No UART signal from ESP32-C3-Mini (Custom PCB)

[gianfry7]

<t>Hello,<br/>
<br/>
I have this design for a custom PCB (attached the electrical schematics and the layout).<br/>
The PCB is a flex PCB, so it's really thin, with a stiffener below the ESP32.<br/>
<br/>
Unofrtunately, despite seeming to be powered on (the board draws about 14mA when powered, versus 5 mA with the ESP32 unsoldered), I am not able to program the board. I am using a standard USB-to-UART external converter which works just fine with other boards with the same chip.<br/>
<br/>
I event tried to attach an oscilloscope probe to the exposed TXD0 and RXD0 pin. When powered, I get no signal on the TXD0 pin, while I would expect the chip printing the same message as nothing is found in flash memory, so it should be rebooted continuously (at least this is what was happening with other boards). Similarly, when uploading the program, I get no confirmation from the board and the uploading fails.<br/>
<br/>
The only differences I had with this board versus other boards are the following:<br/>
- The substrate is not FR4 but polyamide, but I have X-Ray analysis of it and solder joints seem fine.<br/>
- I swapped the I2C interface from GPIO9/GPIO10 to GPIO8/GPIO9. I know GPIO8 and GPIO9 are strapping pins, but they are both pulled up. I can bring GPIO9 to ground by closing a couple of jumpers to enter download mode, but still I can't get the traditional "waiting for download" message on UART (no signal on UART whatsoever).<br/>
<br/>
Could it be that, since the power converter takes a while to bring the voltage to 3V3, the GPIO8/9 are seen as floating?<br/>
It is strange tough, because I know there is a bit of slack between when the chip wakes up and the reading of the strapping pins. It is also true that the strapping pins are pulled up by a resistance, but the EN/3V3 pins are not. Also, the board has the same behavior when supplying directly 3V3 (instantly, without any conversion).<br/>
<br/>
Should I go back to using GPIO9/GPIO10 instead of GPIO8/GPIO9? Am I right in assuming that on power-up I should see something printed on the UART TXD0 line?</t>

[ESP_Sprite]

You cannot connect EN directly to Vcc; it needs a power-on reset network. Please check the hardware design guidelines for more info.