Datasheet

149
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
To obtain best accuracy the coefficient k should be measured using two temperature calibra-
tions. Using offset calibration, set k = 1.0, where k = (1024*1.07mV/°C)/1.1V~1.0 [1/°C].
18.10 Register Description
18.10.1 ADMUX – ADC Multiplexer Selection Register
• Bit 7:6 – REFS1:REFS0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in Table 18-3 on page 149. If
these bits are changed during a conversion, the change will not go in effect until this
conversion is complete (ADIF in ADCSR is set).
Special care should be taken when changing differential channels. Once a differential channel
has been selected, the stage may take as much as 25 ADC clock cycles to stabilize to the new
value. Thus conversions should not be started within the first 13 clock cycles after selecting a
new differential channel. Alternatively, conversion results obtained within this period should be
discarded.
The same settling time should be observed for the first differential conversion after changing
ADC reference (by changing the REFS1:0 bits in the ADMUX register).
For channels where differential gain is used (i.e., the gain stage), using VCC or an optional
external AREF higher than (VCC - 1V) is not recommended, as this will affect ADC accuracy.
The internal voltage reference may not be connected to the AREF pin if an external voltage is
already being applied to it. The internal voltage reference is connected to the AREF pin when
REFS1:0 is set to the value "11".
• Bits 5:0 – MUX5:0: Analog Channel and Gain Selection Bits
The value of these bits selects which combination of analog inputs are connected to the ADC.
In the case of differential input, gain selection is also made with these bits. Selections on Table
18-4 on page 150 show values for single-ended channels and where the differential channels
and the offset calibration selections are located. Selecting the single-ended channel ADC8
enables the temperature measurement. See Table 18-4 on page 150 ffor details. If these bits
are changed during a conversion, the change will not go into effect until this conversion is
complete (ADIF in ADCSRA is set).
Bit 76543210
0x07 (0x27) REFS1 REFS0 MUX5 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 Value 0 0 0 0 0 0 0 0
Table 18-3. Voltage Reference Selections for ADC
REFS1 REFS0 Voltage Reference Selection
00V
CC
used as analog reference, disconnected from PA0 (AREF).
01
External Voltage Reference at PA0 (AREF) pin, Internal Voltage Reference
turned off.
1 0 Internal 1.1V Voltage Reference.
1 1 Reserved.