Pulse-Width Modulator for Motor Control (PWMMC)
Control Logic Block
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Pulse-Width Modulator for Motor Control (PWMMC) 169
9.10.2 PWM Counter Modulo Registers
The PWM counter modulus registers (PMODH and PMODL) hold a
12-bit unsigned number that determines the maximum count for the
up/down or up-only counter. In center-aligned mode, the PWM period
will be twice the modulus (assuming no prescaler). In edge-aligned
mode, the PWM period will equal the modulus. See Figure 9-34 and
Figure 9-35.
To avoid erroneous PWM periods, this value is buffered and w ill no t be
used by the PWM generator until the LDOK bit has been set and the next
PWM load cycle begins.
NOTE: When reading this register, the value read is the buffer (not necessarily
the value the PWM generator is currently using).
Because of the equals-comparator architecture of this PWM, the
modulus = 0 case is considered illegal. Therefore, the m od ulus r egiste r
is not reset, and a modulus value of 0 will result in waveforms
inconsistent with the other modulus waveforms. If a modulus of 0 is
loaded, the counter will continually count down from $FFF. This
operation will not be tested or guaranteed (the user should consider it
illegal). However, the dead-time constraints and fault conditions will still
be guaranteed.
Address: $0028
Bit 7654321Bit 0
Read: 0000
Bit 11 Bit 10 Bit 9 Bit 8
Write:
Reset:0000XXXX
= Unimplemented X = Indeterminate
Figure 9-34. PWM Counter Modulo Register High (PMODH)
Address: $0029
Bit 7654321Bit 0
Read: Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Write:
Reset:XXXXXXXX
X = Indeterminate
Figure 9-35. PWM Counter Modulo Register Low (PMODL)