Re: NTSC VIC-II timing

From: Segher Boessenkool <>
Date: Wed, 21 Jun 2017 14:08:43 -0500
Message-ID: <>
On Wed, Jun 21, 2017 at 06:46:08PM +0100, smf wrote:
> On 21/06/2017 18:29, wrote:
> >... 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.
> A light pen is a simple sensor that triggers when a brighter than normal 
> light is detected. The VIC just latches the x & y position & fires an 
> IRQ when it is triggered, because it assumes that if the light pen can 
> see the gun then it must be over the current pixel.
> Adding something to VIC to ignore pulses outside the visible portion of 
> the display would be a waste of space.

IIRC the LP stuff can be triggered only once per frame, this makes it
a bit inconvenient to use for non-lightpen uses like we are discussing
here.  It would be nice if we could just read out the current X coor
directly, but nope.


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Received on 2017-06-21 20:00:02

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