Hello,

I have been monitoring the PIClist for a few weeks, and decided to post
about some trouble I am having with the USART on the 16F873. I have written
some code to write to an LCD as the start of a larger project, and I am now
working on the serial communications link. I first print the initialization
message on the LCD, and send the same data out the USART to the PC at the
same time. This works fine, but I can't transmit data back to the PIC from
the PC.

I am using a 16F873 running at 3.4MHz with a TC232 IC as the level
converter. I checked the signals going into the PIC (on RC7) with a 'scope,
and the levels appear to be fine. I am not sure about the actual bit
pattern, but by going up and down the alphabet on the keyboard I can see
binary counting going on, so I think it is sending the right data. I am
using the old "Terminal" program, but I tried Hyperterminal also. It is
running at 1200 baud. I have a troubleshooting LED that should turn on when
it enters the ISR, but it never goes on when I send it characters. Below is
the relevant part of the code:

------------------------------------------
 org 0
 goto Init

 org 4
 goto Interrupt

 __CONFIG _BODEN_OFF & _CP_OFF & _XT_OSC & _PWRTE_OFF & _WDT_OFF & _LVP_OFF
 include "P16F873.inc"

Interrupt:
    bcf           INTCON,GIE
    btfss        INTCON,GIE
    goto        $ - 2
    bsf          PORTB,LED    ; turn on LED
    call         Delay15ms
    call         Delay15ms
    call         Delay15ms
    call         Delay15ms
    bcf         PORTB,LED
    btfsc      PIR1,RCIF       ; see if RX flag is set
    call        RS232_Receive  ; if it is, call RS232_Receive
    retfie

    org 20
Init:
    clrf           INTCON            ; make sure INTCON is cleared
    bsf           INTCON,PEIE   ; turn on interrupts for peripherals
    movlw     0x90
    movwf     RCSTA               ; turn on USART, continuous receive
    bsf           STATUS,    RP0     ; page 1
    clrf          PIE1                     ; clear individual peripheral
interrupt enables
    bsf          PIE1,RCIE          ; turn on USART RX interrupt
    movlw     0x2B
    movwf     SPBRG         ; load 1200 baud into baud rate gen
    movlw     0x20
    movwf     TXSTA         ; turn on serial transmit enable
    bcf          STATUS,RP0     ; page 0
    ; the rest of the LCD init code goes below this

RS232_Receive:          ; called in ISR
    movf       RCREG,w     ; put received data in w
    movwf    RXdata          ; put it in RXdata reg
    call         LCDwrite       ; write char to LCD
    ;movf      RXdata,w      ; put data in w again
    ;xorlw     0x0D             ; see if enter was pressed
    ;btfsc      STATUS,0    ; if it was, go to next instruction
    ;call        ClearLCD      ; clear the display
    return

------------------------------------

If I am doing something wrong, please let me know what it is and how I
should change it.

TIA,

Andy Meng N8MX

www.qsl.net/n8mx


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