ESP 32 (custom board) does not start im flash mode when powered on

[alexander96]

Hello folks, I am currently building a custom board for an ESP32 WROOM-E based on the devkit schematics, everything was working and fine I thought I am nearly finished...
Then the chip for the USB/UART Bridge the CP2102 QFN28 was out of stock and I had to do a new revision with CP2102 QFN24 (which I am still thinking is the exact same chip just different size?)

Well today I built the newest revivision of my board with the QFN24 chip and now I have the problem that my board does not start the programm when powered on. It only starts the programm when the EN button is pressed.

(what is a bit suspect but I dont know if it hast anything to do with it, the new revision takes about 10 secs to connect to the same WIFI while the old one was connected nearly instant)

I am annoying at the end and now I have to find the problem all over again and fix it so I would be very happy if annyone of you has a suggestion.

So what could cause this problem:

- I fucked something up on my pcb design? (I couldnt find any difference except the other chip?)
- I fucked something up while soldering the components on it?
- I fucked something up with the Software / initialization of the ESP32 (I uploaded the exact same code to both revisions)

I measured the GIPO PIN 0 when the USB power is plugged on on both revisions and its instantly at nearly 3,3V at both PCB's

Sincerly Alexander

[alexander96]

So it looks like I have the same problem mentioned here:
viewtopic.php?f=12&t=20138

The ESP32 only starts when I desolder the 0,1uF condensator parallel to the boot button (named C3 in my schematics)?

I uploaded a part of my pcb schematics as a picture is there anything I did wrong?

If I change C4 to a higher value i doesnt work too. (I tried 4,7uF)
I still have to remove C3.

Why does it work in my previous PCB design and not now?

Is it a problem If I just leave C3 not connected (so I dont place this cap on the board) or could cause this other problems?


Okay so I digged a little bit deeper:
in the current hardware design guidelines it says:

To ensure the power supply to the ESP32 chip during power-up, it is advised to add an RC delay circuit at the
CHIP_PU pin. The recommended setting for the RC delay circuit is usually R = 10 kΩ and C = 1 µF. However,
specific parameters should be adjusted based on the power-up timing of the power supply and the power-up and
reset sequence timing of the chip

These values refer to C4 and R7 in my schematics? Am I right with this?
I was currently using 0,1uF instead of 1uF because it has this value on the public devkit Schematics I found which seem to be a little bit old.
since I changed C4 from 0,1uF to 4,7uf ( I dont have 1uF here atm) it still does not work?
So is there even a chance it works with 1uF?

What else can I try (change R7 maybe?)


I hope anyone of you has an idea.

Sincerly Alexander

[alexander96]

Ok so I digged even deeper:

There seem to be a difference between the CP2012 QFN24 and QFN28 package:

The QFN24 Version has a seperate VIO AND VDD pin the QFN28 does not have a VIO pin.

On the Schematics for the devkit (with the QFN28) therefore the RSTB pin pullup resistor is connected to VDD (like the manual of the CP2102 says it) but when using the QFN24 package the pullup resistor for the RSTB pin has to be connected with VIO and not VDD.

Thats what I think I was doing wrong in my PCB design I connected it the same way as then QFN28 with VDD and not VIO.

quote from the CP2102 manual:

In all cases, a 1 kΩ pull-up on the RSTb pin is recommended. This pull-up should be tied to VIO on devices that have it. On devices
where VIO is connected to VDD or devices that do not have VIO, this pull-up should be tied to VDD. The RSTb pin will be driven low
during power-on and power failure reset events.

That would probably explain why the old version worked and the new version does not work anymore but im not good enough in this stuff to say that this "error" results in the ESP not starting in flash mode.

Does anybody of you know if this mistake will result in my problem?