Rainbow Electronics ATtiny861_V - Manual (Page 143)

143
2588B–AVR–11/06
ATtiny261/461/861
Figure 19-1. Analog to Digital Converter Block Schematic
19.3 Operation
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
CC
, the voltage on the AREF pin or an internal 1.1V / 2.56V voltage reference.
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 MUX5..0 bits in
ADMUX. Any of the 11 ADC input pins ADC10..0 can be selected as single ended inputs to the
ADC. The positive and negative inputs to the differential gain amplifier are described in
Table
19-4 .
If differential channels are selected, the differential gain stage amplifies the voltage difference
between the selected input pair by the selected gain factor, 1x, 8x, 20x or 32x, according to the
setting of the MUX5..0 bits in ADMUX and the GSEL bit in ADCSRB. 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
8-BIT DATA BUS
15 0
ADC MULTIPLEXER
SELECT (ADMUX)
ADC CTRL. & STATUS
REGISTER A (ADCSRA)
ADC DATA REGISTER
(ADCH/ADCL)
MUX2
ADIE
ADATE ADSC ADEN ADIF
ADIF
MUX1 MUX0
ADPS0 ADPS1 ADPS2
MUX3
CONVERSION LOGIC
10-BIT DAC
+
-
SAMPLE & HOLD
COMPARATOR
MUX DECODER
MUX4
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
REFS0 REFS1 ADLAR
+
-
CHANNEL SELECTION
GAIN SELECTION
ADC[9:0]
ADC
MULTIPLEXER OUTPUT
GAIN
AMPLIFIER
INTERNAL 1.18V
REFERENCE
PRESCALER
SINGLE ENDED /
DIFFERENTIAL SELECTION
AGND
POS.
INPUT
MUX
NEG.
INPUT
MUX
ADC10
ADC9
ADC8
VCC
AREF
TEMPERATURE
SENSOR
INTERNAL 2.56/1.1V
REFERENCE
MUX
ADC CTRL. & STATUS
REGISTER B (ADCSRB)
REFS2 GSEL MUX5