ADC not working through IO35 using MicroPython

[glennra]

I have the ESP23-2432S028R and would like to use the ADC via pin IO35 on connector P3.

I have a 9V battery connected via a 300k/100k divider.

Reading from the ADC gives what seems like a cycling value.

For this demo I've used the stock 1.26.1 MicroPython firmware but I get the same result using lvgl_micropython. I have 2 boards and they both have the same behaviour.

Can anyone shed some light on this?

from machine import ADC, Pin
import time

adc = ADC(Pin(35))               # GPIO35 (input-only), ADC1_CH7
adc.width(ADC.WIDTH_12BIT)       # explicit 12-bit resolution (0..4095)
adc.atten(ADC.ATTN_11DB)         # ~0–3.9 V range (use 11 dB for most cases)

while True:
    raw = adc.read()             # 0..4095 (12-bit)
    uv = adc.read_uv()/1000.0
    # Approximate voltage; adjust 3.3 if your board’s ADC reference differs.
    v = (raw / 4095) * 3.3
    print("raw:", raw, " V from raw≈", round(v, 3), "uv", uv)
    time.sleep_ms(100)

raw: 1383  V from raw≈ 1.115 uv 1297.0
raw: 2651  V from raw≈ 2.136 uv 2346.0
raw: 2896  V from raw≈ 2.334 uv 2524.0
raw: 3376  V from raw≈ 2.721 uv 2820.0
raw: 3431  V from raw≈ 2.765 uv 2812.0
raw: 2896  V from raw≈ 2.334 uv 2501.0
raw: 2784  V from raw≈ 2.244 uv 2396.0
raw: 2559  V from raw≈ 2.062 uv 2231.0
raw: 2010  V from raw≈ 1.62 uv 1735.0
raw: 1216  V from raw≈ 0.98 uv 1099.0
raw: 773  V from raw≈ 0.623 uv 752.0
raw: 112  V from raw≈ 0.09 uv 208.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 320  V from raw≈ 0.258 uv 438.0
raw: 944  V from raw≈ 0.761 uv 936.0
raw: 1294  V from raw≈ 1.043 uv 1208.0
raw: 1725  V from raw≈ 1.39 uv 1583.0
raw: 2363  V from raw≈ 1.904 uv 2105.0
raw: 2839  V from raw≈ 2.288 uv 2453.0
raw: 3211  V from raw≈ 2.588 uv 2728.0
raw: 3422  V from raw≈ 2.758 uv 2806.0
raw: 3009  V from raw≈ 2.425 uv 2555.0
raw: 2752  V from raw≈ 2.218 uv 2366.0
raw: 2553  V from raw≈ 2.057 uv 2218.0
raw: 2123  V from raw≈ 1.711 uv 1819.0
raw: 1328  V from raw≈ 1.07 uv 1180.0
raw: 800  V from raw≈ 0.645 uv 781.0
raw: 312  V from raw≈ 0.251 uv 365.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0
raw: 0  V from raw≈ 0.0 uv 142.0

[glennra]

Just after writing the above I had a suspicion that it may be that the divider I've used doesn't supply enough current. I'll try using lower resistor values.

[glennra]

That didn't help (47k/10k divider)