PIC signal question

Thread Starter

ksquare

Joined Oct 11, 2009
9
The attached pic is a simplified section of a circuit I'm trying to understand. The arrow points to a point where on some boards the voltage is +5v and others it's around +2.19v. The boards with 5v are good but the others not so much. I'm no expert on the PIC but shouldn't the signal from the PIC be either high (5v) or low (~0v)? I don't have access to the PIC code. Thoughts?
sample_drawing.gif
 

Thread Starter

ksquare

Joined Oct 11, 2009
9
Not asking for anyone to do any work. Just wondering if anyone has knowledge of the PIC pin off the top of their head. Already have datasheets for both devices.
 

dl324

Joined Mar 30, 2015
16,677
Just wondering if anyone has knowledge of the PIC pin off the top of their head. Already have datasheets for both devices.
I have no knowledge of that particular part, but digital outputs essentially switch between VSS and VDD. Inputs in well-designed circuits don't load outputs driving them. With a pull-up resistor on that node, you're looking at one of the 3 defects I mentioned.
 

Jon Chandler

Joined Jun 12, 2008
986
I see two possibilities:

● R is much less than 20k, preventing the PIC from pulling down the voltage.

● This cyptic note about pin 1 of the LT part:

Screenshot_20231110_112359_Dropbox.jpg

You'll have to delve further into the data sheet to understand what this means.
 

Thread Starter

ksquare

Joined Oct 11, 2009
9
I was puzzled because it looks like the PIC pin can be an output or an input pin depending on register settings. The PIC datasheet is 496 pages long. Guess I was wondering if there was a PIC expert here. And yes that note is cryptic:)
 

BobTPH

Joined Jun 5, 2013
8,661
How are you measuring the voltage? If you are using a multimeter, it might be switching between 0 and 5V and you are reading the average. Switch to the AC range. If you get any voltage (more than a few millivolts) it is switching.
 

nsaspook

Joined Aug 27, 2009
12,778
I was puzzled because it looks like the PIC pin can be an output or an input pin depending on register settings. The PIC datasheet is 496 pages long. Guess I was wondering if there was a PIC expert here. And yes that note is cryptic:)
My read of the device sheet is that the PIC pin should be in a open-drain configuration if it's used for the shutdown function instead of the normal Totem-Pole configuration for a output pin. You can fake that using the TRIS register for the pin to make the pin normally an input when not needed for the pull-down and using TRIS again to make it an output pin when you do need it to pull-down with the output pin set to logical 0. Without the source code I can't tell what's being done with the PIC, only guess.

1699651190317.png
 

Jon Chandler

Joined Jun 12, 2008
986
I don't believe the PIC is the problem. I believe the PIC is fighting against a small smaller than claimed pullup resistor or the note about pin 1 on the LT chip I mentioned above.
 

nsaspook

Joined Aug 27, 2009
12,778
OP has boards that are good and boards that are bad. That means that it's unlikely that the PIC code or switching regulator configuration are the problem.
With so little information we have no idea what's the root cause of the problem. Incorrect code could absolutely be a cause of flaky behavior from board to board (as per @BobTPH #7) but I didn't even try to find a problem. Only described what the PIC pin configuration should be per the regulator datasheet as asked by the OP in post #6.
 
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Thread Starter

ksquare

Joined Oct 11, 2009
9
BobTPH, I measured first with a DVM and then with a scope. Will replace the LT1054 & see. Yes it could be a firmware fault or a PIC failure. Output of LT1054 is supposed to be +22v but is only 10ish. In trying to find out why, I learned of the difference of voltages on the PIC pin. Not sure I understand how the LT1054 is being used. Circuit is not at all like samples in the datasheet. Guess some engineer got really clever (or lucky - haha). BTW yes this a work project. I never intended my post to be a way of getting someone else to do my work. I guess I looked at as a learning opportunity for whoever was interested in responding. The circuit is part of a special control board in a Tesla Roadster.
 

Thread Starter

ksquare

Joined Oct 11, 2009
9
It was a defective LT1054. Something went bad with an input pin which I don't often see. Probably the reference source. My apologies if my query was not appropriate use of this forum.
 

liaifat85

Joined Sep 12, 2023
48
It was a defective LT1054. Something went bad with an input pin which I don't often see. Probably the reference source. My apologies if my query was not appropriate use of this forum.
Good to know that you found the problem. I was about to tell you to try some test codes. Then I saw this post. No need to apologize here. The forum is all about trying to help each other.
 

nsaspook

Joined Aug 27, 2009
12,778
It was a defective LT1054. Something went bad with an input pin which I don't often see. Probably the reference source. My apologies if my query was not appropriate use of this forum.
Good to hear that but something's not right if you have multiple similar failures of the same chip on several boards.
 
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