Hello,

I have a time critical application where each bit to be sent out
serially is to have duration of 69uS (.02/288 to be precise, but 69uS
will do).  I have a routine for serial transmission, modified from
the embedded control applications book. However, I wish the time for
each bit to be the same duration.  Is there a good way of doing this?
Also when fetching another data byte to be sent and then converting
serially, although I expect some padding can be done when sending
bits to allow for the overhead involved in fetching the next byte.

Can a simple modification be done to the code that I have:


; FILE SAVED  AS MAGIC2.ASM
; MAGIC1.ASM HAS BEEN TRASHED

; FOR PIC 16C84

        CBLOCK 0X00
        INDF,RTCC,PC,STATUS,FSR,PORTA,PORTB
        ENDC

BIT_COUNT       EQU 08H
BYTE_COUNT      EQU 0CH

DATA0           EQU 09H

F       EQU 1
W       EQU 0

Z       EQU 2   ;ZERO FLAG
C       EQU 0   ;CARRY FLAG

#DEFINE TX_0    PORTB,7 ; THIS IS SINE + 0 LINE

;**************IT STARTS HERE*****************

         ORG      0
         GOTO     START

         ORG      30

START   CLRW            ; PUT 0 IN WORK REG
        TRIS    PORTB   ; SET UP PORTB ALL OUTPUTS
        TRIS    PORTA   ;PORT A AS ALL OUTPUTS AS WELL

        MOVLW   0
        MOVWF   BYTE_COUNT

NEXT_BYTE
        MOVLW   .8
        MOVWF   BIT_COUNT       ;START BIT COUNT AT 8 (TO GO)
        MOVLW   .3     ; MODIFY LATER, SORT ONE BIT OUT AT A TIME !
      CALL    MAGIC   ; AFTER WHICH W CONTAINS FIRST BYTE OF DATA
   MOVWF   DATA0

;TRANSMIT SECTION

SEND_ANOTHER_BIT
        RLF     DATA0,F    ; CARRY FLAG TO CONTAIN SERIAL DATA
        BTFSC   STATUS,C
        BSF     TX_0    ;TRANSMIT A ONE
        BTFSS   STATUS,C
        BCF     TX_0    ;TRANSMIT A ZERO
        DECFSZ  BIT_COUNT,F
        GOTO    SEND_ANOTHER_BIT
        GOTO    NEXT_BYTE

MAGIC   ADDWF   PC,F    ; ADD OFFSET TO PC AND GET MAGIC DATA
        RETLW   000h
        RETLW   090h
        RETLW   034h
        RETLW   0C3h
        RETLW   0E6h
        RETLW   0BBh
        RETLW   0DDh
        RETLW   0F7h
        RETLW   0F7h

        END

The program is by no means complete but any help or suggestions, on
the problem of a '1' having duration of 10uS and a '0' of 6uS would
be appreciated, please correct me if these figures are wrong!

NB C3 was chosen for now as all being well this should give a square
wave if repeated 1100 0011 1100 0011 1100 0011.

Many thanks for getting this far, hope very much that you can help.

Cheers

Andy Farrar
asf@ee.port.ac.uk