Chapter 19 Pulse-Width Modulator (S12PWM8B8CV1)
MC9S12XE-Family Reference Manual Rev. 1.19
Freescale Semiconductor
715
channel becomes enabled (PWMEx = 1), the associated PWM counter continues from the count in the
PWMCNTx register. This allows the waveform to continue where it left off when the channel is re-
enabled. When the channel is disabled, writing “0” to the period register will cause the counter to reset on
the next selected clock.
NOTE
If the user wants to start a new “clean” PWM waveform without any
“history” from the old waveform, the user must write to channel counter
(PWMCNTx) prior to enabling the PWM channel (PWMEx = 1).
Generally, writes to the counter are done prior to enabling a channel in order to start from a known state.
However, writing a counter can also be done while the PWM channel is enabled (counting). The effect is
similar to writing the counter when the channel is disabled, except that the new period is started
immediately with the output set according to the polarity bit.
NOTE
Writing to the counter while the channel is enabled can cause an irregular
PWM cycle to occur.
The counter is cleared at the end of the effective period (see
Section 19.4.2.5, “Left Aligned Outputs”
and
Section 19.4.2.6, “Center Aligned Outputs”
for more details).
19.4.2.5
Left Aligned Outputs
The PWM timer provides the choice of two types of outputs, left aligned or center aligned. They are
selected with the CAEx bits in the PWMCAE register. If the CAEx bit is cleared (CAEx = 0), the
corresponding PWM output will be left aligned.
In left aligned output mode, the 8-bit counter is configured as an up counter only. It compares to two
registers, a duty register and a period register as shown in the block diagram in
PWM counter matches the duty register the output flip-flop changes state causing the PWM waveform to
also change state. A match between the PWM counter and the period register resets the counter and the
output flip-flop, as shown in
, as well as performing a load from the double buffer period and
duty register to the associated registers, as described in
Section 19.4.2.3, “PWM Period and Duty”
. The
counter counts from 0 to the value in the period register – 1.
NOTE
Changing the PWM output mode from left aligned to center aligned output
(or vice versa) while channels are operating can cause irregularities in the
PWM output. It is recommended to program the output mode before
enabling the PWM channel.
Table 19-4. PWM Timer Counter Conditions
Counter Clears ($00)
Counter Counts
Counter Stops
When PWMCNTx register written to
any value
When PWM channel is enabled
(PWMEx = 1). Counts from last value in
PWMCNTx.
When PWM channel is disabled
(PWMEx = 0)
Effective period ends
Because
of
an
order
from
the
United
States
International
Trade
Commission,
BGA-packaged
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