Chapter 19 Pulse-Width Modulator (S12PWM8B8CV1)
MC9S12XE-Family Reference Manual , Rev. 1.19
706
Freescale Semiconductor
NOTE
Writing to the counter while the channel is enabled can cause an irregular PWM cycle to occur.
Read: Anytime
Write: Anytime (any value written causes PWM counter to be reset to $00).
19.3.2.13 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.
See
Section 19.4.2.3, “PWM Period and Duty”
for more information.
To calculate the output period, take the selected clock source period for the channel of interest (A, B, SA,
or SB) and multiply it by the value in the period register for that channel:
•
Left aligned output (CAEx = 0)
•
PWMx Period = Channel Clock Period * PWMPERx Center Aligned Output (CAEx = 1)
PWMx Period = Channel Clock Period * (2 * PWMPERx)
For boundary case programming values, please refer to
Section 19.4.2.8, “PWM Boundary Cases”
.
Module Base + 0x000C = PWMCNT0, 0x000D = PWMCNT1, 0x000E = PWMCNT2, 0x000F = PWMCNT3
Module Base + 0x0010 = PWMCNT4, 0x0011 = PWMCNT5, 0x0012 = PWMCNT6, 0x0013 = PWMCNT7
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 19-14. PWM Channel Counter Registers (PWMCNTx)
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
