Re: NTSC VIC-II timing

On 21/06/2017 18:29, silverdr@wfmh.org.pl 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.


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