Hi PIC people,

I have been attempting to interface a 16C84 to an LCD serial backpack as
produced by Scott Edwards Electronics (www.seetron.com).

I have tried many different bit-banging serial programs out there, none of
which seem to work. I realise that the serial backpack needs inverted serial
data - which requires swapping any bcf and bsf opcodes. However, does this
include inverting the start and stop bits?

The following is the simplest program I found that was posted here recently. I
have a scope on Pin 18 observing the output. There data is generated, but then
it suddenly goes dead (after about 2 seconds) - no more output is generated. I
generate a snapshot on the scope, and it shows the output as high for approx
1.4mS and low very briefly for 100uS. There are 8 bits output, but why are the
high and low times so different? According to the source, bit time is 104uS.

The PIC is using  4Mhz crystal in HS mode. The source below is for gpasm ( a
GNU PIC assembler, but it will work in mpasm with some tweaks). Btw - I used a
PIC disassembler to verify that the .hex file corresponds to the assembler
listing!

So my questions are:
1) Why does this program suddenly stop geneating output?
2) Does anyone have code to interface a PIC to a serial-LCD backpack?

Many thanks!
Mark.

; test txbyte routine
; Mark Crosbie 9/9/98
; Sends the letter A to the serial LCD on pin 18 (RA1)
; at 9600, 8N1. Assumes 4Mhz Xtal
;

        include p16c84.inc
        processor 16c84

        __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON

#define serport PORTA
#define dout 1

        org 0

        bsf STATUS, RP0
        bcf TRISA, dout
        bcf STATUS, RP0
loop:
        movlw 170               ; alternating 1 and 0 pattern
        call txbyte
        goto loop
temp    dt 0

;TXBYTE: send 1 byte @ 9600 baud: 1 RAM, 15 (16) ROM, 1043 cycles
;enters with data in w, exits with w trashed.
;Copyright (C) 1995 Dwayne Reid. May be freely used so long as this
;copyright notice is retained.

txbyte                  ; send byte in W at 9600 baud (4MHz osc), 8N1
 movwf temp
 movlw 10               ; 8 data + 1 start + 1 stop (inc for more stop bits)
 clrc                   ; start bit

txloop                  ; bit time = 104.167 uSec
 skpnc
 bsf serport,dout       ; output bit = carry
 skpc
 bcf serport,dout

dloop                   ; 95 clk cycle delay
 goto $+1               ; 2-cycle NOP in 1 instruction!
 addlw b'00010000'      ; increment upper nibble
 skpc                   ; delay = 6n -1 (1 less when falls thru)
 goto dloop             ; loop sixteen times
 addlw -1               ; dec w, valid z, c=1 if w=0 after decrement
 rrf temp,F             ; carry will be set - shifted in as stop bit
 skpz
 goto txloop            ; txloop is 104 clk cycles
 return

--
Mark Crosbie                    http://www.best.com/~mcrosbie
Hewlett-Packard MS 47 LA        mcrosbie@cup.hp.com
19447 Pruneridge Avenue         (408) 447-2308
Cupertino, CA 95014             (408) 447-6766 FAX