Datasheet
508
32133D–11/2011
UC3D
When writing a one to CR.TCLR, the timebase counter and the spread spectrum counter are
reset at their lower limit values and the effective top value of the timebase counter will also be
reset.
25.6.4 Duty Cycle and Waveform Properties
Each PWM channel has its own duty cycle value (DCV) which is write-only and cannot be read
out. The duty cycle value can be changed in two approaches as described in Section25.6.6.
When the duty cycle value is zero, the PWM output is zero. Otherwise, the PWM output is set
when the timebase counter is zero, and cleared when the timebase counter reaches the duty
cycle value. This is summarized as:
Note that when increasing the duty cycle value for one channel from 0 to 1, the number of GCLK
cycles when the PWM waveform is high will jump from 0 to 2. When incrementing the duty cycle
value by one for any other values, the number of GCLK cycle when the waveform is high will
increase by one. This is summarized in Table 25-2.
25.6.5 Waveform Output
PWMA waveforms are output to I/O lines. The output waveform properties are controlled by
Composite Waveform Generation (CWG) register(s). If this register is cleared (by default), the
channel waveforms are out directly to the I/O lines. To avoid too many I/O toggling simultane-
ously on the output I/O lines, every other output PWM waveform toggles on the negative edge of
the GCLK instead of the positive edge.
In CWG mode, all channels are paired and their outputs are XOR’ed together if the correspond-
ing bit of CWG register is set. The even number of output is the XOR’ed output and the odd
number of output is the inverse of its. Each bit of CWG register controls one pair channels and
the least significant bit refers to the lowest number of pair channels.
25.6.6 Updating Duty Cycle Values
25.6.6.1 Interlinked Single Value PWM Operation
The PWM channels can be interlinked to allow multiple channels to be updated simultaneously
with the same duty cycle value. This value must be written to the Interlinked Single Value Duty
Table 25-2. PMW Waveform Duty Cycles
Duty Cycle Value
#Clock Cycles
When Waveform is High
#Clock Cycles
When Waveform is Low
00 ETV+1
12 ETV-1
23 ETV-2
... ... ...
ETV-1 ETV 1
ETV ETV+1 0
PWM Waveform =
low when DCV 0 or TC DCV>=
high when TC DCV and DCV 0≠≤
⎩
⎨
⎧