Atmel ATtiny25, Manual

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123
7598H–AVR–07/09
ATtiny25/45/85
The values described in Table 51 are typical values. However, due to the 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 software calibration requires that a calibration value is measured and stored in a
register or EEPROM for each chip, as a part of the production test. The sofware calibration can
be done utilizing the formula:
where
V
TEMP
is the ADC reading of the temperature sensor signal,
k
is a fixed coefficient and
T
OS
is the temperature sensor offset value determined and stored into EEPROM as a part of produc-
tion test.
18.7.5 ADC Multiplexer Selection Register – ADMUX
• Bit 7..6,4 – REFS2..REFS0: Voltage Reference Selection Bits
These bits select the voltage reference (V
REF
) for the ADC, as shown in Table 18-3 . 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). Whenever these bits are changed, the next conversion will
take 25 ADC clock cycles. If active channels are used, using V
CC
or an external AREF higher
than (V
CC
- 1V) as a voltage reference is not recommended, as this will affect the ADC accuracy.
• Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.
Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the
ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-
sions. For a comple te description of this bit, see
“The ADC Data Register – ADCL and ADCH”
on page 126 .
Temperature k V
TEMP
T
OS
+
⋅
=
Bit 7 6 5 4 3 2 1 0
REFS1 REFS0 ADLAR REFS2 MUX3 MUX2 MUX1 MUX0 ADMUX
Read/Write R R/W R/W R R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 18-3. Voltage Reference Selections for ADC
REFS2 REFS1 REFS0 Voltage Reference (V
REF
) Selection
0 0 0 V
CC
used as Voltage Reference, disconnected from PB0 (AREF).
0 0 1
External Voltage Reference at PB0 (AREF) pin, Internal Voltage
Reference turned off.
0 1 0
Internal 1.1V Voltage Reference without external bypass capacitor,
disconnected from PB0 (AREF).
0 1 1
Internal 1.1V Voltage Reference with external bypass capacitor at
PB0 (AREF) pin.
1 1 0
Internal 2.56V Voltage Reference without external bypass
capacitor, disconnected from PB0 (AREF).
(1)
1. The device requires a supply voltage of 3V in order to generate 2.56V reference voltage.
1 1 1
Internal 2.56V Voltage Reference with external bypass capacitor at
PB0 (AREF) pin.
(1)