Re: NTSC VIC-II timing

From: Gerrit Heitsch <>
Date: Wed, 21 Jun 2017 19:44:34 +0200
Message-ID: <>
On 06/21/2017 07:29 PM, wrote:
>> On 2017-06-21, at 19:17, Gerrit Heitsch <> wrote:
>>>> Excellent fun!  It sees if the light pen interrupt can be triggered
>>>> between raster lines x'136 and 2; if so, it's a 6569R1, if not, not.
>>> That's very interesting too! How does one trigger LP IRQ from within software alone? Is it a kind of raster position / CIA port combination?
>> Well, the LP Input is connected to the joystick ports and shares a Pin with PB4 of the CIA on U1.
> Hardware side sure. What I meant was...
>> So you can trigger LP IRQs via software.
> ... that I never used LP IRQs but thought that it would trigger only when the button line is grounded within a specific timing window so some kind of synchronisation with raster position would be required for the IRQ to happen. But that's a different subject anyway.

No, the LP IRQ just latches the current X and Y position of the electron 
beam when you get the high to low transition on the LP line. And you 
also get an IRQ to notify you of that event.

The idea was to have a light pen, meaning a photo cell held against the 
screen (needs a CRT, won't work on a TFT) and detect when the beam comes 
by. With the right software you could draw on the screen with it.

So you can just trigger an LP event including IRQ by software via PB4 
and then check the registers. Doing this at different times (use $D012 
to determine when) will help you differentiate between a 659R1 and the 
other PAL-VICs.


       Message was sent through the cbm-hackers mailing list
Received on 2017-06-21 18:04:07

Archive generated by hypermail 2.2.0.