You may want to look at
http://www.piclist.com/../microchip/delays
for a nice tutorial on pic delays.

---
James Newton (PICList Admin #3)
mailto:jamesnewton@piclist.com 1-619-652-0593
PIC/PICList FAQ: http://www.piclist.com or .org

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of David E Arnold
Sent: Tuesday, April 04, 2000 19:18
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: Proof of Concept( Custom Duty Cycle Serial Port ) with
PIC16F84...


I barely know pic code, it looks like your just doing a for loop
for a set amount of times. How do you know what the delay will be from this?
This depends on the crystal speed right? Faster crystal=shorter delay?

Is this just semi psuedo code? some of these instructions don't quite look
like
F84.
I didn't find SKPZ or SKPNC in the datasheet.

I think from looking at your delay loop you're calculating with a 4mhz clock
and
this loop takes 5 * 100 instruction cycle=500uS at 4mhz.  Is this correct?
Which
part of the code
is doing the 10us on time?

In pseudo language: is your routine doing this following steps:
set port bit on for 10us
clear port bit
delay 416us
repeat for all 8 bits.

What does "goto $+1"  do?

thanks,
-Dave

Andrew Warren <fastfwd@IX.NETCOM.COM> on 04/04/2000 04:37:17 PM
To:   PICLIST@MITVMA.MIT.EDU
cc:    (bcc: David E Arnold/SYBASE)
Subject:  Re: Proof of Concept( Custom Duty Cycle Serial Port ) with
      PIC16F84...

David E Arnold <PICLIST@MITVMA.MIT.EDU> wrote:

> does anyone know if it's possible with "bit banging" or "PWM" to
> make software serial port (TTL level not rs232 voltages - no
> max232 required) that outputs 8N1 format ( 8 data, 1 stop, 1 start
> bit, no parity bit ) but with reduced duty cycle.
>

    Dave:

    Sure, it's possible.

> What I mean by reduced duty cycle is, the bit on time _IS NOT_
> 100% duty cycle like most serial ports. It is something like 2%
> duty cycle.
> ....
> Can the PIC16F84 even make a 2uS pulse?

    Yes.

> For 2400bps this gives a 416uS bit period and a duty cycle of 2% for
> 10uS when there is a logical 1.
>
> If anyone knows if this would be possible with a PIC and perhaps even
> has some code I would be very greatful.

    ; ENTER WITH BYTE TO BE TRANSMITTED IN "BYTE".

    TWOPERCENT:

        CLRC                ;START BIT IS LO.

        MOVLW   9
        MOVWF   COUNT

    LOOP:

        SKPNC
        BSF     TX
        SKPC
        BCF     TX

        GOTO    $+1
        GOTO    $+1
        GOTO    $+1
        NOP

        BCF     TX

        CALL    DELAYBIT

        RRF     BYTE

        DECFSZ  COUNT
        GOTO    LOOP

        SETC                ;STOP BIT IS HI.

    DELAYBIT:

        MOVLW   99

    DLOOP:

        ADDLW   -1
        SKPZ
        GOTO    DLOOP

        RETURN

    Timing isn't EXACT, but it's way more than close enough and the
    "on" time IS 10 uS.

    -Andy


=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - San Diego, California
=== http://www.geocities.com/SiliconValley/2499