Datasheet

439
Atmel | SMART SAM D20 [DATASHEET]
Atmel-42129K–SAM-D20_datasheet–06/2014
z COUNT8: The 8-bit TC has its own Period register (PER). This register is used to store the period value that can
be used as the top value for waveform generation.
z COUNT16: This is the default counter mode. There is no dedicated period register in this mode.
z COUNT32: This mode is achieved by pairing two 16-bit TC peripherals. This pairing is explained in “Clocks” on
page 436. The even-numbered TC instance will act as master to the odd-numbered TC peripheral, which will act
as a slave. The slave status of the slave is indicated by reading the Slave bit in the Status register
(STATUS.SLAVE). The registers of the slave will not reflect the registers of the 32-bit counter. Writing to any of the
slave registers will not affect the 32-bit counter. Normal access to the slave COUNT and CCx registers is not
allowed.
27.6.2.5 Counter Operations
The counter can be set to count up or down. When the counter is counting up and the top value is reached, the counter
will wrap around to zero on the next clock cycle. When counting down, the counter will wrap around to the top value when
zero is reached. In one-shot mode, the counter will stop counting after a wraparound occurs.
To set the counter to count down, write a one to the Direction bit in the Control B Set register (CTRLBSET.DIR). To count
up, write a one to the Direction bit in the Control B Clear register (CTRLBCLR.DIR).
Each time the counter reaches the top value or zero, it will set the Overflow Interrupt flag in the Interrupt Flag Status and
Clear register (INTFLAG.OVF). It is also possible to generate an event on overflow or underflow when the
Overflow/Underflow Event Output Enable bit in the Event Control register (EVCTRL.OVFEO) is one.
The counter value can be read from the Counter Value register (COUNT) or a new value can be written to the COUNT
register. Figure 27-3 gives an example of writing a new counter value. The COUNT value will always be zero when
starting the TC, unless some other value has been written to it or if the TC has been previously reloaded at TOP value,
because stopped while TC was counting down.
Figure 27-3. Counter Operation
Stop Command
On the stop command, which can be evoked in the Command bit group in the Control B Set register (CTRLBSET.CMD),
the counter will retain its current value. All waveforms are cleared. The counter stops counting, and the Stop bit in the
Status register is set (STATUS.STOP).
Retrigger Command and Event Action
Retriggering can be evoked either as a software command, using the Retrigger command in the Control B Set register
(CTRLBSET.CMD), or as a retrigger event action, using the Event Action bit group in the Event Control register
(EVCTRL.EVACT).
When a retrigger is evoked while the counter is running, the counter will wrap to the top value or zero, depending on the
counter direction..
DIR
COUNT
TOP
COUNTwrittenDirection Change
Period (T)
BOT
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