Atmel XMEGA B, User Manual

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157 XMEGA B [DATASHEET]
8291B–AVR–01/2013
A counter wraparound can occur in any mode of operation when up-counting without buffering, as shown in Figure 13-8 .
This due to the fact that CNT and PER are continuously compared, and if a new TOP value that is lower than current
CNT is written to PER, it will wrap before a compare match happen.
Figure 13-8. Unbuffered dual-slope operation.
When double buffering is used, the buffer can be written at any time and still maintain correct operation. The period
register is always updated on the UPDATE condition, as shown for dual-slope operation in
Figure 13-9 . This prevents
wraparound and the generation of odd waveforms.
Figure 13-9. Changing the period using buffering.
13.7 Capture Channel
The CC channels can be used as capture channels to capture external events and give them a timestamp. To use
capture, the counter must be set for normal operation.
Events are used to trigger the capture; i.e., any events from the event system, including pin change from any pin, can
trigger a capture operation. The event source select setting selects which event channel will trigger CC channel A. The
subsequent event channels then trigger events on subsequent CC channels, if configured. For example, setting the
event source select to event channel 2 results in CC channel A being triggered by event channel 2, CC channel B
triggered by event channel 3, and so on.
CNT
MAX
New TOP written to
PER that is higher
than current CNT
New TOP written to
PER that is lower
than current CNT
"update"
"write"
Counter Wraparound
BOTTOM
CNT
MAX
New Period written to
PERBUF that is higher
than current CNT
New Period written to
PERBUF that is lower
than current CNT
"update"
"write"
New PER is updated
with PERBUF value.
BOTTOM