VBatt oddness on Frankenso

[Simon@FutureProof]

I'm expecting this to be entirely my fault but I am having some odd response from my VBatt channel.

It seems to work fine but any voltage above 5v is saturating the ADC and reading the same in tuner studio.

I have double checked all the resistors match the schematic. The VBatt divider setting works just fine in tuner studio. With the ECU connected to 12v and the VBatt disconnected I have a 1.18v reading on pin PC4 and about 2.36v at W44.

If i connect W44 to that 12v then set the divider to match the reading to 12v all seems ok, until I put 5v on that pin and can still read 12v in tuner studio. 3.3v on W44 then reads 7.9v in tuner studio.

So it seems I only need 5v on W44 to hit 2.44v on pin PC4 which is a bit of an issue when 12v is giving 2.48v. (3.3v is giving 1.66v at PC4).

Is there a mistake in the frankenso schematic? Could I have an incorrect resistor in there?

Edit - Also the 5v for the double schottky is fine.

[kb1gtt]

On the Frankenso schematic, I shows the 12.02V, which should read as about 1.1V out12. With a multi-meter can you probe the circuit and see when the voltage do not match the schematic notes? If you get about 1.1V at either test point P66 or STM32 PC4, then it would appear the problem is with the software. However if you are getting a high voltage, then the problem is hardware.

Hmmm, perhaps the issue is that you should NOT have W44 installed. The 12V comes from W2, and not W44. The W2 will run this signal through the front end resistor divider, while W44 does not include this resistor divider. With W44 removed, and 12V on W2, you should read about 1.1V on PC4.

[Simon@FutureProof]

I currently don't have W44 installed, I was just probing to the via for that jumper. However I don't think I have W2 in place either as I was working on the premise that VBatt is on INP12 via W44. Edit - ignore that boxxxks obviously W2 is installed or I wouldn't have any power to the board.

I assume there are no schematic changes from R0.4 to R0.5.3? I recall a thread about it being roughly the same so no update to the schematic but best to check.

Re-examining the schematic when it is not 2am and I might have been a bit silly here, your point about 12v on one side of R320 showing ~ 1.1v at the discovery pin is correct when I have nothing on W44.
I did have a mistake which can be seen on the build thread where R320 was a 392 resistor instead of a 393 resistor, this has been fixed from my first checks without W44 connected.

I think everything is actually working perfectly as intended and I have overthought this, thinking I need to add a VBatt reference that is already internal to the board.

If nothing this is all important notes to put on the build documentation and why I decided to build it myself, nothing teaches you about how/why something works like having to create it yourself.

[Simon@FutureProof]

Just to round this off incase anyone finds it in future with a similar issue.

I checked the board last night and it seems it was actually working as intended. My initial tests failed because a resistor was in correct on the first stage voltage divider. Instead of a 3.9:1 ratio on the divider I had a 0.39:1 ratio.

With the voltage divider corrected and the VBatt coefficient set to ~9.50 I have a working VBatt reading.

There is no need to connect a separate VBatt input via W44 but if this is required for any reason then the way to do it is to remove R320 and put the 39000 ohm resistor in as the jumper for W44. This would give the same result as the internal VBatt reference.