Jonathan,
I have your code revisited and no failure detected.
The only principal difference compared to my UART rx/tx coding is the
initializing of RC6/7 port pins. I'm programming TxD as output and RxD as
input. I know, there was a thread on this list discussing that. Many
piclister has initialized the ports as input or output (i didn't remember).
Try my initializing coding, it's an iddea.
Frank

> -----UrsprŸngliche Nachricht-----
> Von: Jonathan Evans [mailto:evansjon@LIVERPOOL.AC.UK]
> Gesendet: Mittwoch, 19. April 2000 11:16
> An: PICLIST@MITVMA.MIT.EDU
> Betreff: RS232 / Comm problem with PIC16F877
> 
> 
> Hello all,
> 
> We have a PIC16F877 ICD development board (from Microchip) 
> which we use
> with MPLAB (v5). We have been trying to develop the first stage of
> serial line communication protocol for the PIC but are having the
> most annoying problem - and I'm going round in circles with it.
> 
> I cannot seem to successfully receive characters. 
> Transmission has proved
> to be fine, but reception gives a constant FERR (framing error). I
> understand that this is caused by the
> "reception of STOP bit as 0 instead of 1".
> 
> I have attached a file (commtest.asm) to try and illustrate 
> what we are
> doing, and the register configuration we are trying. The 
> program tries to
> simply echo each character back to the RS232 line as it is 
> received (we
> are using a standard MAX233 device for the TTL->RS232 voltage 
> conversion).
> 
> 
> 
> ; ===============================================================
> ; commtest.asm
> ; ===============================================================
> 
>  list p=16f877
> 
>  ; Include file, change directory if needed
>   include "p16f877.inc"
> 
> 
>  ; Start at the reset vector
>   org 0x000
>   nop
> Start
> 
> 
> Main
> 
>    call InitComm
>    goto EchoTest
> 
> InitComm
>       ; At first we didn't have the the PORTC / TRISC
>       ; code (because the Microchip code doesn't
>       ; mention them - but we have included them to try
> 
>    bcf STATUS,5     ; select BANK 0
>    bcf STATUS,6
>    movlw 0x00       ; Set PORTC to all ZEROS
>    movwf PORTC
> 
>    bsf STATUS,5     ; Select BANK 1
>    bcf STATUS,6
> 
>    movlw 0xC0       ; Select PORTC bits 6 & 7 to USART
>    movwf TRISC
> 
>       ; We have tried a range of baud rates, included 300, 9600, 19.2K
>       ; at both high and low clock rates (BRGH)
>    movlw   0x0B
>    movwf SPREG      ;SPERG = 11 (for 19.2K high speed)
>    movlw 0x24       ;Set TXSTA for Async, High speed, 8 bit data
>    movwf TXSTA
>    bcf PIE1,TXIE    ;Disable Transmission Interrupt
>    bcf PIE1,RCIE    ;Disable Receive Interrupt
> 
> 
>    bcf STATUS,5     ; Select BANK 0
>    bcf STATUS,6
> 
>    movlw 0x90       ; Set RXSTA for Async, 8 bit data
>    movwf RCSTA
> 
>     return
> 
> 
> EchoTest
>    bcf STATUS,5      ; Select BANK 0
>    bcf STATUS,6
> 
> RCIFloop
>    btfss PIR1,RCIF   ; Loop, while RCIF = 0
>    goto RCIFloop
> 
>    btfss RCSTA,FERR  ; Check for Frame-Error
>    goto NoFRAMEError ; If no FERR then skip
> 
>    movf RCREG,0      ; Copy RCREG -> w
> 
>    ;bcf RCSTA,CREN   ; Tried this ... doeen't make difference
>    ;bsf RCSTA,CREN   ; Clears error (flag CREN 0 then 1)
>    goto TXIFloop     ; Jump to Transmission Loop
> 
> NoFRAMEError
>    movf RCREG,0      ; Copy RCREG -> W
> 
> TXIFloop
>    btfss PIR1,TXIF   ; Loop, while TXIF = 0
>    goto TXIFloop
> 
>    movwf TXREG       ; Copy W -> TXREG
>    goto RCIFloop     ; Jump to start of loop
> 
>    end
> ; ===============================================================
> 
> 
> 
> We have noticed the following which might help to identify 
> the problem:
> 
> * We have confirmed the baud rate is correct using a scope and logic
> analyser (we are using a clock freq of 3.6864 MHz).
> 
> * We seem to have no problem with transmission, all 
> characters are correctly
> received.
> 
> * The busy-wait on the RCIF flag seems to work correctly. The 
> program will
> wait forever until a character is sent to the PIC, at which point the
> program continues immediately. This seems to indicate the 
> detection of the
> START bit is OK.
> 
> * The value of the RCREG is *always* 0x00. This is suspicious 
> because I
> understand we should see the bit pattern that was received, 
> even if it is
> marked as possible framing error.
> 
> * Because of the above, it has occured to me that since every 
> bit is seen as
> 0, then the STOP might also and hence the FERR. However, I 
> don't understand
> why every bit is seen as 0 ... especially when the START bit 
> transition from
> 1 -> 0 is detected OK.
> 
> * It has also occured to me that the "constant" zero problem might be
> because the direction register of PORTC is incorrect (TRISC). However,
> looking around similar code and playing with the value seems 
> not to have
> helped.
> 
> * We have played around with 9 bit data and 2 stop bits and 
> confirmed with
> the logic analyser that the timing of the stop bit sample seems to be
> correct (i.e. after the data has ended and the line has 
> returned to logic
> HIGH).
> 
> 
> Sorry for the length of this, but I am at a complete loss as to how to
> proceed further. Any help or advice that anyone could give 
> would be very
> gratefully received.
> 
> Many thanks in advance,
> 
> Jonathan
> 
> ====================================================================
> Dr Jonathan Evans
> Department of Electrical Engineering and Electronics
> The University of Liverpool               E-Mail: evansjon@liv.ac.uk
> LIVERPOOL   L69 3GJ                       Fax: (+44)-(0)151-794-4540
> United Kingdom (UK)                       Tel: (+44)-(0)151-794-4602
> ====================================================================
>