Serial Communication

Purpose:
To establish a serial communication link between the PC and the 8051.

Description:
Serial communication is often used either to control or to receive data from an embedded microprocessor. Serial communication is a form of I/O in which the bits of a byte begin transferred appear one after the other in a timed sequence on a single wire. Serial communication has become the standard for intercomputer
communication. In this lab, we'll try to build a serial link between 8051 and PC using RS232.

RS232C
The example serial waveforms in Fig 1 show the waveform on a single conductor to transmit a byte (0x41) serially. The upper waveform is the TTL-level waveform seen at the transmit pin of 8051. The lower waveform shows the same waveform converted to RS232C levels. The voltage level of the RS232C are used to assure error-free transmission over greater distances than would be possible with TTL levels.
As shown in Fig 1, each byte is preceded by a start bit and followed by one stop bit. The start and stop bits are used to synchronize the serial recivers. The data byte is always transmitted least-significant-bit first. For error checking it is possible to include a parity bit as well, just prior to the stop bit. The bits are transmitted at specific time intervals determined by the baud rate of the serial signal. The baud rate is the reciprocal of the time to send 1 bit. Error-free serial communication requires that the baud rate, number of data bits, number of stop bits, and presence or absence of a parity bit be the same at the transmitter and at the receiver.

Using the Serial Port
8051 provides a transmit channel and a receive channel of serial communication. The transmit data pin (TXD) is specified at P3.1, and the receive data pin (RXD) is at P3.0. The serial signals provided on these pins are TTL signal levels and must be boosted and inverted through a suitable converter(LT1130CN is used in this lab) to comply with RS232 standard.
All modes are controlled through SCON, the Serial CONtrol register. The SCON bits are defined as SM0, SM1, SM2, REN, TB8, RB8, TI, RI from MSB to LSB. The timers are controlled using TMOD, the Timer MODe register, and TCON, the Timer CONtrol register.

Register Descriptions

SCON bit definitions
SCON Serial Control Register
(msb) (lsb)
-----------------------------
|SMO|SM1|SM2|REN|TB8|RB8|TI|RI|
-----------------------------
SMO, SM1, SM2 Serial Mode COntrol Bits
SM0 SM1 Mode Baud Rate
-----------------------
0 0 0 fosc/12
0 1 1 variable
1 0 2 fosc/32 or fosc/64
SM2 Multiprocessor Mode Control Bit
1 = Multi-processor mode
0 = Normal mode
REN Receiver Enable Bit
1 = Receive Enable
0 = Receive Disabled
TB8 9th Transmit Bit
Enabled only in modes 2 and 3
RB8 9th Bit Received
Used in modes 2 and 3
RI, TI Serial Interrrupts
RI is set to indicate receipt of a serial word and TI
is set to indicate completion of a serial transmission.
-----------------------------------------------------------------------------
TMOD Timer Mode Register
------------------------------
|Gate|C/T|M1|M0|Gate|C/T\|M1|M0|
------------------------------
|<-Timer 1----><---Timer 0 --->
Gate Gating Control.
0= Timer enabled
1 = Timer enabled if INTx\ is high
C/T\ Counter or Timer Selector
0 = Internal count source (clock/12)
1 = External count source (Tx pin)
M1, M0 Mode Control
M1 M0 Mode
----------------------------------
0 0 Mode 0, 13 bit count mode
0 1 Mode 1, 16 bit count mode
1 0 Mode 2, Auto reload mode
1 1 Mode 3, Multiple mode
-----------------------------------------------------------------------------
TCON Timer Control Receiver Register
---------------------------
|TF1|TR1|TF0|TR0| | | | |
---------------------------
<-Timer Controls><-Unused for timers
TRx Timer x run control
0 = Timer not running
1 = Timer running
TFx Timer x flag
0 = timer has not rolled over
1 = timer has rolled over
----------------------------------------------------------------------------
Formula to load the value of TH1 corresponding to required baud rate
Clock Frequency (12 MHz)
----------------------- = Baud Rate
12 x 32 x (256-TH1)

RS232 connector

PCs have 9pin/25pin male SUB-D connectors. The pin layout is as follows (seen from outside your PC):


Name (V24) 25pin 9pin Dir Full name Remarks
--------------------------------------------------------------------------
TxD 2 3 o Transmit Data
RxD 3 2 i Receive Data
RTS 4 7 o Request To Send
CTS 5 8 i Clear To Send
DTR 20 4 o Data Terminal Ready
DSR 6 6 i Data Set Ready
RI 22 9 i Ring Indicator
DCD 8 1 i Data Carrier Detect
GND 7 5 - Signal ground
- 1 - - Protective ground Don't use this one
for signal ground!

• The most important lines are RxD, TxD, and GND. Others are used with modems, printers and plotters to indicate internal states.

Apparatus Required:
1. 0.1 μF capacitors(6)
2. LT1130CN
3. connector and cable
4. 5V power supply
5. Philips PDS51 development board

Program:
#pragma SMALL DB OE
#include
unsigned char ReceiveSerial() {
unsigned char c;
TMOD = 0x20; /* configure timer for the correct baud rate */
TH1 = 0xe6; /* 1200 bps for 12 MHz clock */
TCON = 0x00; /* Set timer to not running */
SCON = 0x50; /* Set Serial IO to receive and normal mode */
TR1 = 1; /* start timer to Receive */
while( (SCON & 0x01) == 0 ) /* wait for receive data */;
c = SBUF;
return c;
}

void SendSerial(unsigned char c) {
/* initialize..set values for TMOD, TH1 and TCON */
/* set the Tx interrupt in SCON to indicate sending data */
/* start timer */
/* write character to SBUF */
/* wait for completion of sent data */
}
void main(void) {
unsigned char c;
while( 1 ) {
/* Use ReceiveSerial to read in a character 'c' */
/* Do some computation on 'c' */
/* Send the result using SendSerial() */
}
}