Serial Comm in 8051

8/3/2019 Serial Comm in 8051

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Serial

Communication


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Basics of serial communicationParallel: expensive - short distance

fast

no modulation

Serial :cheaper long (two different cities by modem)-slow


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Packaging DataStart and stop bits

In asynchronous transmissionWhen there is no transfer the signal is highTransmission begins with a start (low) bitLSB firstFinally 1 stop bit (high)Data transfer rate (baud rate) is stated in bps


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2 methods, asynchronous and synchronous

synchronousmethod transfers a block of data

(characters) at a time

asynchronousmethod transfers a single byteat a time

Uses special IC chips called UART (universalasynchronous receiver-transmitter) andUSART (universal synchronousasynchronous

receiver-transmitter) 8051 chip has a built-in UART


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RS232 pins


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Data communicationclassification


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Serial control (SCON) Register

SM2: used for multi processor communication

REN: receive enable (by software enable/disable)

TB8: transmit bit8

RB8: receive bit 8

TI : transmit interrupt flag set by HW after send , clear by SW

RI : receive interrupt flag set by HW after received ,clear by SW

SM0 RITIRB8TB8RENSM2SM1

7 6 5 4 3 2

1 0

SM0 SM1 MODE operation transmit rate0 0 0 shift register fixed (xtal/12)0 1 1 8 bit UART variable (timer1)

1 0 2 9 bit UART fixed (xtal/32 orxtal/64)1 1 3 9 bit UART variable (timer1)

SM0: mode specifierSM1: mode specifier


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Mode of operation Mode 0 :

Serial data enters and exits through RxD

TxD outputs the shift clock.

8 bits are transmitted/received(LSB first)

The baud rate is fixed a 1/12 the oscillator frequency.

Application Port expansion

8051

TXD

RXD Shift registerclk

data


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Timing of send in mode 0One machine cycle

oscillator cycle

RXD (data)

TXD (clock pulse)

MOVSCON,#0001xxxxBWait: JNB RI,WAIT

CLR RIMOV A,SBUF

MOV

SCON,#0001xxxxBWait: JNB TI,WAITCLR TIMOV SBUF,A


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Mode of operation Mode 1

Ten bits are transmitted (through TxD) or received (through RxD)

(A start bit (0), 8 data bits (LSB first), and a stop bit (1) )

On receive, the stop bit goes into RB8 in SCON

the baud rate is determined by the Timer 1 overflow rate.

Timer1 clock is 1/32 machine cycle (MC=1/12 XTAL)

Timer clock can be programmed as 1/16 of machine cycle

Transmission is initiated by any instruction that uses SBUF asa destination register.


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Mode of operation Mode 2 :

Eleven bits are transmitted (through TxD), received (throughRxD) A start bit (0) 8 data bits (LSB first) A programmable 9th data bit and a stop bit (1)

On transmit, the 9th bit (TB8) can be assigned 0 or 1. On receive, the 9the data bit goes into RB8 in SCON. the 9th can be parity bit The baud rate is programmable to 1/32 or 1/64 the oscillator

frequency in Mode 2 by SMOD bit in PCON register

Mode 3 Same as mode 2

But may have a variable baud rate generated from Timer 1.


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Mode of operation


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What is SMOD

Bit 7 of PCON register If SMOD=1 double baud rate

PCON is not bit addressable

How to set SMOD

Mov a, pcon

Setb acc.7

Mov pcon,a


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Power control register


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Power control

A standard for applications where powerconsumption is critical

two power reducing modes Idle

Power down


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Baud rate in the 8051

serial communications of the 8051 withthe COM port of the PC

must make sure that the baud rate of the8051 system matches the baud rate of the

PC's COM port can use Windows HyperTerminal

program


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PC Baud Rates


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Baud rate in the 8051

baud rate in the 8051 is programmable

done with the help of Timer 1

relationship between the crystal frequency and thebaud rate in the 8051

8051 divides the crystal frequency by 12 to get themachine cycle frequency

XTAL = 11.0592 MHz, the machine cycle frequency is921.6 kHz

8051's UART divides the machine cycle frequency of921.6 kHz by 32 once more before it is used by Timer

1 to set the baud rate

921.6 kHz divided by 32 gives 28,800 Hz

Timer 1 must be programmed in mode 2, that is 8-bit,auto-reload


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Table 104 Timer 1 TH1 Register Values for Various Baud Rates


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Example:

With XTAL = 11.0592 MHz, find the TH1

value needed to have the following baud

rates. (a) 9600 (b) 2400 (c) 1200 machine cycle frequency

= 11.0592 MHz / 12 = 921.6 kHz

Timer 1 frequency provided by 8051UART= 921.6 kHz / 32 = 28,800 Hz

(a) 28,800 / 3 = 9600 where -3 = FD (hex)

(b) 28,800 / 12 = 2400 where -12 = F4 (hex)

(c) 28,800 / 24 = 1200 where -24 = E8 (hex)

Serial Comm in 8051

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