import machine
import time

# [Ed note: Feb 10, 2021 from hippy in raspberrypi.org/forums t=301152]
# [Ed note: ADC VREF can be read using ADC as either ADC(3) or ADC(29) but...]
VSYS_ADC_INPUT = 3      # 3 or 29

# Using ADC(3)  - Pin 29 needs to be set as input with no pulls.
# Using ADC(29) - Pin 29 is automatically set as required.

if VSYS_ADC_INPUT == 3:
  Pin = machine.Pin
  Pin(29, Pin.IN)
  vsysChannel = machine.ADC(3)
else:
  vsysChannel = machine.ADC(29)  # [Ed note: safest]
  
while True:
  adcReading  = vsysChannel.read_u16()
  adcVoltage  = (adcReading * 3.3) / 65535
  vsysVoltage = adcVoltage * 3
  vbusVoltage = vsysVoltage + 0.275 # At 100mA
  print(adcReading >> 4, adcReading, adcVoltage, "|", vbusVoltage, "|", vsysVoltage)
  time.sleep(1)

Example output        VBUS       VSYS

2016 32263 1.624596 | 5.148788 | 4.873788
2019 32311 1.627013 | 5.156039 | 4.881039
2009 32151 1.618956 | 5.131869 | 4.856869
2013 32215 1.622179 | 5.141537 | 4.866537

# [Ed note: if you change first line from 3 to 29 then rerun, results should not vary much 2021EMay20]

/Source04_Read_VSYS.txt stored at http://ephotocaption.com/a/164/Source04_Read_VSYS.txt