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
critical!.
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: lu2aoq@yahoo.com
wrk: Arg. Assoc. for Space Tech. ham: TCP/IP high speed group HSG
file: /Techref/scenix/lib/io/osi2/serial/rs232at9600on16F84noint.htm, 4KB, , updated: 2000/5/3 11:17, local time: 2024/11/20 03:48,
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