High resolution mode?

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marcelk
Posts: 377
Joined: 13 May 2018, 08:26

Re: High resolution mode?

Post by marcelk » 05 Jul 2019, 17:48

We do have an inverted clock, but it's also created by an inverter (U1). You can try and characterise a 74x241. One of its enable pins is positive, the other negative. You might be lucky if their propagations are the same, but a quick first glance at the 74LS and 74HCT data sheets is discouraging. Still, this should be a simple test.

74LS241.png
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PurpleGirl
Posts: 19
Joined: 09 Sep 2019, 08:19

Re: High resolution mode?

Post by PurpleGirl » 09 Oct 2019, 15:24

Would capacitors help smooth over the transition? And would the final chip need to be faster or slower? There are faster TTL-compatible chips, but if the signal before that is not changing fast enough, might that make it worse?

alastair
Posts: 1
Joined: 10 Oct 2019, 14:28

Re: High resolution mode?

Post by alastair » 10 Oct 2019, 14:48

The fundamental issue here is the additional state. You only want two states: "Pixel 1" (state A) or "Pixel 2" (state B), but there is also a third state: "Update Pixels" (state C). You can try and adjust the timing and hide state C in the A->B or B->A state transition. This doesn't work though since the rate at which the logic changes from 0->1 and1->0 is different. You might be able to align one of these transitions, but the other will show up. Also, this requires critical timing adjustment that will change as the chips warm up.

The only solution is to add an additional register after the A/B selector using the faster dot clock (12.5 MHz). This will sample the A and B states and eliminate state C from the output. It's a simple fix but does require an additional chip

PS. I building something very similar and have been working though all these issues on a breadboard for the last 6 months :x

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