Rainbow Electronics ATmega128L - Manual (Page 227)

227
ATmega128(L)
2467B–09/01
In Single Conversion mode, always select the channel before starting the conversion.
The channel selection may be changed one ADC clock cycle after writing one to ADSC.
However, the simplest method is to wait for the conversion to complete before changing
the channel selection.
In Free Running mode, always select the channel before starting the first conversion.
The channel selection may be changed one ADC clock cycle after writing one to ADSC.
However, the simplest method is to wait for the first conversion to complete, and then
change the channel selection. Since the next conversion has already started automati-
cally, the next result will reflect the previous channel selection. Subsequent conversions
will reflect the new channel selection.
When switching to a differential gain channel, the first conversion result may have a
poor accuracy due to the required settling time for the automatic offset cancellation cir-
cuitry. The user should preferably disregard the first conversion result.
ADC Voltage Reference The reference voltage for the ADC (V
REF
) indicates the conversion range for the ADC.
Single ended channels that exceed V
REF
will result in codes close to 0x3FF. V
REF
can be
selected as either AV
CC
, internal 2.56V reference, or external AREF pin.
AV
CC
is connected to the ADC through a passive switch. The internal 2.56V reference is
generated from the internal bandgap reference (V
BG
) through an internal amplifier. In
either case, the external AREF pin is directly connected to the ADC, and the reference
voltage can be made more immune to noise by connecting a capacitor between the
AREF pin and ground. V
REF
can also be measured at the AREF pin with a high impedant
voltmeter. Note that V
REF
is a high impedant source, and only a capacitive load should
be connected in a system.
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 2.56 V as reference selection. The first ADC conversion result after
switching reference voltage source may be inaccurate, and the user is advised to dis-
card this result.
If differential channels are used, the selected reference should not be closer to AV
CC
than indicated in Table 137 on page 316.
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:
1. Make sure that the ADC is enabled and is not busy converting. Single Con-
version Mode must be selected and the ADC conversion complete interrupt
must be enabled.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a con-
version once the CPU has been halted.
3. 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 con-
version 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.