NXP Semiconductors MPC5644A, Reference Manual, Page: 674 / 1735

Configurable Enhanced Modular IO Subsystem (eMIOS200)
MPC5644A Microcontroller Reference Manual, Rev. 6
674 Freescale Semiconductor
Figure 22-29. MCB Mode A1 Register Update in Up/Down Counter Mode
22.5.1.1.8 Output pulse width and frequency modulation buffered (OPWFMB) mode
This mode (MODE[0:6] = 10110b0) provides waveforms with variable duty cycle and frequency. The
internal channel counter is automatically selected as the time base when this mode is selected. The A1
register indicates the duty cycle and B1 register the frequency. Both A1 and B1 registers are double
buffered to allow smooth signal generation when changing the registers values on the fly. 0% and 100%
duty cycles are supported.
At OPWFMB mode entry the output flip-flop is set to the value of the EDPOL bit in the EMIOS_CCR[n].
In order to provide smooth and consistent channel operation this mode differs substantially from the
OPWFM mode. The main differences reside in the A1 and B1 registers update, on the delay from the A1
match to the output pin transition and on the range of the internal counter values which starts from 0x1 up
to B1 register value.
If when entering OPWFMB mode exiting from GPIO mode the internal counter value is not within that
range then the B match will not occur causing the channel internal counter to wrap at the maximum counter
value which is 0xFF_FFFF for a 24-bit counter. After the counter wrap occurs it returns to 0x1 and resume
normal OPWFMB mode operation. Thus in order to avoid the counter wrap condition make sure its value
is within the 0x1 to B1 register value range when the OPWFMB mode is entered.
When a match on comparator A occurs the output register is set to the value of EDPOL. When a match on
comparator B occurs the output register is set to the complement of EDPOL. B1 match also causes the
internal counter to transition to 0x1, thus restarting the counter cycle.
Only values greater than 0x1 are allowed to be written to B1 register. Loading values other than those leads
to unpredictable results.
Figure 22-30 describes the operation of the OPWFMB mode regarding output pin transitions and A1/B1
registers match events. Note that the output pin transition occurs when the A1 or B1 match signal is
deasserted which is indicated by the A1 match negedge detection signal. If register A1 is set to 0x4 the
output pin transitions four counter periods after the cycle had started, plus one system clock cycle. Note
that in the example shown in
Figure 22-30 the internal counter prescaler has a ratio of two.
A1 value0x000006
A2 value0x000006 0x000005
0x000006
0x000005
A1 load signal
Counter = 2
EMIOSCNT[n]
TIME
write to A2
match A1
match A1
write to A2
0x000001
0x000005
0x000006
0x000006
cycle n
cycle n+1
cycle n+2
Prescaler ratio = 2