Chapter 19 Pulse-Width Modulator (S12PWM8B8CV1)
MC9S12XE-Family Reference Manual Rev. 1.19
Freescale Semiconductor
713
and the counter value and causes the state of the output to change during the period. The starting polarity
of the output is also selectable on a per channel basis. Shown below in
is the block diagram
for the PWM timer.
Figure 19-19. PWM Timer Channel Block Diagram
19.4.2.1
PWM Enable
Each PWM channel has an enable bit (PWMEx) to start its waveform output. When any of the PWMEx
bits are set (PWMEx = 1), the associated PWM output signal is enabled immediately. However, the actual
PWM waveform is not available on the associated PWM output until its clock source begins its next cycle
due to the synchronization of PWMEx and the clock source. An exception to this is when channels are
concatenated. Refer to
Section 19.4.2.7, “PWM 16-Bit Functions”
for more detail.
NOTE
The first PWM cycle after enabling the channel can be irregular.
On the front end of the PWM timer, the clock is enabled to the PWM circuit by the PWMEx bit being high.
There is an edge-synchronizing circuit to guarantee that the clock will only be enabled or disabled at an
edge. When the channel is disabled (PWMEx = 0), the counter for the channel does not count.
Clock Source
T
R
Q
Q
PPOLx
From Port PWMP
Data Register
PWMEx
To Pin
Driver
Gate
8-bit Compare =
PWMDTYx
8-bit Compare =
PWMPERx
CAEx
T
R
Q
Q
8-Bit Counter
PWMCNTx
M
U
X
M
U
X
(Clock Edge
Sync)
Up/Down
Reset
Because
of
an
order
from
the
United
States
International
Trade
Commission,
BGA-packaged
product
lines
and
part
numbers
indicated
here
currently
are
not
available
from
Freescale
for
import
or
sale
in
the
United
States
prior
to
September
2010:
S12XE
products
in
208
MAPBGA
packages
