> If I do this on a 12f629:

> CMCON = 7
> TRISIO = 0

> LED_0 = GPIO.0

> LED_0 = 1


> ... it doesn't work.

Try this and see how it works.

ANSEL = 0 ' Set all digital
CMCON = 7 ' Analog comparators off
TRISIO = 0
LED_0 VAR GPIO.0

Loop:
LED_0 = 1
PAUSE 250
LED_0 = 0
PAUSE 250
GOTO Loop