Datasheet

Voltage on ADC3 is 300mV, voltage on ADC2 is 500mV.
ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270
ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts the result:
ADCL = 0x70, ADCH = 0x02.
28.8. Register Description
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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