I would like to suggest you to narrow down the range to locate the bug. You
may bypass all communication-unrelated codes and focus on the serial port
operation. Just to verify the TX/RX is correct. One thing can try is keeping
sending an identical printable char to the receiver and let the receiver
display it as directly and quickly as possible. I still suspect the
receiving port is unintentionally accessed if the waveform to the receive
pin is correct.

Raymond

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jennifer Loiacono
Sent: January 24, 2002 10:32 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: PIC18 USART RX Data Corruption


The value is transmitted (also via USART - possibly a clue?) to a serial
LCD.  The code is listed below.  All references to AARGBx, BARGBx, and
FXDxxxxxx registers/routines are part of the standard math libraries
provided by Microchip.  I have used them extensively, and they never SEEM to
cause any other problems.


;generic display routine

SERDISP BTFSS   TXSTA,TRMT              ;if transmit shift register full
        GOTO    SERDISP                 ;loop until ready
        BSF     TXSTA,TXEN              ;enable transmission
        MOVFF   CHARIN,TXREG            ;move new character into buffer
        RETURN


;display input value
LIMDISP                                 ;position cursor
        MOVLW   0xFE                    ;instruction character
        MOVWF   CHARIN
        CALL    SERDISP
        MOVLW   0x80                    ;position 1,0
        MOVWF   CHARIN
        CALL    SERDISP
        MOVLW   0x4C                    ;L
        MOVWF   CHARIN
        CALL    SERDISP
        MOVLW   0x49                    ;I
        MOVWF   CHARIN
        CALL    SERDISP
        MOVLW   0x4D                    ;M
        MOVWF   CHARIN
        CALL    SERDISP
        MOVLW   0x3A                    ;:
        MOVWF   CHARIN
        CALL    SERDISP

        ;divide by powers of 10 to get one decimal character at a time
        MOVFF   RCREG,AARGB0            ;move limit into dividend register
        MOVLW   0x64                    ;100d
        MOVWF   BARGB0                  ;move to divisor register
        CALL    FXD0808U                ;unsigned 8x8 division
        MOVLW   0x16                    ;load high byte of PC for table
lookup vector
        MOVWF   PCLATH                  ;program counter latch high
        MOVF    AARGB0,W                ;move quotient to wreg for code
lookup
        CALL    ASC_TBL                 ;lookup hex code for ascii character
        MOVWF   CHARIN                  ;move value to character holding
register
        CALL    SERDISP                 ;display character
        MOVFF   REMB0,AARGB0            ;move remainder to use as next
divisor
        MOVLW   0x0A                    ;10d
        MOVWF   BARGB0                  ;move to divisor register
        CALL    FXD0808U                ;unsigned 8x8 division
        MOVLW   0x16                    ;load high byte of PC for table
lookup vector
        MOVWF   PCLATH                  ;program counter latch high
        MOVF    AARGB0,W                ;move quotient to wreg for code
lookup
        CALL    ASC_TBL                 ;lookup hex code for ascii character
        MOVWF   CHARIN                  ;move value to character holding
register
        CALL    SERDISP                 ;display character
        MOVLW   0x16                    ;load high byte of PC for table
lookup vector
        MOVWF   PCLATH                  ;program counter latch high
        MOVF    REMB0,W                 ;move remainder to wreg for code
lookup
        CALL    ASC_TBL                 ;lookup hex code for ascii character
        MOVWF   CHARIN                  ;move value to character holding
register
        CALL    SERDISP                 ;display character

        MOVLW   0x20                    ;space
        MOVWF   CHARIN
        CALL    SERDISP

        RETURN


----- In Response To -----

Jen,

How do you 'print' out the data received?  By outputing them to LEDs?
Transmitting the received data again to a PC?  I am trying to see if
it's possibly the code.


Rudy


> The transmission is from PIC to PIC - one USART to another.  The
sending PIC
> is a 16F627.  The scope readings show perfect 5V signals at 9600 baud.
> There is no parity bit, only 8 data bits, plus start and stop bit.
>
> Jen
>
> > > Despite accurate signal levels, debugging RCREG immediately
> > after data is
> > > received reveals that the byte seems to flip back and forth.
> > >
> > > e.g., a byte equal to 4 (dec) will oscillate between 4 and
> > 32.  A byte
> > equal
> > > to 1 will oscillate between 1 and 128, etc.
> > >
> > > Again, this value is being read immediately after reception
> > (triggered by
> > RX
> > > interrupt), so it is highly unlikely that program code
> > could be altering
> > the
> > > register after reception.
> > >
> > > Has anyone seen a problem like this before, or perhaps have
> > any ideas as
> > to
> > > its cause?  I have been stumped on this for a while, so ANY
> > help would be
> > > greatly appreciated!
> >

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