Alright, I've read the 8051 data book and searched through the archives and still haven't found anything that identifies the cause of my failed attempt at a rudementary serial routine. My apologies for having to list my code (and my stupidity).

I set up my serial port as follows:

clr SM0 ;Mode 1
setb SM1;Mode 1
clr SM2 ;ensure no multi-proc comm
setb REN;Rx enable
clr TB8 ;Set later for address Tx
clr RB8 ;Ensure clear
clr TI ;Ensure Tx int is clear
clr RI ;Ensure Rx int is clear
;SMOD in PCON
mov A,PCON
clr ACC.7
mov PCON,A ;set for 9.6 kBaud
;TMOD
;Not required in mode 2
setb ES ;Enable serial interrupt
I set up T1 as follows:

mov TMOD,#20h ;mode 2
mov TH1,#0fdh ;9.6 kBaud
mov TL1,TH1 ;pre-load TL1 BRG
setb TR1 ;enable T1
clr ET1 ;the data book says to
;clr the T1 int. Must
;test this
The Serial ISR code is as follows:

ljmp Ser_ISR
reti

Ser_ISR:
jnb RI,Tx ;If not RI, TI
mov @R0,SBUF ;Load RBuff w/ SBUF
inc R7 ;Increment Rx count
clr RI
sjmp Ser_ISR1 ;return to RI/TI

Tx:
clr TI
setb sent
Ser_ISR1:
ret

The failing code is as follows:
(line #s added for clarity only)

337 mov SBUF,#'A'
338 nop
339 nop
340 nop
341 jnb sent,$
342 clr sent
343 mov SBUF,#'B'
344 nop
345 nop
346 nop
347 jnb sent,$
348 clr sent
349 mov SBUF,#'C'
350 nop
351 nop
352 nop
353 jnb sent,$
354 clr sent
355 nop
356 nop
357 nop
358 mov SBUF,#'D'
359 jnb sent,$
360 clr sent
361 nop
362 nop
363 nop
364 mov SBUF,#'E'
365 jnb sent,$
366 clr sent
367
368 Loop:
369 nop
370 nop
371 nop
372 sjmp Loop


I set up HyperTerminal as follows:

8N1 and no flow control

My observations:

- When I execute this with ET1 cleared and a BP at line 350, hyperterminal shows "AB" and the program hangs

- When I execute this with ET1 set and a BP at line 350, hyperterminal shows "ABC" and stops execution as set.

- When I set a BP at line 355 (ET1 set), hyperterminal shows "A" and the program hangs.

- When I set a BP at line 369, hyperterminal shows "ABC" and hangs.

- When I set a BP within the ser ISR after clr RI (within a bunch of nops) hyperteminal shows "ABC" and the program hangs (RI is never set). I can watch the serial data from the PC on my scope connected to the MAX232 output connected to the 8051 Rx.

- When I comment out all of the "mov SBUFs" and "jnb sents" and set a BP after clr RI within the ser ISR, the location pointed to by @R0 contains the correct ASCII byte sent to it by hyperterminal.

I wrote this to get comfortable with the serial interrupt operation so I only write "ABCDE" to hyperterminal and then wait to accept single bytes from hyperterminal.

As I read the data book, TI is set once bit 8 is shifted to the output. So, any suggestions as to why my seemingly straight forward code fails when transmitting?

Thanks, in advance.

Jeff Biss