Rainbow Electronics ATmega128RFA1 - Manual (Page 425)

425
8266A-MCU Wireless-12/09
ATmega128RFA1
Table 27-7. Correlation Between Input Voltage and Output Codes
V
ADCn
Read Code Corresponding Decimal Value
V
ADCm
+ V
REF
/ GAIN 0x1FF 511
V
ADCm
+ 0.999 V
REF
/ GAIN 0x1FF 511
V
ADCm
+ 0.998 V
REF
/ GAIN 0x1FE 510
… … …
V
ADCm
+ 0.001 V
REF
/ GAIN 0x001 1
V
ADCm
0x000 0
V
ADCm
- 0.001 V
REF
/ GAIN 0x3FF -1
… … …
V
ADCm
- 0.999 V
REF
/ GAIN 0x201 -511
V
ADCm
- V
REF
/ GAIN 0x200 -512
Example:
ADMUX = 0xED (ADC3 - ADC2, 10x gain, 1.6V reference, left adjusted result)
The voltage on ADC3 is 300 mV; the voltage on ADC2 is 425 mV.
ADCR = 512 * 10 * (300 - 425) / 1600 = -400 = 0x270.
ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right
adjusts the result: ADCL = 0x70, ADCH = 0x02.
27.9 Internal Temperature Measurement
The on-chip temperature can be measured using a special setup of the A/D converter
inputs. The integrated temperature sensor provides a linear, medium-accurate voltage
proportional to the absolute temperature (in Kelvin). This voltage is first amplified with
the programmable gain amplifier and then processed with the A/D converter. A low
frequency of the conversion clock must be selected due to the nature of the input
signal. The absolute accuracy of the temperature measurement is limited by
manufacturing tolerances, noise from supply and ground voltages and the exactness of
the reference voltage. The following table summarizes the preferred setup of the
temperature measurement:
Table 27-8. Recommended ADC Setup for Temperature Measurement
Parameter Register Recommended Setup
ADC Channel ADMUX,
ADCSRB Select the Temperature Sensor, MUX4:0 = 01001;
MUX5 = 1;
ADC Clock ADCSRA Select a clock frequency of 500kHz or lower;
V
REF
ADMUX Select the internal 1.6V reference voltage;
Start-up time ADCSRC Standard requirement of 20µs is sufficient;
Tracking time ADCSRC Setting ADTHT = 0 is sufficient;
The A/D conversion result ADC
TEMP
will always be a positive number. The ideal result
can be calculated when using the internal 1.6V reference voltage according to the
following equation:
C ADC
TEMP
° ⋅ + = / 885 . 0 4 . 241 θ
Similar the Celsius-temperature θ can be extracted from the A/D conversion result with
this formula: