4.17.7 Peripheral and Generic Clock Controller

The PMC controls the clocks of the embedded peripherals by means of the Peripheral Control register (PMC_PCR). With this register, the user can enable and disable the different clocks used by the peripherals:

• Peripheral clocks (periph_clk[PID]), routed to every peripheral and derived from MCK. It is mandatory to enable this clock before using a peripheral.
• Generic clocks (GCLK[PID]), routed to selected peripherals only (refer to the “Peripheral Identifiers” table). These clocks are independent of the core and bus clocks (CPU_CLK, MCK and periph_clk[PID]). They are generated by selection and division of available sources. The list of available source clocks depends on the peripheral. Refer to the description of each peripheral to know available sources and limitations to be applied to GCLK[PID] compared to periph_clk[PID].

To configure a peripheral’s clocks, PMC_PCR.CMD must be written to ‘1’ and PMC_PCR.PID must be written with the index of the corresponding peripheral. All other configuration fields must be correctly set.

To read the current clock configuration of a peripheral, PMC_PCR must be first accessed with PMC_PCR.CMD bit written to ‘0’ and PMC_PCR.PID field written with the index of the corresponding peripheral. This write does not modify the configuration of the peripheral. The PMC_PCR can then be read to know the configuration status of the corresponding PID.

The status of the peripheral clock activity can be read in the Peripheral Clock Status registers (PMC_CSRx).

The status of the peripheral generic clock activity can be read in the Generic Clock Status registers (PMC_GCSRx).

When a peripheral or a generic clock is disabled, it is immediately stopped. These clocks are disabled after a reset. The source and the division ratio of generic clocks must not be modified while the peripheral is enabled. The generic clock configuration must be set before the peripheral is enabled.

To stop a peripheral clock, it is recommended that the system software wait until the peripheral has executed its last programmed operation before disabling the clock. This is to avoid data corruption or erroneous behavior of the system.