@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?
I’m having this same problem with my L29HDA Mozihao board but I tested the connection using a CH430G USB to TTL and it worked perfect so why would we need to change out the resistor?
The issue is that the output of the L29H may have limited drive capability and with the extra resistor in series may not have the drive capability to drive all kinds of inputs. Quectel themselves do not recommend any resistor in series, so having that resistor on the Mozihao board just reduces the drive capability and could cause issues with some inputs.
I’ll be, it worked. My ESP32 is finally receiving and forwarding data through my server socket. For those that want to know what I did, I soldered a jumper cable from one end of the resistor to the other end. I’ll be doing this to my LC29HEA module as well.
Glad to hear that. FWIW, I only noticed the reduced drive on certain esp32 pins, not all pins. But not having any idea why Mozihao put such a high value (1K) resistor in series, the right thing to do is to replace it with something like a 22 ohm resistor, or if you don’t have access to SMD resistors, do what you did, which is just short it out.
I’m not sure who posted it (maybe @bamarcant), but if you look at this thread, in the list of links at the top, there is a link to a Mega folder. The Mozihao schematic is in there…
