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ATtiny48/88
selection. Since the next conversion has already started automatically, the next result will reflect
the previous channel selection. Subsequent conversions will reflect the new channel selection.
17.6.2
ADC Voltage Reference
The ADC reference voltage (V
REF) indicates the conversion range for the ADC. Single ended
channels that exceed V
REF will result in codes close to 0x3FF. VREF can be selected as either
AV
CC, or internal 1.1V reference. The internal 1.1V reference is generated from the internal
bandgap reference (V
BG) through an internal amplifier.
The first ADC conversion result after switching reference voltage source may be inaccurate, and
the user is advised to discard this result.
17.7
ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode. This reduces
noise induced from the CPU core and other I/O peripherals. The noise canceler can be used
with ADC Noise Reduction and Idle mode. To make use of this feature, the following procedure
should be used:
Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must
be selected and the ADC conversion complete interrupt must be enabled.
Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the
CPU has been halted.
If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake
up the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt
wakes up the CPU before the ADC conversion is complete, that interrupt will be executed,
and an ADC Conversion Complete interrupt request will be generated when the ADC
conversion completes. The CPU will remain in active mode until a new sleep command is
executed.
Note that the ADC will not automatically be turned off when entering other sleep modes than Idle
mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-
ing such sleep modes to avoid excessive power consumption.
17.8
Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in
Figure 17-8 An analog
source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regard-
less of whether that channel is selected as input for the ADC. When the channel is selected, the
source must drive the S/H capacitor through the series resistance (combined resistance in the
input path).
The ADC is optimized for analog signals with an output impedance of approximately 10k
Ω or
less. If such a source is used, the sampling time will be negligible. If a source with higher imped-
ance is used, the sampling time will depend on how long time the source needs to charge the
S/H capacitor, which can vary widely. The user is recommended to only use low impedance
sources with slowly varying signals, since this minimizes the required charge transfer to the S/H
capacitor.
In order to avoid distortion from unpredictable signal convolution, signal components higher than
the Nyquist frequency (f
ADC/2) should not be present. The user is advised to remove high fre-
quency components with a low-pass filter before applying the signals as inputs to the ADC.