MEGA128CAN Datasheet, PDF (271/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

AT90CAN128
Changing Channel or
Reference Selection
diately after the previous conversion completes, and since CK_ADC2 is high at this time, all
automatically started (i.e., all but the first) Free Running conversions will take 14 ADC
clock cycles.
If differential channels are used and conversions are started by Auto Triggering, the
ADC must be switched off between conversions. When Auto Triggering is used, the
ADC prescaler is reset before the conversion is started. Since the stage is dependent of
a stable ADC clock prior to the conversion, this conversion will not be valid. By disabling
and then re-enabling the ADC between each conversion (writing ADEN in ADCSRA to
“0” then to “1”), only extended conversions are performed. The result from the extended
conversions will be valid. See “Prescaling and Conversion Timing” on page 268 for tim-
ing details.
The gain stage is optimized for a bandwidth of 4 kHz at all gain settings. Higher frequen-
cies may be subjected to non-linear amplification. An external low-pass filter should be
used if the input signal contains higher frequency components than the gain stage band-
width. Note that the ADC clock frequency is independent of the gain stage bandwidth
limitation. E.g. the ADC clock period may be 6 µs, allowing a channel to be sampled at
12 kSPS, regardless of the bandwidth of this channel.
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a tem-
porary register to which the CPU has random access. This ensures that the channels
and reference selection only takes place at a safe point during the conversion. The
channel and reference selection is continuously updated until a conversion is started.
Once the conversion starts, the channel and reference selection is locked to ensure a
sufficient sampling time for the ADC. Continuous updating resumes in the last ADC
clock cycle before the conversion completes (ADIF in ADCSRA is set). Note that the
conversion starts on the following rising ADC clock edge after ADSC is written. The user
is thus advised not to write new channel or reference selection values to ADMUX until
one ADC clock cycle after ADSC is written.
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 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).
The settling time and gain stage bandwidth is independent of the ADHSM bit setting.
4250E–CAN–12/04
271