Synchronous Channels

Some channels can be linked together as synchronous channels. They have the same source clock, the same period, the same alignment and are started together. In this way, their counters are synchronized together.

The synchronous channels are defined by the SYNCx bits in the PWM Sync Channels Mode Register (PWM_SCM). Only one group of synchronous channels is allowed.

When a channel is defined as a synchronous channel, the channel 0 is also automatically defined as a synchronous channel. This is because the channel 0 counter configuration is used by all the synchronous channels.

If a channel x is defined as a synchronous channel, the fields/bits for the channel 0 are used instead of those of channel x:

CPRE in PWM_CMR0 instead of CPRE in PWM_CMRx (same source clock)
CPRD in PWM_CPRD0 instead of CPRD in PWM_CPRDx (same period)
CALG in PWM_CMR0 instead of CALG in PWM_CMRx (same alignment)

Modifying the fields CPRE, CPRD and CALG of for channels with index greater than 0 has no effect on output waveforms.

Because counters of synchronous channels must start at the same time, they are all enabled together by enabling the channel 0 (by the CHID0 bit in PWM_ENA register). In the same way, they are all disabled together by disabling channel 0 (by the CHID0 bit in PWM_DIS register). However, a synchronous channel x different from channel 0 can be enabled or disabled independently from others (by the CHIDx bit in PWM_ENA and PWM_DIS registers).

Defining a channel as a synchronous channel while it is an asynchronous channel (by writing the bit SYNCx to ‘1’ while it was at ‘0’) is allowed only if the channel is disabled at this time (CHIDx = 0 in PWM_SR). In the same way, defining a channel as an asynchronous channel while it is a synchronous channel (by writing the SYNCx bit to ‘0’ while it was ‘1’) is allowed only if the channel is disabled at this time.

The UPDM field (Update Mode) in the PWM_SCM register selects one of the three methods to update the registers of the synchronous channels:

Method 1 (UPDM = 0): The period value, the duty-cycle values and the dead-time values must be written by the processor in their respective update registers (respectively PWM_CPRDUPDx, PWM_CDTYUPDx and PWM_DTUPDx).The update is triggered at the next PWM period as soon as the bit UPDULOCK in the PWM Sync Channels Update Control Register (PWM_SCUC) is set to ‘1’.
Method 2 (UPDM = 1): The period value, the duty-cycle values, the dead-time values and the update period value must be written by the processor in their respective update registers (respectively PWM_CPRDUPDx, PWM_CDTYUPDx and PWM_DTUPD). The update of the period value and of the dead-time values is triggered at the next PWM period as soon as the bit UPDULOCK in the PWM_SCUC register is set to ‘1’. The update of the duty-cycle values and the update period value is triggered automatically after an update period defined by the field UPR in the PWM Sync Channels Update Period Register (PWM_SCUP).

Method 3 (UPDM = 2): Same as Method 2 apart from the fact that the duty-cycle values of ALL synchronous channels are written by the DMA Controller. The user can choose to synchronize the DMA Controller transfer request with a comparison match (see Section 7.3 “PWM Comparison Units”), by the fields PTRM and PTRCS in the PWM_SCM register. The DMA destination address must be configured to access only the PWM DMA Register (PWM_DMAR). The DMA buffer data structure must consist of sequentially repeated duty cycles. The number of duty cycles in each sequence corresponds to the number of synchronized channels. Duty cycles in each sequence must be ordered from the lowest to the highest channel index. The size of the duty cycle is 16 bits.

Table 1. Summary of the Update of Registers of Synchronous Channels
Register	UPDM = 0	UPDM = 1	UPDM = 2
Period Value (PWM_CPRDUPDx)	Write by the processor
Period Value (PWM_CPRDUPDx)	Update is triggered at the next PWM period as soon as the bit UPDULOCK is set to ‘1’
Dead-Time Values (PWM_DTUPDx)	Write by the processor
Dead-Time Values (PWM_DTUPDx)	Update is triggered at the next PWM period as soon as the bit UPDULOCK is set to ‘1’
Duty-Cycle Values (PWM_CDTYUPDx)	Write by the processor	Write by the processor	Write by the DMA Controller
Duty-Cycle Values (PWM_CDTYUPDx)	Update is triggered at the next PWM period as soon as the bit UPDULOCK is set to ‘1’	Update is triggered at the next PWM period as soon as the update period counter has reached the value UPR.
Update Period Value (PWM_SCUPUPD)	Not applicable	Write by the processor
Update Period Value (PWM_SCUPUPD)	Not applicable	Update is triggered at the next PWM period as soon as the update period counter has reached the value UPR.