NXP Semiconductors freescale semiconductor MC13211, Справочное руководство

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MCU Timer/PWM (TPM Module)
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor 17-7
but when CPWMS = 1 the counter needs a valid match to the modulus register somewhere other than at
$0000 in order to change directions from up-counting to down-counting.
Figure 17-3 shows the output compare value in the TPM channel registers (multiplied by 2), which
determines the pulse width (duty cycle) of the CPWM signal. If ELSnA = 0, the compare match while
counting up forces the CPWM output signal low and a compare match while counting down forces the
output high. The counter counts up until it reaches the modulo setting in TPM1MODH:TPM1MODL, then
counts down until it reaches zero. This sets the period equal to two times TPM1MODH:TPM1MODL.
Figure 17-3. CPWM Period and Pulse Width (ELSnA = 0)
Center-aligned PWM outputs typically produce less noise than edge-aligned PWMs because fewer I/O pin
transitions are lined up at the same system clock edge. This type of PWM is also required for some types
of motor drives.
Because the HCS08 is a family of 8-bit MCUs, the settings in the timer channel registers are buffered to
ensure coherent 16-bit updates and to avoid unexpected PWM pulse widths. Writes to any of the registers,
TPM1MODH, TPM1MODL, TPM1CnVH, and TPM1CnVL, actually write to buffer registers. Values are
transferred to the corresponding timer channel registers only after both 8-bit bytes of a 16-bit register have
been written and the timer counter overflows (reverses direction from up-counting to down-counting at the
end of the terminal count in the modulus register). This TPM1CNT overflow requirement only applies to
PWM channels, not output compares.
Optionally, when TPM1CNTH:TPM1CNTL = TPM1MODH:TPM1MODL, the TPM can generate a TOF
interrupt at the end of this count. The user can choose to reload any number of the PWM buffers, and they
will all update simultaneously at the start of a new period.
Writing to TPM1SC cancels any values written to TPM1MODH and/or TPM1MODL and resets the
coherency mechanism for the modulo registers. Writing to TPM1CnSC cancels any values written to the
channel value registers and resets the coherency mechanism for TPM1CnVH:TPM1CnVL.
17.5 TPM Interrupts
The TPM generates an optional interrupt for the main counter overflow and an interrupt for each channel.
The meaning of channel interrupts depends on the mode of operation for each channel. If the channel is
configured for input capture, the interrupt flag is set each time the selected input capture edge is
recognized. If the channel is configured for Output Compare or PWM Modes, the interrupt flag is set each
time the main timer counter matches the value in the 16-bit channel value register. See the
Chapter 12,
PERIOD
PULSE WIDTH
COUNT =
COUNT = 0
OUTPUT
COMPARE
(COUNT UP)
OUTPUT
COMPARE
(COUNT DOWN)
COUNT =
TPM1MODH:TPM
TPM1C
TPM1MODH:TPM
2 x
2 x