Datasheet

113
8024A–AVR–04/08
ATmega8HVA/16HVA
Power-off mode. Note that the bandgap voltage reference must be enabled and disabled sepa-
rately, see ”BGCCR – Bandgap Calibration C Register” on page 118.
Figure 20-2. Voltage ADC Conversion Diagram
To perform a V-ADC conversion, the analog input channel must first be selected by writing to the
VADMUX register. When a logical one is written to the V-ADC Start Conversion bit VADSC, a
conversion of the selected channel will start. The VADSC bit stays high as long as the conver-
sion is in progress and will be cleared by hardware when the conversion is completed. When a
conversion is in progress, the V-ADC Data Register - VADCL and VADCH will be invalid. If the
System Clock Prescaler setting is changed during a V-ADC conversion, the conversion will be
aborted. If a different data channel is selected while a conversion is in progress, the ADC will fin-
ish the current conversion before performing the channel change. When a conversion is finished
the V-ADC Conversion Complete Interrupt Flag – VADCCIF is set. One 12-bit conversion takes
519 µs to complete from the start bit is set to the interrupt flag is set. The V-ADC Data Register -
VADCL and VADCH will be valid until a new conversion is started. To ensure that correct data is
read, both high and low byte data registers should be read before starting a new conversion.
20.3.1 Configuring PA1 and PA0 for V-ADC operation
When one of the single ended channels ADC0 or ADC1 is used as analog input to the VADC,
either PA0 or PA1 are used as signal ground (SGND). When ADC0/1 is selected as input chan-
nel, PA1/0 is automatically switched to SGND.
The use of PA1 and PA0 as SGND is efficient for the thermistor configuration shown in ”Operat-
ing Circuit” on page 162. Both termistors, RT1 and RT2, are connected through a common
divider resistor, R1, to PA0 and PA1 respectively.
Both PA0 and PA1 have very high input impedance when used as ADC inputs, which makes it
possible to connect two thermistors in the configuration, shown in ”Operating Circuit” on page
162. However, input impedance is limited and if high accuracy is required, only one thermistor
should be connected between PA0 and PA1. If two termistors are connected, the configuration
is as follows:
• When measuring RT1, PA1 should be used as input channel and PA0 is automatically switched
to SGND.
• When measuring RT2, PA0 should be used as input channel and PA1 is automatically switched
to SGND.
Start Conversion
Interrupt
Conversion Result
INVALID DATA
INVALID DATA
OLD DATA
VALID DATA