Hi TQFP. First off, I must commend you on a very well put together question. The debugging experiments well explained, the thought process well put together.
Thanks. I was trying to keep it as short as possible and not pull in unnecessary details. I was worried it was getting too long.
I noticed you mentioned "old computers" - maybe implying linear supplies,
Classic home computers from the early to mid 80's mostly. They are old, but I'm always surprised at how much what we consider "modern technology" they have in them (SMD parts, 8/8mil trace/space, etc). Most of the systems have switching supplies, but some are linear. In this particular case the supply is a switcher.
and I read a bit about that in this app note
https://www.ti.com/lit/an/slyt079/slyt079.pdf
In any case, have a look at the app note - it will probably give you the insights you need.
cheers!
Ah yes, very nice, thanks! Google is great, but you still have to get the search right.
I was off looking for power supply info, LDO with inductors, etc. and didn't think about FPGA power up sequencing. The only crappy part at this point is, I have a pile of 250 boards in front of me and a design change now would !@#$. I hate learning the hard way...
Matthew
---------- Post added at 09:49 ---------- Previous post was at 09:38 ----------
1.
Connecting a coil is risky. It can generate a high voltage kick if current is reduced by inserting a high impedance, such as when power is disconnected either by you or internal circuitry.
Good to know! I think an inductor is not going to be a silver bullet for me anyway.
2.
Are there large capacitors on your board? Do they charge up in the first moments of power on? That could pull down supply V at a crucial time.
Only what is required according to the LDO datasheets. Each LDO has one 1uf ceramic on the input, and each output also has one 1uf ceramic (5 total in the regulator section). Each IC has at least one 0.1uf ceramic, and the FPGA has eight 0.1uf ceramics on it, again according to the FPGA (Spartan 3E) datasheet. No electrolytic caps on my board, and nothing over 1uf.
3.
Is there a lot of ripple at the supply rails when your board powers up? That could prevent proper initialization even if the ripple subsides quickly.
I'm not sure, but the 5VDC supply is from the host system, and by the time the voltage gets to the socket of an IC I assume it is as clean as it is going to get. The system power supply is a switcher, and I'm not sure how to capture the power on events without something like a storage scope.
4.
Does your oscilloscope have a vertical threshold setting, so it will begin a trace just as the input crosses that threshold? That is a way to capture the first moments of power-on.
Or, try leaving it on free run. The trace should linger onscreen so you can see what happened at power-on.
It probably does (Tek2465B), and it certainly has more features than I have ever needed so far. I'll use your suggestions and see if I can figure something out.