![](http://datasheet.mmic.net.cn/90000/MQ83C154DXXX-12-883R_datasheet_2377128/MQ83C154DXXX-12-883R_155.png)
155
2588F–AVR–06/2013
ATtiny261/461/861
assuming calibration at room temperature. Better accuracies are achieved by using two
temperature points for calibration.
The values described in
Table 15-2 are typical values. However, due to process variation the
temperature sensor output voltage varies from one chip to another. To be capable of achieving
more accurate results the temperature measurement can be calibrated in the application soft-
ware. The sofware calibration can be done using the formula:
T = k * [(ADCH << 8) | ADCL] + T
OS
where ADCH and ADCL are the ADC data registers, k is the fixed slope coefficient and T
OS is
the temperature sensor offset. Typically, k is very close to 1.0 and in single-point calibration the
coefficient may be omitted. Where higher accuracy is required the slope coefficient should be
evaluated based on measurements at two temperatures.
15.13 Register Description
15.13.1
ADMUX – ADC Multiplexer Selection Register
Bits 7:6 – REFS1:REFS0: Voltage Reference Selection Bits
These bits together with the REFS2 bit from the ADC Control and Status Register B (ADCSRB)
select the voltage reference for the ADC, as shown in
Table 15-3.If these bits are changed during a conversion, the change will not go in effect until this conver-
sion is complete (ADIF in ADCSR is set). Also note, that when these bits are changed, the next
conversion will take 25 ADC clock cycles.
Table 15-2.
Temperature vs. Sensor Output Voltage (Typical Case)
Temperature
-40
C+25 C+85 C
ADC
230 LSB
300 LSB
370 LSB
Bit
7
6
543210
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
ADMUX
Read/Write
R/W
Initial Value
0
000000
Table 15-3.
Voltage Reference Selections for ADC
REFS2
REFS1
REFS0
Voltage Reference Selection
X0
0
VCC used as voltage reference,
disconnected from AREF
X0
1
External voltage reference at AREF pin,
internal voltage reference turned off
0
1
0
Internal 1.1V voltage reference
0
1
Reserved
110
Internal 2.56V voltage reference,
without external bypass capacitor,
disconnected from AREF
111
Internal 2.56V voltage reference,
with external bypass capacitor at AREF pin