Operation - Atmel ATtiny25 Manual

18.2

Operation

ATtiny25/45/85
112
Figure 18-1. Analog to Digital Converter Block Schematic
8-BIT DATA BUS
ADC CTRL. & STATUS B
REGISTER (ADCSRB)
V
CC
AREF
INTERNAL 1.1V/2.56V
REFERENCE
TEMPERATURE
SENSOR
ADC4
ADC3
INPUT
ADC2
MUX
ADC1
ADC0
NEG.
INPUT
MUX
The ADC converts an analog input voltage to a 10-bit digital value through successive approxi-
mation. The minimum value represents GND and the maximum value represents the voltage on
V , the voltage on the AREF pin or an internal 1.1V / 2.56V voltage reference.
CC
The voltage reference for the ADC may be selected by writing to the REFS2..0 bits in ADMUX.
The VCC supply, the AREF pin or an internal 1.1V / 2.56V voltage reference may be selected as
the ADC voltage reference. Optionally the internal 1.1V / 2.56V voltage reference may be decou-
pled by an external capacitor at the AREF pin to improve noise immunity.
The analog input channel and differential gain are selected by writing to the MUX3..0 bits in
ADMUX. Any of the four ADC input pins ADC3..0 can be selected as single ended inputs to the
ADC. ADC2 or ADC0 can be selected as positive input and ADC0, ADC1, ADC2 or ADC3 can
be selected as negative input to the differential gain amplifier.
If differential channels are selected, the differential gain stage amplifies the voltage difference
between the selected input pair by the selected gain factor, 1x or 20x, according to the setting of
the MUX3..0 bits in ADMUX. This amplified value then becomes the analog input to the ADC. If
single ended channels are used, the gain amplifier is bypassed altogether.
ADC CONVERSION
COMPLETE IRQ
ADC MULTIPLEXER ADC CTRL. & STATUS A
SELECT (ADMUX) REGISTER (ADCSRA)
MUX DECODER
10-BIT DAC
SINGLE ENDED / DIFFERENTIAL SELECTION
GAIN
AMPLIFIER
+
-
INTERRUPT
FLAGS
ADTS[2:0]
15
ADC DATA REGISTER
(ADCH/ADCL)
TRIGGER
SELECT
PRESCALER
CONVERSION LOGIC
SAMPLE & HOLD
COMPARATOR
-
+
ADC MULTIPLEXER
OUTPUT
0
7598H–AVR–07/09