program for pic12f675

it features two AC (50Hz) dimmer circuit drivers

dim levels can be set from a PC using a simple serial protocol
alternatively
dim levels can be set from an analog dial (both dimmers will have the same
level)

This is the pin usage, pins are named with their function as
in the datasheet.

pin 1: VDD (5V)
pin 2: GP5 -> dimmer1
pin 3: GP4 -> dimmer0
pin 4: GP3 <- PC serial data (input)
pin 5: INT <- zero crossing detector, active high
pin 6: GP1 <- PC serial clock (input)
pin 7: AN0 <- potentiometer 10K from 0V to 5V
pin 8: VSS (0V)

[PIC16F627 testing]

pin 14: VDD (5V)
pin 18: RA1 -> dimmer1
pin 17: RA0 -> dimmer0
pin  9: RB3 <- PC serial data (input)
pin  6: INT <- zero crossing detector, active high
pin 12: RB6 <- PC serial clock (input)
pin  5: VSS (0V)


program is (after initialisation) completely interrupt driven

- timer0 interrupt will occur 100Hz*256 (256 dim levels, zero crossing 100 per
second)
- INT interrupt will occur at 100Hz (whenever a zero crossing is detected, rising edge)
- A/D converter will signal if A/D conversion is ready
- GP3 will generate an interrupt on change (timer1 is used for timeout on recv)

processor can not be put to sleep, because timer0 will stop when asleep