please dont rip this site
with a 4Mhz Xtal, and the code that I use, I can
get 9600 baud Tx or Rx not both, because I'm not using interrupts to keep
the routine as simple as possible, you can include it almost in any program
with minor modifications, I know that here in the list are better ones but
this one is very simple :-).

Serial TX routine:
; *************************************************
; **               transmit routine              **
; **   W must have the data to be transmitted    **
; *************************************************
; Note: _tx is the bit used for transmit data, must be set as output.
;       baudrate is a constant see initialization.
;       you must declare as registers the follow: txreg, delay & count
send            movwf   txreg
                bcf     _tx             ;send start bit
                movlw   baudrate
                movwf   delay
                movlw   .9
                movwf   count
txbaudwait      decfsz  delay
                goto    txbaudwait
                movlw   baudrate
                movwf   delay
                decfsz  count
                goto    sendnextbit
                movlw   .9
                movwf   count
                bsf     _tx             ;send stop bit
                retlw   0
sendnextbit     rrf     txreg
                btfss   _c              ;check next bit to tx
                goto    setlo
                bsf     _tx             ;send a high bit
                goto    txbaudwait
setlo           bcf     _tx             ;send a low bit
                goto    txbaudwait
; end
; *********************************************
; **              Serial RX routine          **
; ** before call this one, delay = baudrate  **
; *********************************************
; same as below but _rx must be declared as pin, and configured as input.
; also declare rcreg as register
; count must have the value 9 before call this routine (needed only the
; first time you call it).
receive         btfsc   _rx
                goto    $-1
                movlw   baudrate
                movwf   delay
rxbaudwait      decfsz  delay
                goto    $-1
                movlw   baudrate
                movwf   delay
                decfsz  count
                goto    recvnextbit
                movlw   .9
                movwf   count
                retlw   0
recvnextbit     bcf     _c
                btfsc   _rx
                bsf     _c
                rrf     rcreg
                goto    rxbaudwait

Remember that when you call receive routine, it will keep looping until a
low state (start bit) can be detected, then it will get the 8 bits and
exit routine, if you are planning to receive many bytes, you must call this
one again, as much as characters to receive you are especting.

with some programming you can modify that to do other jobs in the mean time,
but keep in mind the timming factor, at 4Mhz and high baud rates it's very

there is no parity check, only 8N1 format.

At the start of the program you must declare the follow:

; definitions...
count           equ             0x10
delay           equ             0x11
txreg           equ             0x12
rcreg           equ             0x13
clockrate       equ             .2000000   ;Xtal value (2Mhz in this case)
fclk            equ             clockrate/4
baudrate        equ             ((fclk/.2400)/3-2) ;2400 is the baud rate

for 4mhz clockrate must be .4000000, etc.
but remember that not all combinations work, you must done this calculation
by hand to make shure that the baudrate value will not be more than FF or
it won't work, in this case the value is : 2e6/4= 5e5/2400 = 208/3 = 69-2
=67.44\  , the value for the baudrate constant will be 67, remember
that if the decimal component is more that .5 you must round up the number
to avoid errors, also, the smaller value for baudrate, the higher posibility
of reception error, so if the number is bigger, will work better.

I hope that it can be useful for you, anyway if doesn't work write me and
i will try to help you...

see you.

   Leandro J. Laporta (LU2AOQ)     mail:
 wrk: Arg. Assoc. for Space Tech.   ham: TCP/IP high speed group HSG



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