Rainbow Electronics ATmega32L - Manual (Page 206)

206
ATmega32(L)
2503C–AVR–10/02
If Auto Triggering is used, the exact time of the triggering event can be indeterministic.
Special care must be taken when updating the ADMUX Register, in order to control
which conversion will be affected by the new settings.
If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If
the ADMUX register is changed in this period, the user cannot tell if the next conversion
is based on the old or the new settings. ADMUX can be safely updated in the following
ways:
1. When ADATE or ADEN is cleared.
2. During conversion, minimum one ADC clock cycle after the trigger event.
3. After a conversion, before the interrupt flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next
ADC conversion.
Special care should be taken when changing differential channels. Once a differential
channel has been selected, the gain stage may take as much as 125 µs to stabilize to
the new value. Thus conversions should not be started within the first 125 µs after
selecting a new differential channel. Alternatively, conversion results obtained within this
period should be discarded.
The same settling time should be observed for the first differential conversion after
changing ADC reference (by changing the REFS1:0 bits in ADMUX).
ADC Input Channels When changing channel selections, the user should observe the following guidelines to
ensure that the correct channel is selected:
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 AVCC, internal 2.56V reference, or external AREF pin.
AVCC 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