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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! I'm porting Linux to Apple Silicon Macs at @AsahiLinux. http://patreon.com/marcan  | http://github.com/sponsors/marcan 

Tokyo, Japan
marcan.st
Joined May 2009

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    1. a pile of unfinished projects in a trenchcoat‏ @ferristweetsnow 20 Sep 2019
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      Digital logic folks: I've been seeing a lot of 4/8-bit computer projects using TTL logic. From my limited FPGA experience I know that meeting timing is critical for a functioning design, yet on these simple projects, nobody seems to talk about this.

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    2. a pile of unfinished projects in a trenchcoat‏ @ferristweetsnow 20 Sep 2019
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      These designs are all fairly low freq, so is it a non-issue because the propagation delay through the wires is much faster than the delay through the digital logic? Can a design like this be routed such that these constraints are violated? At what frequency is this even relevant?

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    3. a pile of unfinished projects in a trenchcoat‏ @ferristweetsnow 20 Sep 2019
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      I guess a case that I would have assumed may be problematic is routing a clock signal to a "far" away (a breadboard or two) register, where some output from another register to that register takes a short path (through a gate or two) before reaching the input.

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    4. a pile of unfinished projects in a trenchcoat‏ @ferristweetsnow 20 Sep 2019
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      I guess one thing I could do to figure this out myself is check out TTL register setup/hold timings and find some propagation speeds for typical wires or whatever and do some basic calculations on some simple circuit models, but it would be cool if someone could clarify this. :)

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    5. a pile of unfinished projects in a trenchcoat‏ @ferristweetsnow 20 Sep 2019
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      Maybe @ben_eater, @eevblog, @benheck would know :)

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    6. Graham Sutherland (Polynomial^DSS)‏ @gsuberland 20 Sep 2019
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      Replying to @ferristweetsnow @ben_eater and

      /cc @marcan42 for FPGA wizardry

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    7. Hector Martin‏ @marcan42 21 Sep 2019
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      Replying to @gsuberland @ferristweetsnow and

      The answer is yes, it matters just like on an FPGA, and you can certainly get into sticky situations with setup/hold times. But as long as all your logic is the same type it should work well together in the slow case, and then it's just a matter of how high you can clock.

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      Hector Martin‏ @marcan42 21 Sep 2019
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      Replying to @marcan42 @gsuberland and

      Discrete logic designs are rarely run at anywhere past a MHz or two, while FPGA designs are often pushing 100MHz. It's also much harder to simulate a physical circuit to come up with a theoretical timing model, because you don't know things like wire capacitance.

      4:12 AM - 21 Sep 2019
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      • a pile of unfinished projects in a trenchcoat
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        2. Hector Martin‏ @marcan42 21 Sep 2019
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          Replying to @marcan42 @gsuberland and

          On an FPGA the physical layout is fairly uniform and you can derive a "safe" timing model that is consistent (in practice you can often go much faster than the timing report says, but no guarantees). With breadboard designs, well, just try and see.

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        3. Hector Martin‏ @marcan42 21 Sep 2019
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          Replying to @marcan42 @gsuberland and

          Even at slow clocks though, you can definitely have timing and signal integrity problems. For example, bad clock routing can cause ringing/reflections and double-clocking of logic. In general mixing fast and slow logic can be a bad idea.

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