ATA5771 ATMEL Corporation, ATA5771 Datasheet - Page 191

• Manufacturer Part Number: ATA5771
• Manufacturer: ATMEL Corporation

16.6.2
16.7
16.8
8006G–AVR–01/08
ADC Noise Canceler
Analog Input Circuitry
ADC Voltage Reference
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.
The reference voltage for the ADC (V
ended channels that exceed V
either V
after switching reference voltage source may be inaccurate, and the user is advised to discard
this result.
The ADC features a noise canceler that enables conversion during sleep mode to reduce 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:
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.
The analog input circuitry for single ended channels is illustrated in
analog source applied to ADCn is subjected to the pin capacitance and input leakage of that pin,
regardless 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 resis-
tance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10kΩ 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. With slowly varying signals the user is recommended to
use sources with low impedance, only, since this minimizes the required charge transfer to the
S/H capacitor.
Signal components higher than the Nyquist frequency (f
distortion from unpredictable signal convolution. The user is advised to remove high frequency
components with a low-pass filter before applying the signals as inputs to the ADC.
• Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must
• Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the
• If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake
be selected and the ADC conversion complete interrupt must be enabled.
CPU has been halted.
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.
CC
, or internal 1.1V reference, or external AREF pin. The first ADC conversion result
REF
will result in codes close to 0x3FF. V
REF
) indicates the conversion range for the ADC. Single
ADC
/2) should not be present to avoid
Figure 16-8 on page
ATtiny24/44/84
REF
can be selected as
140. An
139