Pulse-Width Modulator (S12PWM8B8CV1)
MC9S12XHY-Family Reference Manual, Rev. 1.01
Freescale Semiconductor
441
13.4
Functional Description
13.4.1
PWM Clock Select
There are four available clocks: clock A, clock B, clock SA (scaled A), and clock SB (scaled B). These
four clocks are based on the bus clock.
Clock A and B can be software selected to be 1, 1/2, 1/4, 1/8,..., 1/64, 1/128 times the bus clock. Clock SA
uses clock A as an input and divides it further with a reloadable counter. Similarly, clock SB uses clock B
as an input and divides it further with a reloadable counter. The rates available for clock SA are software
selectable to be clock A divided by 2, 4, 6, 8,..., or 512 in increments of divide by 2. Similar rates are
available for clock SB. Each PWM channel has the capability of selecting one of two clocks, either the
pre-scaled clock (clock A or B) or the scaled clock (clock SA or SB).
The block diagram in
Figure 13-18
shows the four different clocks and how the scaled clocks are created.
13.4.1.1
Prescale
The input clock to the PWM prescaler is the bus clock. It can be disabled whenever the part is in freeze
mode by setting the PFRZ bit in the PWMCTL register. If this bit is set, whenever the MCU is in freeze
mode (freeze mode signal active) the input clock to the prescaler is disabled. This is useful for emulation
in order to freeze the PWM. The input clock can also be disabled when all eight PWM channels are
disabled (PWME7-0 = 0). This is useful for reducing power by disabling the prescale counter.
Clock A and clock B are scaled values of the input clock. The value is software selectable for both clock
A and clock B and has options of 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, or 1/128 times the bus clock. The value
selected for clock A is determined by the PCKA2, PCKA1, PCKA0 bits in the PWMPRCLK register. The
value selected for clock B is determined by the PCKB2, PCKB1, PCKB0 bits also in the PWMPRCLK
register.
13.4.1.2
Clock Scale
The scaled A clock uses clock A as an input and divides it further with a user programmable value and
then divides this by 2. The scaled B clock uses clock B as an input and divides it further with a user
programmable value and then divides this by 2. The rates available for clock SA are software selectable to
be clock A divided by 2, 4, 6, 8,..., or 512 in increments of divide by 2. Similar rates are available for clock
SB.
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