Rainbow Electronics ATmega8HVD - Manual (Page 96)

96
8052B–AVR–09/08
ATmega4HVD/8HVD
17.7 ADC Conversion Result
After the conversion is complete (ADIF is high), the conversion result can be found in the ADC
Result Registers (ADCL, ADCH).
For single ended conversion, the result is
where V
IN
is the voltage on input pin and V
REF
the voltage reference. 0x000 represents analog
ground, and 0x3FF represents 5 times the reference voltage minus one LSB.
17.7.1 Temperature Measurement using the internal temperature sensor – TBD
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single ended ADC channel. Selecting the ADC Temperature channel by writing the ADTEMP
bit in ADCSRA register enables the temperature sensor. 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 tempera-
ture as described in
Table 17-2 on page 96 . The voltage sensitivity is approximately 1 mV / °C
and the accuracy of the temperature measurement is +/- 10°C.
The values described in Table 17-2 on page 96 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 is measured
and stored in a register or EEPROM for each chip, as a part of the production test. The sof-
ware calibration can be done utilizing the formula:
where V
TEMP
is the ADC reading of the temperature sensor signal, k is a fixed coefficient (TBD,
1.07 mV/°C) and T
OS
is the temperature sensor offset value determined and stored into
EEPROM as a part of production test.
ADC
V
IN
1024
⋅
5 V
REF
⋅
-------------------------- =
Table 17-2. Temperature vs. Sensor Output Voltage (Typical values)
Temperature (°C) -45 °C +27 °C +105 °C
Voltage (mV) 225 mV 297 mV 327 mV
Temperature k V
TEMP
T
OS
+
⋅
=