Improve ADC input handling for TTGO LoRa32 V1

[matthias-bs]

Actually VBAT is provided to the ADC and should be used as uplink data and for checking the supply voltage.
There is no separate supply voltage measurement by default.

[matthias-bs]

In https://github.com/LilyGO/TTGO-LORA32/blob/master/schematic1in6.pdf on sheet 4, there are two input dividers - R42/R43 and R44/R45 for VBAT connecting to IO35. Chances are that one of them has wrong labels and is actually connected to VBUS and a different ADC input (IO34 would be a good candidate).

Opened issue LilyGO/TTGO-LORA32#35

[matthias-bs]

@mczakk Is it possible to connect a power supply to the battery connector and check if you can get a plausible voltage reading from IO34?

[matthias-bs]

If IO34 is not connected:
Please try this circuit and report if the supply voltage (with and without connected battery ) can be measured

                    100k       100k
JP1 Pin2 - GND  o--/\/\/\-–|--/\/\/\------o +5V - JP1 - Pin 1
                           |
                           `--------------> IO34 (ADC in) - JP2 Pin 10 

[matthias-bs]

@franki29 Maybe you can help to figure this out?

[mczakk]

@mczakk Is it possible to connect a power supply to the battery connector and check if you can get a plausible voltage reading from IO34?

I'll be honest, I have no idea! Don't even know where the battery connector is. Or how I would get a power supply to it 😱

[franki29]

@franki29 Maybe you can help to figure this out?

Hi, took me some time to get the right parts to test.
Found https://github.com/anii1211/ESP32_ADC-Accuracy_improvement/tree/main/ESP32-ADC-Accuracy-Improvement-main
and https://esp32-server.de/adc/ for testing.

To test it I took a normal ESP 32 for testing and a voltage regulator bord.
As I can not use first example with the extrenal voltage regulator (no serial connection possible) I used the second one.
I used a 220 K Ohm resistor between +5 V and AC6 (pin 34) and a potentiometer between - and pin 34.
Than I messure the voltage with a voltage messure device between regulator output and adjust the potentiometer to get same result from ESP32 and my messure device. Changing now the voltage I could see it at my esp32.
Hope this helps.

[matthias-bs]

@franki29 Thanks for the quick answer, but my question was targeted to the TTGO LoRa32 specific power supply and ADC input circuit design.

In general, I know how to use the ADCs and how to get calibrated conversions.

[franki29]

Oh sorry, I did not read the full issue, sorry. I have a board like this one https://polluxlabs.net/arduino-tutorials/daten-per-lora-senden-und-empfangen-mit-dem-ttgo-lora32-board/ and I think it is a clone. There are no resistors between the Battery connector and the ADC, none of them. (Measured by a Ohm Meter) So basically to add two 100K like you proposed in this issue should work.