Datasheet

148
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
The voltage of the positive pin must always be larger than the voltage of the negative pin or
otherwise the voltage difference is saturated to zero. The result is presented in one-sided
form, from 0x000 (0d) through 0x3FF (+1023d). The GAIN is either 1x or 20x.
18.8.3 Bipolar Differential Conversion
If differential channels and a bipolar input mode are used, the result is
where V
POS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,
and V
REF the selected voltage reference. The result is presented in two’s complement form,
from 0x200 (-512d) through 0x1FF (+511d). The GAIN is either 1x or 20x. Note that if the user
wants to perform a quick polarity check of the result, 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.
As default the ADC converter operates in the unipolar input mode, but the bipolar input mode
can be selected by writing 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.
18.9 Temperature Measurement
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single-ended ADC8 channel. Selecting the ADC8 channel by writing the MUX5:0 bits in the
ADMUX register to "100010" 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 temperature 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 Table 51. The voltage sensitivity is approxi-
mately 1mV/°C, and the accuracy of the temperature measurement is ±10°C after offset
calibration. Bandgap is always calibrated, and its accuracy is only guaranteed between 1.0V
and 1.2V
The values described in Table 18-2 on page 148 are typical values. However, due to the pro-
cess 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 software. The software calibration requires that a calibration value be mea-
sured and stored in a register or EEPROM for each chip as a part of the production test. The
software calibration can be done utilizing the formula:
T = {[(ADCH << 8) | ADCL] - TOS} / k
where ADCn are the ADC data registers, k is a fixed coefficient and T
OS
is the temperature
sensor offset value determined and stored into EEPROM as a part of the production test.
ADC
V
POS
V
NEG
–()512⋅
V
REF
-------------------------------------------------------
GAIN⋅=
Table 18-2. Temperature vs. Sensor Output Voltage (Typical Case)
Temperature / °C -40°C +25°C +85°C +125°C
Voltage / mV 243mV 314mv 380mV 424mV