is same as
MOV A,R4 ;which means copy R4 into A

MOV A,7 ;is same as

MOV A,2 ;is the same as

MOV A,0 ;is the same as

MOV R2,#5 ;R2=05
MOV A,2 ;copy R2 to A (A=R2=05)
MOV B,2 ;copy R2 to B (B=R2=05)
MOV 7,2 ;copy R2 to R7
;since “MOV R7,R2” is invalid

SFR Registers & Their Addresses
MOV 0E0H,#55H ;is the same as
MOV A,#55H ;which means load 55H into A (A=55H)

MOV 0F0H,#25H ;is the same as
MOV B,#25H ;which means load 25H into B (B=25H)

MOV 0E0H,R2 ;is the same as

MOV 0F0H,R0 ;is the same as
MOV B,R0 ;which means copy R0 into B

Stack and Direct Addressing Mode
• Only direct addressing is allowed for stack

Register Indirect Addressing Mode

• Only R0 & R1 can be used

MOV A,@R0 ;move contents of RAM location whose
;address is held by R0 into A
MOV @R1,B ;move contents of B into RAM location
;whose address is held by R1

Advantage of Register Indirect Addressing
• Looping not possible in direct addressing

Index Addressing Mode & On-chip ROM
Access

•Limitation of register indirect addressing:
•8-bit addresses (internal RAM)
•DPTR: 16 bits
•MOVC A, @A+DPTR ; “C” means
program (code) space ROM

• Instructions Registers
– Arithmetic
– Logic
– Data transfer
– Boolean
– Program branch

Quick reference chart

Arithmetic/Logic operations

Carry
borrow

MOV Instruction
• MOV destination, source ; copy source to
dest.
• MOV A,#55H ;load value 55H into reg. A
MOV R0,A ;copy contents of A into R0
;(now A=R0=55H)
;(now A=R0=R1=55H)
;(now A=R0=R1=R2=55H)
MOV R3,#95H ;load value 95H into R3
;(now R3=95H)
MOV A,R3 ;copy contents of R3 into A
;now A=R3=95H

Notes on Programming
• Value (proceeded with #) can be loaded
directly to registers A, B, or R0 – R7
– MOV R5, #0F9H
• If values 0 to F moved into an 8-bit
register, the rest assumed all zeros
– MOV A, #5
• A too large value causes an error
– MOV A, #7F2H

ADD Instruction
• ADD A, source ;ADD the source operand
;to the accumulator
• MOV A, #25H ;load 25H into A
MOV R2,#34H ;load 34H into R2
ADD A,R2 ;add R2 to accumulator
;(A = A + R2)

Structure of Assembly Language
ORG 0H ;start (origin) at location 0
MOV A,#0 ;load 0 into A
ADD A,R5 ;add contents of R5 to A
;now A = A + R5
ADD A,R7 ;add contents of R7 to A
;now A = A + R7
ADD A,#12H ;add to A value 12H
;now A = A + 12H
HERE: SJMP HERE ;stay in this loop
END ;end of asm source file
Sample of an Assembly Language Program

Interrupt Enable Register :

• EA : Global enable/disable.
• --- : Undefined.
• ET2 :Enable Timer 2 interrupt.
• ES :Enable Serial port interrupt.
• ET1 :Enable Timer 1 interrupt.
• EX1 :Enable External 1 interrupt.
• ET0 : Enable Timer 0 interrupt.
• EX0 : Enable External 0 interrupt.

• Six interrupts in the 8051

– 1 reset interrupt, when the reset pin is activated, the 8051
jumps to address location 0000
– 2 timer interrupts
– 2 external hardware interrupts
– pin 12 (P3.2) and 13 (P3.3) in port 3 are for the external
hardware interrupts
– 1 serial communication interrupt that belongs to both
receive and transmit
– a limited number of bytes is set aside for each interrupt

SJCET

• Enabling and disabling an interrupt

– upon reset all interrupts are disabled

– interrupts must be enabled by software

– IE register (interrupt enable) is responsible
for enabling and disabling the interrupts

– IE is a bit-addressable register

SJCET

• Steps in enabling an interrupt

1. EA must be set to 1
2. set the relevant bits in IE register to high

– EA = 0, no interrupt will be responded to,
even if the relevant bit in the IE register is
high

SJCET

• Interrupt priority upon reset

SJCET

INTERFACING TO EXTERNAL MEMORY

SJCET

SEMICONDUCTOR MEMORY
• Memory capacity
• The number of bits that a
semiconductor memory chip can store is
called chip capacity.
• It can be in units of Kbits (kilobits),
Mbits (megabits), and so on.

SJCET

Memory organization

• Memory chips are organized into a
number of locations within the IC.

• Each location can hold 1 bit, 4 bits, 8
bits, or even 16 bits, depending on how
it is designed internally.
SJCET

Speed

• The speed of the memory chip is commonly referred
to as its access time.

• The access time of memory chips varies from a few
nanoseconds to hundreds of nanoseconds, depending
on the IC technology used in the design and process.

SJCET

ROM (read-only memory)

• ROM is a type of memory that does not
lose its contents when the power is
turned off.

• For this reason, ROM is also called
nonvolatile memory.

SJCET

PROM (programmable ROM)

• PROM is programmed by blowing the
fuses.

• If the information burned into PROM is
wrong, that PROM must be discarded
since its internal fuses are blown
permanently.

SJCET

Flash memory EPROM
• flash memory can be programmed while it
is in its socket on the system board, it is
widely used to upgrade the BIOS ROM of
the PC.

• flash memory is semiconductor memory
with access time in the range of 100 ns
compared with disk access time in the
range of tens of milliseconds.
SJCET

Mask ROM

• Mask ROM refers to a kind of ROM in
which the contents are programmed by
the IC manufacturer.

• Mask ROM is used when the needed
volume is high (hundreds of thousands)
and it is absolutely certain that the
contents will not change.
SJCET

RAM (random access memory)

• RAM memory is called volatile memory
since cutting off the power to the IC
results in the loss of data.

SJCET

SRAM (static RAM)

• Storage cells in static RAM memory are made of flip-
flops and therefore do not require refreshing in
order to keep their data. This is in contrast to DRAM.

SJCET

DRAM (dynamic RAM)

• uses a capacitor to store each bit
• requires constant refreshing due to
leakage

SJCET

The 8051 is designed for small systems, in order to reduce the
number of pins used, time multiplexing of signals is used. For
example, port 0 and p2 are used as input/output ports or memory
interface pins.
related hardware pins are as follows:

(1)Address and data bus time-multiplexing for external memory
devices

(2) External Program memory read: Hardware pins involved:
port 0 and port 2 (P0.1~P0.7, P2.1~P2.7 = 16 bits), and /ALE,
/PSEN

(3) External Data memory read/write: Hardware pins involved:
port 0 and port 2 (P0.1~P0.7, P2.1~P2.7 = 16 bits), and /ALE,
/PSEN, /RD, /WR

SJCET

8051 microcontroller notes continuous

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