Atmel XMEGA B, User Manual

Page: 178 / 415

178 XMEGA B [DATASHEET]
8291B–AVR–01/2013
The peripheral clock (clk
PER
) is fed into the common prescaler (common for all timer/counters in a device). A selection of
prescaler outputs from 1 to 1/1024 is directly available. In addition, the whole range of time prescalings from 1 to 2
15
is
available through the event system.
The clock selection (CLKSEL) selects one of the clock prescaler outputs or an event channel for the high-byte counter
(HCNT) and low-byte counter (LCNT). By using the event system, any event source, such as an external clock signal, on
any I/O pin can be used as the clock input.
By default, no clock input is selected, and the counters are not running.
14.5 Counter Operation
The counters will always count in single-slope mode. Each counter counts down for each clock cycle until it reaches
BOTTOM, and then reloads the counter with the period register value at the following clock cycle.
Figure 14-3. Counter operation.
As shown in Figure 14-3 , the counter can change the counter value while running. The write access has higher priority
than the count clear, and reloads and will be immediate.
14.5.1 Changing the Period
The counter period is changed by writing a new TOP value to the period register. Since the counter is counting down, the
period register can be written at any time without affecting the current period, as shown in
Figure 14-4 on page 178 . This
prevents wraparound and generation of odd waveforms.
Figure 14-4. Changing the period.
14.6 Compare Channel
Each compare channel continuously compares the counter value with the CMPx register. If CNT equals CMPx, the
comparator signals a match. For the low-byte timer/counter, the match will set the compare channel's interrupt flag at the
next timer clock cycle, and the event and optional interrupt is generated. The high-byte timer/counter does not have
compare interrupt/event.
CNT
BOTTOM
MAX
"reload"
TOP
CNT written
CNT
MAX
New TOP written to
PER that is higher
than current CNT
New TOP written to
PER that is lower
than current CNT
"reload"
"write"
BOTTOM