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Purpose of R3 in WROVER-B

Posted: Mon Jun 22, 2020 4:00 am
by raspy00135
I use ESP32 LyraT.
I have two version of boards, one is WROVER, another one is WROVER-B. Both are v4.3 boards.

Then I noticed I cannot connect UART0 between devices, TX to RX, because of R3(499R) connected to TXD0.
It makes a huge problem to me. TXD0 has very weak drive capability because of this. It cannot override RXD0's pull-up.
Why did it make this incompatible change, and what's the purpose of R3?

I see this resistor in recent version of WROVERs. Unfortunately I don't see any idea to work around this other than circuit change.

Re: Purpose of R3 in WROVER-B

Posted: Mon Jun 22, 2020 5:04 am
by ESP_Angus
Hi raspy,

Sorry for the inconvenience this change has caused.

This resistor was added to suppress 80MHz harmonics in the TXD0 output signal. You can refer to "1.2.2.How can I suppress the harmonics of 80 MHz? " in the ESP32 FAQ document for the cause which led to this change.

The internal pullup on RXD0 is probably not the reason you can no longer make this connection. On the LyraT, the CP2102N USB to Serial chip is also actively driving the RXD0 pin on the WROVER module from its TXD pin. This may have worked before, but maybe by chance (the two push-pull drivers were "fighting" each other and the WROVER TXD0 signal was stronger than the CP2102N TXD signal.)

The simplest circuit change would be to modify R11 (0 ohm resistor in LyraT v4.3) which connects CP2102N TXD to RXD0. You could either remove it entirely or replace it with a relatively high resistance value. Then the CP2102N will "lose" again.

Alternatively, if you don't need to talk directly to the ESP32 boot ROM then you could remap the ESP-IDF serial console and/or change the UART driver to use a different pin assignment.

Re: Purpose of R3 in WROVER-B

Posted: Mon Jun 22, 2020 1:46 pm
by raspy00135
Thanks for the reply, unfortunately both suggestion won't work for my case, the change was killer.
I need to change my daughter board circuit design.