PIC MCU to PC Serial Communication Problem

max232picrs232

I was just trying to send continuous stream of letter 'G' from my PIC MCU (PIC18F452) to my laptop.

Here is my assembly code that i am using : 

#include <p18f452.inc>

    CONFIG  OSC=XT,WDT=OFF,PWRT=OFF
    ORG     0H
    MOVLW   D'12'
    MOVWF   SPBRG
    MOVLW   B'00100000'
    MOVWF   TXSTA
    BCF     TRISC,TX
    BSF     RCSTA,SPEN
    OVER    MOVLW   A'G'
    S1      BTFSS   PIR1,TXIF
    BRA     S1
    MOVWF   TXREG
    BRA     OVER
    END

I design the circuit in proteus and simulate it with realterm and hyperterminal i get the desired output.
But when i try to implement the hardware i face some problem. I do not get output at Realterm just when i connect my source. But at the instant i remove the source (unplug just ground or live wire) i get the output (G string).
Can any one tell me what is the problem?
Devices that i use:
01. PIC18F452
02. 8MHz Crystal Oscillator,
03. MAX232 with five 10uF capacitor
04. USB to RS232 cable.
Here is my connection diagram:
PIC MCU serial communication

Best Answer

If the PIC is a 3.3V one you might find the MAX232 is borderline working with the levels on its TX pin. You might need to use the 3V version of the MAX232.

Otherwise you need to put a digital oscilloscope on pin3 of your P1 and check if you have your data there, if yes measure the width of a bit and make sure the baud rate is what you are expecting. (1 divided by width of one bit in seconds will give you close to the actual baud rate).

If no data, then check on pin 25 of U1 and see if you have data there. If not there might be a software bug.

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