ESP32 Forum

We have a very strange problem.

We have connected Teensy 3.2 board (3.3V levels) Tx, Rx and GND to ESP32 development board. and made simple communication test SW.

The communication fails, except it work if we disconnect ground and power Esp32 from battery. i.e. only Rx and Tx connected.

How is this possible? and why is it not working when GND is connected?

Hi, KimKim
Has this problem been solved? If not, can you provide a picture showing how you connect the two boards and the battery?

thanks !!

Hi ESP_houwenxiang ,

I have the same problem as KimKim. When I'm connecting GND of SIM800L to GND of ESP32, the ESP32 start to not working and it starts be invisible from ArduinoIDE (connecting problems when trying to upload a program (Connecting........_____....._____....._____....._____)). When I'm disconnecting GND it instantly starting work and upload the program.

Also I've tried to test if esp32 itself working when GND is connected - I've put some loop that writes in the Serial Monitor function into esp32 and after connecting GND it stops writing, after disconnecting - start to writing again. So I'm pretty sure that the UART GND breaks something.

I connected the modules exacly like on this picture (excluding other devices)

https://content.instructables.com/ORIG/ ... 111d5be699

Also power supply of SIM800L is 3.7V Li-Poly battery. SIM800L works fine, it indicates by LED that is connecting to the network, so power supply is correct. The only problem is no UART connection between modules without GND between modules.

GPIO2 must be unconnected or low when entering download mode. Your SIM800L is connected to that pin; my guess would be that it pulls the pin high preventing the ESP32 from properly entering download mode.