Hello, *MAGIC* PIC_listers !

I'm trying for many days to make a Serial routine for a 16c84, it seems
to
work on MPLAB simulator, but in reality ... %-(

I must say that i'v a LCD(HD44780) in my application, so i can see
what i get ...

First, i'v tryed to connect with a serie resistor straight to Tx(tested:
82k,200k,382k) and a common Ground; noisy results.

Now, i'v wired it with a MAX232; noisy results too.


I wanna do this in ISR (interrupt), like that I'm free of this in MAIN
program.

I thought about few methods to do this :

- I could set TMR0 to 0xFF, triggered by RA4/T0CLK input to make 
  a T0IF(timer overflow), etc...
 * Not good for my purpose, cause i need TMR0 for a base time.

- I could set interrupt on PORTB change.
 * Not recommended.(I heared this, near here)

- I could sample the Rx pin.
 * Good, but wasted time to sample each bit, as many it was
super-sampled.(3,4 times ?)

- And i could set RB0/INT for interrupt on transition, and calculate
  time between each of them.
 * This is my choice ...


I want to implement a classic 8,N,1 @ 1200
(1bit start, 8bit data lsb/msb, no parity, 1bit stop, at 1200 baud.)

May be i'm in wrong, for this case it's,

      start             stop        (hope ASCII drawing good.)
1______   _______________   _____
       |_|_|_|_|_|_|_|_|_|_|
0         1 2 3 4 5 6 7 8

(1) set first for falling edge interrupt(INTEDG=0)

(2) interrupt(INTF)
     - next INTF on rising edge(INTEDG=1)
     - at least(+/- 1.2% ?), 10*(1/1200)/second later must come a rising
edge.(STOP bit)

(3) interrupt(INTF)
     - next INTF on falling edge(INTEDG=0)
     - calculate time between (2) & (3), to count how many 1 bit
     - n*bit=time/(1/1200s), including at least START bit.
     - if n*bit=10, a byte received.(start+'11111111'+stop)

(4) interrupt(INTF)
     - next INTF on rising edge(INTEDG=1)
     - calculate time between (3) & (4), to count how many 0 bit
     - n*bit=time/(1/1200s).
     - if n*bit>=10, error, STOP bit must be on rising edge !

(etc) until 10 bits counted, do/calculate, INTF rising/falling time.


To calculate time(wrong ?):

My ISR routineÓ is shared with TMR0 overflow, so each time TMR0 roll
over,
 it reloaded with a pre-calculated value: (CLK/4)/prescaler/(256-value)=
interrupt/s

The wanted result(?) is 4 times the baud rate, ~4800 Hz

4194304/4 /1 /218= 4809 int/s, then Reload value = (256-218) = 38 

;***********************************************************************************

                __CONFIG _XT_OSC & _CP_OFF & _WDT_OFF & _PWRTE_ON


                org 0h
                goto MAIN

                org 4h

_Interrupt:     push                  ; MACRO save W & STATUS

                btfsc INTCON, INTF    ; test if transition on RB0/INT
                goto _RxINT           ; an INTF occur, go to serial
                                      ; else it's a TMR0 overflow
                bcf INTCON, T0IF      ; clear TMR0 overflow bit

              ; some code here

                movlw Reload          ; The pre-calculated value = 38,
cf upper text
                addwf TMR0, f         ; to reload TMR0 + elapsed time
>from T0IF

                incf RxTIMER          ; this is for serial, +1 every
1/4800s
                                      ; cleared, on each transition in
_RxINT
                                      ; to be divided by 4, to count bit
@ 1200 bps

                pop                   ; MACRO restore W & STATUS
                retfie                ; Exit ISR

_RxINT:         bcf INTCON, INTF      ; clear INTF transition bit

             ;  some code here

                pop                   ; MACRO restore W & STATUS
                retfie                ; Exit ISR
;***********************************************************************************

- Do you estimate this method accurate ? 

- Is my vision of serial transmit good ?

- What about TMR0 reload and interrupt latency ?

- Do you think i could have noise from LCD, pin not tied(set to output),
or serial(PC) ?

- Or what ? (may be i'm a dude ?, hope not !)


Please, switch my light !


*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*Happy X_MAS lovely people !*
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*


* life is a joke, but PICs are serious ...