MicrocontrollersLab 8

COMMUNICATING WITH PC USING SERIAL COMMUNICATION IN 8051

Objectives of the LabSerial Communication Types

Registers governing Serial communication

Using timer1 to set baud rate

Hardware necessities for serial communication

Deciding Pins or Ports to use

Use withcaution

If EA high*The pins for this purpose are by default P3.0(RXD) and P3.1(TXD) .

Serial Communication Serial communication has the advantage over parallel

communication w.r.t number of wires used but the trade-off done is on speed of communication.

The quantity that decides the usage of serial or parallel communication is the distance, the data needs to cover.

Synchronous method transfers a block of data at a time while asynchronous transfers a single byte at a time.

Serial communication is classified into following types.

SCON Register

SM0 SCON.7 Serial Port Mode specifier

SM1 SCON.6 Serial Port Mode specifier

SM2 SCON.5 Used for multiprocessor communication

REN SCON.4 Set/Cleared by software to enable/disable reception

TB8 SCON.3 Not widely used

RB8 SCON.2 Not widely used

TI SCON.1 Transmit Interrupt, Set by HW after the data is successfully sent, cleared by SW. (At start, keep it high, so we know that the previous data was sent)

RI SCON.0 Receive Interrupt, Set by HW after the data is successfully received, cleared by SW.

SM0 and SM1SM0 SM1 Mode Description Baud Rate

0 0 0 Shift Register Fixed fXTAL/12

0 1 1 8-bit UART Variable (Set by Timer1)

1 0 2 9-bit UART Fixed (fXTAL/12 or fXTAL/64)

1 1 3 9-bit UART Variable (Set by Timer 1)

UART: Universal Asynchronous Receiver TransmitterBaud Rate: Speed (Bits per Second)

SBUF and TMODSBUF is a very important register in serial communication. It

is not bit addressable and is dual purpose serial read/write.When transmitting, data to be transmitted should be in SBUF.When receiving, data will be received in SBUF.

TMOD register is used to set variable baud rate using timer1.We will use in 8-bit auto-reload as count is mostly <255.To find the TH0 value against baud rate, following is done.

Baud Rate=Timer1 OV flow rate ÷ 32 => OV= B.R. × 32e.g. B.R.= 1200 bps OV=1200*32=38400 b/sDelay generated by timer1 should be Value in timer1=counts=

Try fXTAL = 12 MHz

SMOD bit (PCON.7) and Universal Baud Rates

PCON is not bit addressable, so to clear PCON.7, useAnl PCON,#01111111b, and to set Orl PCON,#10000000b

Pseudo-Code for Transmitting and Receiving Data through serial port

1. Move value in SMOD using mode1, keeping REN=1, RI=0 and TI=1.2. Configure TMOD for timer1, mode 2 and load value in TH1 and TL1

for desired baud rate and run timer by setting TR1 high.3. Use SMOD bit as desired.4. For transmission of data,

Clear TI. Move data in SBUF Then wait till TI gets high i.e. Data is transmitted.

5. For reception of data, Wait till RI gets high i.e. Data is received. Clear RI Move data from SBUF in desired register.

Data Transmit/Receive Protocol To transmit a character use mov a,#‘X’. During reception, ASCII of a character will be received. To transmit integer, first make it ASCII, add a,#30h. During reception, use clr c & subb a,#30h to make integer. To display a line, use look-up table.mov dptr,#line1again:mov a,#0movc a,@a+dptrinc dptrjz exit1 clr ti

mov sbuf,ajnb ti,$ jmp againexit1:

line1: db “Hello World”,0ah,0dh,0

Proteus Devices needed in this Lab1. AT89c51

2. Virtual Terminal (TXD, RXD, use in cross-connection)

Virtual Terminal Settings:

Baud Rate= as desired. Rest settings as default

Serial communication with PCCommunicating serially in software is much easier and doesn’t

require electronic equipment and any debugging but hardware serial communication with PC is quite complicated.

First of all, we need to use hyper-terminal in windows XP. It’s location is in ‘Start>Programs>Accessories>Communication

>Hyper-terminal.’ (Settings=> Bits per second: B.R. , Data bits=8, Parity=None, Flow-Control=None)Then, we have to use a serial cable. (Straight/Cross)Then, a level converter IC.And finally the microcontroller coded with serial communication.

Serial communication with PC

Level converter IC

Serial Port and Cable

Serial cables have serial ports on both sides of it. If internally pin2 and pin3 of one portare short with pin2 and pin3 of other port it is straight cable, if pin2 of one port is connected to pin3 of other port and pin3 of one port is connected to pin2, this is cross cable.

RS-232 to TTL converter(MAX-232)

As we know, that the microcontroller level low is ‘0 volts’ and level high is ‘5 volts’, these are called TTL levels.

RS-232 levels are different than TTL levels and thus we need a conversion of levels.

In RS-232, a level low is represented by ‘+3~+25 volts’ where as a level high is represented by ‘-3~-25 volts’.

*MAX-233 doesn’t require capacitors but is costly.

Debugging Stages:1. First of all check the serial-cable, whether it is straight or

cross.2. Then check the serial-cable, whether it is functioning or not.

For this short pins 2 & 3 and write something in hyper-terminal, if data is echoed, the cable is fine.

3. Then check the level converter IC, by shorting its TXD and RXD pins on the side of μC.

4. μC 8051 should already be checked by LED test.5. The program should be verified in PROTEUS.

Final Remark on μC 8051 LAB:Small Size, Low-Cost, Same Function: AT89C2051

Lab TasksSerially communicate with PC. If key pressed is that of

your workstation number then display ‘Workstation#xy’ otherwise display ‘Invalid input’.

Quiz Next Week of Timers.

1. Last Week Submissions: Project SW, HW and Report (ADC0804 submission)

2. Vivas from Project and μC Lab3. Lab Manual Submission