attiny25v ATMEL Corporation, attiny25v Datasheet - Page 137

• Manufacturer Part Number: attiny25v
• Description: Microcontroller With 2/4/8k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

17.11.2
17.11.3
17.12 Temperature Measurement
2586K–AVR–01/08
Unipolar Differential Conversion
Bipolar Differential Conversion
If differential channels and an unipolar input mode are used, the result is
where V
and V
139). The voltage on the positive pin must always be larger than the voltage on the negative pin
or otherwise the voltage difference is saturated to zero. The result is presented in one-sided
form, from 0x000 (0d) to 0x3FF (+1023d). The GAIN is either 1x or 20x.
As default the ADC converter operates in the unipolar input mode, but the bipolar input mode
can be selected by writting the BIN bit in the ADCSRB to one. In the bipolar input mode two-
sided voltage differences are allowed and thus the voltage on the negative input pin can also be
larger than the voltage on the positive input pin. If differential channels and a bipolar input mode
are used, the result is
where V
and V
0x200 (-512d) through 0x000 (+0d) to 0x1FF (+511d). The GAIN is either 1x or 20x.
However, if the signal is not bipolar by nature (9 bits + sign as the 10th bit), this scheme loses
one bit of the converter dynamic range. Then, if the user wants to perform the conversion with
the maximum dynamic range, the user can perform a quick polarity check of the result and use
the unipolar differential conversion with selectable differential input pairs (see the Input Polarity
Reversal mode ie. the IPR bit in the ADCSRB register on page 135). When the polarity check is
performed, it is sufficient to read the MSB of the result (ADC9 in ADCH). If the bit is one, the
result is negative, and if this bit is zero, the result is positive.
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single ended ADC4 channel. Selecting the ADC4 channel by writing the MUX3..0 bits in ADMUX
register to “1111” enables the temperature sensor. The internal 1.1V reference must also be
selected for the ADC reference source in the temperature sensor measurement. When the tem-
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 /
ibration. Typically, the measurement accuracy after a single temperature calibration is ±
REF
REF
POS
POS
the selected voltage reference. The result is presented in two’s complement form, from
the selected voltage reference (see
is the voltage on the positive input pin, V
is the voltage on the positive input pin, V
ADC
ADC
=
=
(
------------------------------------------------------- - GAIN
(
---------------------------------------------------- - GAIN
V
V
POS
POS
°
C and the accuracy depends on the method of user cal-
–
–
V
V
V
V
REF
Table 17-3 on page 138
REF
NEG
NEG
) 1024
) 512
⋅
⋅
NEG
NEG
the voltage on the negative input pin,
⋅
the voltage on the negative input pin,
⋅
ATtiny25/45/85
and
Table 17-4 on page
Table 17-2
10°
137
C,