[Coco] CoCo 1/2 VDG & SAM interaction.

[Darren A]

On Sun, May 31, 2015 at 3:56 PM, Zippster wrote:

> Ah, interesting.  Does anyone know what the input CB1 on the PIA gets used
> for?
>
>

There are two PIAs.  For each of them, CB1 is used as an interrupt source
for the CPU.  One is connected to the CART pin of the cartridge connector
and can be used to assert FIRQ* for the cartridge auto-start interrupt.
The other is connected to the FS* output of the VDG and can be used to
assert IRQ* on each frame of video. The 60Hz TIMER function in Extended
Basic uses this for incrementing the value and Disk Basic uses it for the
Motor shut-off timer.

- Darren