Author Topic: Reset circuit?  (Read 507 times)

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Offline MarsupilamiTopic starter

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Reset circuit?
« on: June 03, 2023, 07:20:34 pm »
Hi,

What do you think this is doing? I'm really missing something here.
This is a PCIe adapter board, and the input signal is the PCIe reset.

I tried to reverse engineer based on the board:

It would make a lot of sense IF resistor R4 was populated.
But it isn't:




What's even more interesting is that this is a similar PCIe board from another vendor, and it has a same looking circuitry but doesn't even have a location for what's R4 on the first one. You can see that the collector of Q2 and the base of Q1 is directly connected without anything else. There is no via visible from the other side either.

(R5->R3; R3->R2)


Did I misidentified the transistor as 3904? (Marking 1AM?)  :-//
« Last Edit: June 03, 2023, 09:28:53 pm by Marsupilami »
 

Offline JustMeHere

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Re: Reset circuit?
« Reply #1 on: June 03, 2023, 09:05:10 pm »
It looks like they are taking about this type of Darlington Pair in this document.  See the Sziklai Darlington section.

https://www.electronics-tutorials.ws/transistor/darlington-transistor.html
 
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Offline MarsupilamiTopic starter

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Re: Reset circuit?
« Reply #2 on: June 03, 2023, 09:16:51 pm »
It looks like they are taking about this type of Darlington Pair in this document.  See the Sziklai Darlington section.

Yeah, but that has one NPN and one PNP transistor, which makes sense. On this circuit, at least as far as the markings go they are identical.

I'm starting to have the suspicion, that NVMe doesn't actually need the PCIe reset so it's always high, and on the purple board they did an easy DNP to set it, while the red board might be a knock off, and whoever copied the circuit had no idea what they were doing so they didn't add the optional of the R4 pullup, even though that makes the whole transistor thing moot. I don't know. It doesn't make much sense.  :wtf:
 

Offline Benta

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Re: Reset circuit?
« Reply #3 on: June 03, 2023, 09:24:56 pm »
Your circuit is a pull-up resistor (R3). That's it.
 
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Offline MarsupilamiTopic starter

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Re: Reset circuit?
« Reply #4 on: June 03, 2023, 09:38:46 pm »
Here's another one. This time an M.2 form factor.


The resistors are even have the same reference designation, and what do you know, R4 is populated.
 

Offline MarsupilamiTopic starter

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Re: Reset circuit?
« Reply #5 on: June 03, 2023, 09:39:48 pm »
Your circuit is a pull-up resistor (R3). That's it.

That's what I see too, but then why do they put the transistors there?  :palm:
 


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