![](http://datasheet.mmic.net.cn/90000/MEGA32M1-15AZ_datasheet_3507761/MEGA32M1-15AZ_237.png)
237
7647F–AVR–04/09
ATmega16/32/64/M1/C1
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no
external voltage is applied to the AREF pin, the user may switch between AV
CC and 2.56V as
reference selection. The first ADC conversion result after switching reference voltage source
may be inaccurate, and the user is advised to discard this result.
AREF pin is alternate function with ISRC Current Source output. When current source is
selected, the AREF pin is not connected to the internal reference voltage network. See AREFEN
If differential channels are used, the selected reference should not be closer to AV
CC than indi-
18.6
ADC Noise Canceler
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:
a.
Make sure the ADATE bit is reset.
b.
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.
c.
Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion
once the CPU has been halted.
d.
If no other interrupts occur before the ADC conversion completes, the ADC inter-
rupt 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 com-
plete, 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 be automatically 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.
If the ADC is enabled in such sleep modes and the user wants to perform differential conver-
sions, the user is advised to switch the ADC off and on after waking up from sleep to prompt an
extended conversion to get a valid result.
18.6.1
Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in
Figure 18-8. 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 10 k
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, with can vary widely. The user is recommended to only use low impedant sources
with slowly varying signals, since this minimizes the required charge transfer to the S/H
capacitor.