13 register description, 1 admux – adc multiplexer selection register, Attiny43u – Rainbow Electronics ATtiny43U User Manual
Page 126
126
8048B–AVR–03/09
ATtiny43U
perature sensor is enabled, the ADC converter can be used in single conversion mode to
measure the voltage over the temperature sensor.
The measured voltage has a linear relationship to the temperature as described in
The sensitivity is approximately 1 LSB /
°
C and the accuracy depends on the method of user cal-
ibration. Typically, the measurement accuracy after a single temperature calibration is ±
10
°
C,
assuming calibration at room temperature. Better accuracies are achieved by using two
temperature points for calibration.
The values described in
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.
16.13 Register Description
16.13.1
ADMUX – ADC Multiplexer Selection Register
• Bit 7 – Res: Reserved Bit
This bit is reserved and must always be written zero.
• Bit 6 – REFS: Reference Selection Bit
This bit selects the voltage reference for the ADC, as shown in
. If this bit
is changed during a conversion, the change will not go in effect until this conversion is complete
(ADIF in ADCSR is set).
• Bits 5:3 – Res: Reserved Bits
These bits are reserved and will always read zero.
Table 16-2.
Temperature vs. Sensor Output Voltage (Typical Case)
Temperature
-40
°
C
+25
°
C
+85
°
C
ADC
230 LSB
300 LSB
370 LSB
Bit
7
6
5
4
3
2
1
0
–
REFS
–
–
–
MUX2
MUX1
MUX0
ADMUX
Read/Write
R/W
R/W
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Table 16-3.
Voltage Reference Selections for ADC
REFS
Voltage Reference Selection
0
V
CC
used as analog reference.
1
Internal 1.1V Voltage Reference.