The APB divider determines the relationship between the processor clock (CCLK) and the
clock used by peripheral devices (PCLK). The APB divider serves two purposes. The first
is to provide peripherals with the desired PCLK via APB bus so that they can operate at
the speed chosen for the ARM processor. In order to achieve this, the APB bus may be
slowed down to 12 to 14 of the processor clock rate. Because the APB bus must work
properly at power-up (and its timing cannot be altered if it does not work since the APB
divider control registers reside on the APB bus), the default condition at reset is for the
APB bus to run at 14 of the processor clock rate. The second purpose of the APB divider
is to allow power savings when an application does not require any peripherals to run at
the full processor rate. Because the APB divider is connected to the PLL output, the PLL
remains active (if it was running) during Idle mode.
clock used by peripheral devices (PCLK). The APB divider serves two purposes. The first
is to provide peripherals with the desired PCLK via APB bus so that they can operate at
the speed chosen for the ARM processor. In order to achieve this, the APB bus may be
slowed down to 12 to 14 of the processor clock rate. Because the APB bus must work
properly at power-up (and its timing cannot be altered if it does not work since the APB
divider control registers reside on the APB bus), the default condition at reset is for the
APB bus to run at 14 of the processor clock rate. The second purpose of the APB divider
is to allow power savings when an application does not require any peripherals to run at
the full processor rate. Because the APB divider is connected to the PLL output, the PLL
remains active (if it was running) during Idle mode.
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