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Input Multiplexer
12-4
The positive input channel and the negative input channel are selected in the
ADC Multiplexer register (ADMUX, SFR D7h). The high four bits of ADMUX
(bits 4 through 7) select the positive channel, while the low four bits (bits 0
through 3) select the negative channel. The ADMUX SFR has the following
definition:
7 6 5 4 3 2 1 0 Reset Value
SFR D7
H
INP3 INP2 INP1 INP0 INN3 INN2 INN1 INN0 01
H
INP3-0 (bits 7-4)—Input Multiplexer Positive Channel. This bit selects the
positive signal input.
INP3 INP2 INP1 INP0 Positive Input
0 0 0 0 AIN0 (default)
0 0 0 1 AIN1
0 0 1 0 AIN2
0 0 1 1 AIN3
0 1 0 0 AIN4
0 1 0 1 AIN5
0 1 1 0 AIN6
0 1 1 1 AIN7
1 0 0 0 AINCOM
1 1 1 1 Temperature Sensor (requires ADMUX = FF
H
)
INN3-0 (bits 3-0)—Input Multiplexer Negative Channel. This bit selects the
negative signal input.
INN3 INN2 INN1 INN0 Negative Input
0 0 0 0 AIN0
0 0 0 1 AIN1 (default)
0 0 1 0 AIN2
0 0 1 1 AIN3
0 1 0 0 AIN4
0 1 0 1 AIN5
0 1 1 0 AIN6
0 1 1 1 AIN7
1 0 0 0 AINCOM
1 1 1 1 Temperature Sensor (requires ADMUX = FF
H
)
Therefore, to select AIN1 as the positive channel and AIN6 as the negative
channel, the following assignment would be made to the ADMUX register:
ADMUX = 0x16h; // 0001=AIN1, 0110=AIN6
By default, ADMUX defaults to 01
H
at power up, so AIN0 is the default positive
input and AIN1 is the default negative input.