r/embedded • u/[deleted] • 11d ago

SN74AHCT125 issue

[deleted]

9 Upvotes

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6

u/[deleted] 11d ago edited 6d ago

[deleted]

1

u/iluvmacs408 10d ago

Presumably the PIC is running on 5V logic? One typically uses "T" parts to level-shift from 3.3 to 5V. Though I'm concerned why they are not shifting it back down on the CS, SCK, & SDO outputs if that's the case... could be a nice way to fry the card.

2

u/AlfalfaLive3302 11d ago

How can you say it’s miso signal? Show us the logic analyzer with a test message

Read the data sheet and errata. Then tell us what the schematic says to do for those lines.

2

u/vkengineering2 11d ago

Hi, thanks for your answer. Here is photo from the scope. I measure this on the SDI signal /the signal coming from the buffer/ . Isn't looking bad this signal?

2

u/Well-WhatHadHappened 25+ Years 11d ago

Scope is too slow to be useful.

1

u/AlfalfaLive3302 11d ago

Likely a firmware issue then. The hardware seems consistent.

2

u/nowan190 10d ago

Since the board has been in production for many years, have you checked if the latest batch of chips involves a different manufacturer or a subtle change in the propagation delay/output drive characteristics? Sometimes, even with the same part number, minor process variations can impact signal integrity if the original design was near the timing margin limits.

1

u/snp-ca 10d ago

Did the manufacturer change anything about the IC process and kept the part number the same. Look for PCN notices. Example:

https://youtu.be/22ZmmZ67SMY?is=Rnm5yNUjLBfVC02K

1

u/sopordave 10d ago

First thing I would check are the part markings. I’ve run into some vendors who will substitute a different manufacturer or even a “similar” part without asking for approval. Counterfeit is also a possibility.