Pulse-Width Modulator (S12PWM8B8CV2)
MC9S12VRP Family Reference Manual Rev. 1.3
NXP Semiconductors
285
according to the polarity bit. The counter is also cleared at the end of the effective period (see
Section 9.4.2.5, “Left Aligned Outputs”
Section 9.4.2.6, “Center Aligned Outputs”
for more details).
When the channel is disabled (PWMEx = 0), the PWMCNTx register does not count. When a channel
becomes enabled (PWMEx = 1), the associated PWM counter starts at the count in the PWMCNTx
register. For more detailed information on the operation of the counters, see
.
In concatenated mode, writes to the 16-bit counter by using a 16-bit access or writes to either the low or
high order byte of the counter will reset the 16-bit counter. Reads of the 16-bit counter must be made by
16-bit access to maintain data coherency.
NOTE
Writing to the counter while the channel is enabled can cause an irregular
PWM cycle to occur.
1
This register is available only when the corresponding channel exists and is reserved if that channel does not exist. Writes to
a reserved register have no functional effect. Reads from a reserved register return zeroes.
Read: Anytime
Write: Anytime (any value written causes PWM counter to be reset to $00).
9.3.2.11
PWM Channel Period Registers (PWMPERx)
There is a dedicated period register for each channel. The value in this register determines the period of
the associated PWM channel.
The period registers for each channel are double buffered so that if they change while the channel is
enabled, the change will NOT take effect until one of the following occurs:
•
The effective period ends
•
The counter is written (counter resets to $00)
•
The channel is disabled
In this way, the output of the PWM will always be either the old waveform or the new waveform, not some
variation in between. If the channel is not enabled, then writes to the period register will go directly to the
latches as well as the buffer.
NOTE
Reads of this register return the most recent value written. Reads do not
necessarily return the value of the currently active period due to the double
buffering scheme.
Section 9.4.2.3, “PWM Period and Duty”
for more information.
7
6
5
4
3
2
1
0
R
Bit 7
6
5
4
3
2
1
Bit 0
W
0
0
0
0
0
0
0
0
Reset
0
0
0
0
0
0
0
0
Figure 9-12. PWM Channel Counter Registers (PWMCNTx)
Summary of Contents for MC9S12VRP64
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Page 236: ...S12S Debug Module S12DBGV2 MC9S12VRP Family Reference Manual Rev 1 3 236 NXP Semiconductors ...
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