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11bit ADC data from SAMD51 (M4) boards #11391

Description

@kevinjwalters

CircuitPython version and board name

`10.2.1` on Adafruit Feather M4 Express

Code/REPL

My code is too complex but the essence is `value` from `AnalogIn` (on `A5`) shifted by 4 bits.

Behavior

The values when converted from CircuitPython's 0-65535 using >> 4 to 0-4095 (12bit) values are all even.

Description

From my recent ADC testing which I was reviewing I am only seeing even values from an M4 board's ADC in CircuitPython when it's shifted to a 12 bit value. This suggests it's either set to 11 bits or something is zeroing the lsb??

Additional information

This is some of my raw data.

1311612915047,"triangle","B",0,"C","max",0,"[-1]",-0.0001875,"[8,8,6,8,6,4,8,8,6,6,6,6,6,6,6,6,6,6,8,6,6,6,6,6,6,8,8,6,8,6,6,8,8,6,6,6,6,6,6,6]"
1311659362795,"triangle","B",0,"C","max",0,"[-1]",-0.0001875,"[6,6,6,6,6,6,6,4,6,6,6,6,6,8,6,6,6,6,6,6,6,8,6,6,10,6,8,6,8,6,8,6,6,6,8,6,6,6,6,6]"
1311688812260,"triangle","B",0,"C","max",0,"[-1]",-0.0001875,"[8,8,8,6,6,6,6,6,6,8,6,6,12,6,6,6,6,6,6,6,6,6,6,6,6,6,6,8,6,6,8,6,6,6,6,8,6,8,6,6]"
1311718200689,"triangle","B",0,"C","max",0,"[-1]",-0.0001875,"[8,6,6,6,6,8,6,8,6,6,6,6,6,6,6,6,6,8,6,6,6,6,6,6,8,6,6,8,8,6,8,8,6,6,8,6,6,6,6,6]"
1311747283938,"triangle","B",0,"C","max",0,"[-1]",-0.0001875,"[8,8,6,6,6,6,6,6,6,8,6,6,8,6,6,6,6,6,4,6,6,6,6,6,8,6,6,6,6,6,14,6,6,8,6,6,6,6,4,6]"
1311785400394,"triangle","B",0,"C","max",0,"[0]",0.001125,"[6,8,8,6,8,6,6,6,6,6,6,8,6,6,6,6,6,8,6,8,6,6,6,6,6,6,6,8,6,8,6,6,6,6,8,8,6,6,6,8]"
1311832366948,"triangle","B",0,"C","max",0,"[1]",0.0024375,"[10,8,8,8,10,8,10,8,8,10,8,8,8,8,6,6,8,8,8,8,8,8,10,10,8,8,10,12,10,6,8,8,8,8,0,8,8,10,6,8]"
1311864807136,"triangle","B",0,"C","max",0,"[2]",0.00375,"[10,10,10,10,8,10,10,10,10,10,8,10,10,10,10,12,10,10,10,10,10,10,10,10,12,8,8,10,10,10,10,10,10,10,10,10,10,10,10,10]"
1311904510501,"triangle","B",0,"C","max",0,"[3]",0.004875,"[12,12,12,12,12,12,10,12,12,12,12,12,12,12,10,12,14,12,10,12,12,12,12,10,12,12,12,12,12,12,12,12,10,12,10,12,12,12,12,10]"
1311944824225,"triangle","B",0,"C","max",0,"[4]",0.006374998,"[14,12,14,12,14,12,12,12,12,14,12,12,12,12,14,14,12,14,12,14,12,12,12,14,14,12,14,14,12,14,14,16,14,14,14,14,14,12,14,12]"
1311985290540,"triangle","B",0,"C","max",0,"[5]",0.0075,"[16,14,14,16,16,16,14,14,14,20,14,14,14,14,16,16,14,10,14,14,14,14,14,16,16,14,14,16,14,14,14,16,16,16,14,14,16,14,14,14]"
1312028137218,"triangle","B",0,"C","max",0,"[6]",0.0088125,"[18,16,16,16,16,18,16,16,16,16,16,16,16,16,16,16,16,16,16,18,16,16,16,16,16,16,16,18,20,18,16,16,14,16,16,16,16,22,16,16]"
1312068511965,"triangle","B",0,"C","max",0,"[7]",0.0103125,"[18,18,18,18,18,18,18,20,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,18,20,18,18,18,16,18,18,14,18]"
1312108795171,"triangle","B",0,"C","max",0,"[8]",0.011625,"[24,20,20,20,18,22,20,20,18,20,20,20,20,20,20,20,20,20,20,18,20,20,20,20,20,20,20,18,20,20,20,20,20,20,20,20,20,22,20,20]"

Note the values in the last field are all even. I have same code running on many other boards and have witnessed no other problems. I am still reviewing my results so there's a small possibility this is an application bug authored by me...

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