Serial Communication
RS-232 WAVEFORM
TTL/CMOS
Serial Logic Waveform
The diagram above, shows the
expected waveform from the UART when using the common 8N1
format. 8N1 signifies 8 Data bits, No Parity and 1 Stop Bit.
The RS-232 line, when idle is in the Mark State (Logic 1). A
transmission starts with a start bit which is (Logic 0). Then
each bit is sent down the line, one at a time. The LSB (Least
Significant Bit) is sent first. A Stop Bit (Logic 1) is then
appended to the signal to make up the transmission.
The data sent using this method, is said
to be framed. That is the data is framed between
a Start and Stop Bit .
RS-232 Voltage levels
1. +3 to +25 volts to signify a
"Space" (Logic
0)
2. -3 to -25 volts for a
"Mark" (logic 1).
3. Any voltage in between
these regions (i.e. between +3 and -3 Volts) is undefined.
The data byte is always
transmitted least-significant-bit first.
The bits are transmitted at
specific time intervals determined by the baud rate of
the serial signal.
This is the signal present on the
RS-232 Port of your computer, shown below.
RS-232
Logic Waveform
RS-232
LEVEL
CONVERTER
Standard
serial interfacing of microcontroller (TTL) with
PC or any
RS232C Standard
device , requires TTL to RS232 Level converter . A
MAX232 is used
for this purpose. It provides 2-channel RS232C port and
requires external 10uF capacitors.
The driver requires a
single supply of +5V .
MAX-232
includes a Charge Pump, which generates +10V and -10V from a
single 5v supply.
MICROCONTROLLER
INTERFACING WITH RS-232 STANDARD DEVICES
-
MAX232 (+5V
-> +-12V converter)
-
Serial port
male 9 pin connector (SER)
SETTING SERIAL
PORT.
SCON
8 bit UART ,RN enabled , TI & RI
operated by program. - 50hex
Timer 1 Count
TH1 = 256 - ((Crystal / 384) / Baud)
-PCON.7 is clear.
TH1 = 256 - ((Crystal / 192) /
Baud)-PCON.7 is set.
so
with PCON.7 is clear we get
timer value = FDhex
CODE EXAMPLE
1. TRANSMITTING 'A'
CONTINUOUSLY
ON SERIAL PORT.
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ASSEMBLY
LANGUAGE |
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C
LANGUAGE
(SPJ SIDE51) |
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START
mov TMOD, #20H ;T1
is mode2
mov TH1, #0fd ;9600 baud
mov SCON, #50H
;8b, 1stop, 1start, REN enabled
anl PCON, #07fh ;To
make SMOD =0
setb TR1 ;start
T1
AGAIN
mov SBUF,
#’A’ ;letter A is transmitted
HERE
jnb TI, HERE
;poll TI until all the bits are transmitted
clr
TI ;clear TI for the next character
sjmp AGAIN
;while(1) |
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#include
<Intel\8052.h>
#include
<standard.h>
#include<stdio.h>
void main
()
{
TMOD = 0x20;
TH1 = 0xfd;
PCON &=
0x7f;
SCON = 0x50;
TCON =0x40;
while
(1) /*continues loop */
{
printf("a"); /*
transmit a along with CR & LF.
}
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2.TO RECEIVE DATA FROM SERIAL
PORT AND SENT IT TO PORT 1.
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|
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C
LANGUAGE
(SPJ SIDE51) |
| |
START:
mov TMOD, #20H ;T1
in mode 2
mov TH1, #-3 ;9600
baud
mov SCON, #50H ;8b,
1start, 1stop
anl PCON, #07fh ;To
make SMOD =0
setb TR1 ;start
T1
AGAIN:
clr RI ;ready to
receive a byte
HERE:
jnb RI, HERE ;wait
until one byte is Rx-ed
mov A, SBUF ;read
the received byte from SBUF
mov P1, A ;display
on P1
sjmp AGAIN ;while
(1)
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#include
<Intel\8052.h>
#include
<standard.h>
#include<stdio.h>
unsigned char a;
void main
()
{
TMOD = 0x20;
TH1 =
0xfd;
PCON &= 0x7f;
SCON = 0x50;
TCON
=0x40;
while (1) /*continues loop
*/
{
a= getchar () ;
P1=a;
}
}
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3. SENDING DATA IN STRING TO SERIAL
PORT
In Assembly Lan. prog. : Data is stored in string at pointer DATA.
0 is appended at end of
string. In transmit subroutine data in string is
transmitted till 0 is detected.
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ASSEMBLY
LANGUAGE
(A51)
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|
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C
LANGUAGE
(SPJ SIDE51) |
| |
.org 0000h
ljmp START
DATA: .db "HI,I AM
MAHESH",0dh,0ah,0 ;0
at end to detect end of string(0d carrage return ,0a
-line
feed)
;********************TRANSMIT******************
TRANSMIT:
clr
A ; clear A to get data
movc A,@A+DPTR ; get data
from string at data pointer
jz EXITSTR ; if data
zero, eos
lcall OUTCHAR ; else send character
inc
dptr ; increment data pointer
sjmp TRANSMIT ;
continue, zero condition will
terminate
EXITSTR:
ret
OUTCHAR:
mov
sbuf,a ; place A into Serial Port 1
Buffer
WAITCHAR:
jnb ti,WAITCHAR ; wait
buffer empty flag is set
clr ti ; clear buffer empty
flag
ret
START:
;******************INITIALISATION*****************
mov
TMOD, #20H ;T1 in mode 2
mov TH1, #-3 ;9600
baud
mov SCON, #50H ;8b, 1start, 1stop
anl PCON,
#07fh ;To make SMOD =0
setb TR1 ;start
T1
;*****************To SEND
DATA*******************
mov dptr,#DATA
lcall
TRANSMIT
sjmp START
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|
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#include
<Intel\8052.h>
#include
<standard.h>
#include<stdio.h>
void main ()
{
TMOD = 0x20;
TH1 =
0xfd;
PCON &= 0x7f;
SCON = 0x50;
TCON
=0x40;
while (1) /*continues loop */
{
printf("HI I AM MAHESH\n"); /* transmit a along with
CR & LF*/
}
} |
EXAMPLE - MOBILE PHONE AND GPS
RECEIVER.
You
can use same circuit for communicating with Mobile phones/GSM
Module or GPS. Communicating with both of these require a
Multiplexer ,which can be implemented using NAND
gates.
GPS serial
output
Most GPS are capable of sending information
through a simple serial link. Only the TXD and GROUND pins
need to be connected . The GPS must be set at 9600 bps
(or 4800) , 8 bits, No Parity, and 1 stop
bit.
NAND gate as 2:1 Mux. which connects Rx of GSM
modem or GPS receiver according to select bit logic level (pin
P1.0 of uC)
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