Atmel ATmega128 User Manual

242
2467S–AVR–07/09
ATmega128
Example:
ADMUX = 0xED (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)
Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.
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.
ADC Multiplexer
Selection Register –
ADMUX
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in Table 97. If these bits are
changed during a conversion, the change will not go in effect until this conversion is complete
(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external
reference voltage is being applied to the AREF pin.
•
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.
Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the
ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing
Table 96. Correlation Between Input Voltage and Output Codes
V
ADCn
Read code Corresponding decimal value
V
ADCm
+ V
REF
/GAIN 0x1FF 511
V
ADCm
+ 511/512 V
REF
/GAIN 0x1FF 511
V
ADCm
+ 511/512 V
REF
/GAIN 0x1FE 510
... ... ...
V
ADCm
+ 1/512 V
REF
/GAIN 0x001 1
V
ADCm
0x000 0
V
ADCm
- 1/512 V
REF
/GAIN 0x3FF -1
... ... ...
V
ADCm
- 511/512 V
REF
/GAIN 0x201 -511
V
ADCm
- V
REF
/GAIN 0x200 -512
Bit 76543210
REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0 ADMUX
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Table 97. Voltage Reference Selections for ADC
REFS1 REFS0 Voltage Reference Selection
0 0 AREF, Internal Vref turned off
0 1 AVCC with external capacitor at AREF pin
10Reserved
1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin

Atmel ATmega128 Specifications

General

Brand	Atmel
Model	ATmega128
Category	Microcontrollers
Language	English