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How to even start routing this?
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Joanna_H:

--- Quote from: Ice-Tea on August 02, 2019, 08:21:13 am ---Take the advice of others.. Go to a 4-layer board. If this is a paying job, the extra cost will be offset easily by the saving of labor and the probably saving of a respin because of performance/EMI issues.

Use a calculator from a local (Eurocircuits or somesuch) or an offshore supplier (such as PCBway) and you'll soon figure out in most cases going 2 layers just isn't worth it..

--- End quote ---

Yeah, I'm starting to think 4 layer may be the way to go.... lol
tggzzz:

--- Quote from: Joanna_H on August 02, 2019, 07:57:04 am ---To answer the other questions, this is a simple SVGA 800x600@60Hz Graphics display.  It's a module that I've been putting together to go with a load of other modules I'm creating to make a watch as it works CPU, variable speed up to a max of 1Mhz.   The 40Mhz is required due to that being the specification for the video out.   99% of the logic is standard 74 series, when not it's all TTL compatible outputs, all running at 5V.

--- End quote ---

The 1MHz is completely irrelevant[1] - all that matters is the transition time, which can be <5ns for old school TTL. That corresponds to significant frequency content at >100MHz.

Ground bounce is a real problem due to the low VIL spec of TTL and the currents in switching transients. A good gridded ground is the only way to minimise the consequences.

[1] for a canter through the theory, plus some practical measurements, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/
tggzzz:

--- Quote from: Joanna_H on August 02, 2019, 08:24:55 am ---
--- Quote from: Ice-Tea on August 02, 2019, 08:21:13 am ---Take the advice of others.. Go to a 4-layer board. If this is a paying job, the extra cost will be offset easily by the saving of labor and the probably saving of a respin because of performance/EMI issues.

Use a calculator from a local (Eurocircuits or somesuch) or an offshore supplier (such as PCBway) and you'll soon figure out in most cases going 2 layers just isn't worth it..

--- End quote ---

Yeah, I'm starting to think 4 layer may be the way to go.... lol

--- End quote ---

If you do, make one layer a solid ground plane and another a solid Vcc plane. If they are inner planes then it is easier to probe and modify signals. If they are outer planes then EMI problems will be minimised. Choices, choices :)

Be aware of the maximum length of a track before it has to be treated as a transmission line. With old school TTL (e.g. LS, ALS) you will probably be fine, but modern jellybean logic is a different kettle of fish  (e.g. 74lvc1g* is <1ns, I've measured 300ps)
Joanna_H:
I thought I'd autoroute it just to get a feel of what I've let myself in for......



4 layer board, internal gnd/power planes and top/bottom signal.

I think my head is going to really hurt with this one.
tautech:
Oh and I forgot, for dual, triple, quad and hex packages don't forget you can reassign gates to make routing easier.
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