Greetings-

I've just started playing with microcontrollers and chose the 10F200 because 
of its extreme simplicity.  For a beginner's project, given the approaching 
season, I thought it would be fun to make a simple LED flasher Christmas 
ornament.  I wanted a snowflake pattern with 6 LEDs so I decided to use 2 
10F200s, each controlling 3 LEDs and have them run a very similar program 
which would give the illusion of one device controlling all 6.

The program works and everything looks great until you let it run for 
awhile.  Then things start getting out of sync.  I am using the osccal value 
for each device.  While I didn't expect perfect synchronization I had hoped 
it would last more than a few minutes.

I thought that the code which set the LEDs on or off might take less time in 
some cases, which would throw things off, but I don't think this is the 
case.  The remainder of the code running on the two devices is identical 
aside from the bit pattern stored in W at various steps.  Here's the code to 
set the LEDs on or off.  On input, W contains the pattern in the lower 3 
bits, 1=on, 0=off:

test	equ	h'12'		; LED pattern
lights
	movwf test		; keep pattern
	bcf GPIO,2		; clear all LEDs
	bcf GPIO,1
	bcf GPIO,0
	btfsc test,2		; LED 2 if set
	bsf GPIO,2
	btfsc test,1		; LED 1 if set
	bsf GPIO,1
	btfsc test,0		; LED 0 if set
	bsf GPIO,0

Some might be saying: "Wait, if w already has the bit pattern, why not 
replace all of that with a single movwf GPIO?"  That's what I thought, but 
it doesn't work.  The code has seven patterns to play and with that in place 
the first two seem to repeat endlessly.. odd.  The same thing happens if I 
add a movlw 0 and movwf GPIO ahead of loading the desired pattern into GPIO 
in a single instruction.  I even threw a few nops in there between the two 
as a small delay.  The above plays the patterns I want but eventually the 
two devices get out of sync.  This happens when I power the devices with 2 
AAA cells or use the benchtop power supply at 5V.  My understanding of how 
btfsc works is that if the skip happens it is 2 cycles and if it doesn't it 
is 1 followed by the 1 of the bsf so that regardless of the skip, those two 
instructions always take 2 cycles to execute.

Since the 10F200 doesn't allow for an external oscillator does anyone have 
any ideas on:

1. Why these get our of sync so quickly and how I might prevent that?
2. How to get a single device to control all 6 LEDs itself when only 3 
output lines are available?

Any help appreciated!

Ron


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