@george.gao I have a Mozihao LC29HEA board and noticed that the UART TX output has trouble driving some pins on an esp32 chip. The hardware reference shows the following:
It recommends a direct connection at 3.3v, which is consistent with an esp32. However, the Mozihao module has a 1K resistor in series with the TX output and this is not able to drive certain input pins on the esp32. I have reduced the resistor to 100 ohms, and that resolves the problem, but I was wondering what the actual drive capability of the TX pin is. I noticed the same problem with the Mozihao LG290P module.
Hi @splee
Why do you want to connect a resistor in series? It is recommended to connect directly or choose a 0Ω resistor. We have not tested the driving capability of TX internally. You can test the waveform and check the waveform.
Hi @splee
Theoretically, series resistors are not allowed in TX and RX. I will ask for internal verification on this issue. Please also provide the hardware connection diagram and oscilloscope picture of your test, thank you.
It’s ok, I have removed the resistor. I only bring this up because this is the design on the Mozihao board that many people use. It would be good to have some specification of the drive capability of the TX output pin of the LC29HEA module.
Hello @splee, often I will add a series resistor in my designs in the order of 22ohms. This has the effect to round edges very slightly, enough to reduce RF but not affect the actual signal. Maybe that was the intent and they put the wrong value in there?
