11.10 System Clock Prescaler

The device has a system clock prescaler and the system clock can be divided by configuring the Clock Prescale Register (CLKPR). This feature can be used to decrease the system clock frequency and the power consumption when the requirement for processing power is low. This can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clk_I/O, clk_ADC, clk_CPU, and clk_FLASH are divided by a factor as shown in the CLKPR description.

When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occur in the clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency corresponding to the previous setting nor the clock frequency corresponding to the new setting. The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may be faster than the CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler - even if it were readable, the exact time it takes to switch from one clock division to the other cannot be exactly predicted. From the time the Clock Prescaler Selection bits (CLKPS[3:0]) values are written, it takes between T1 + T2 and T1 + 2 * T2 before the new clock frequency is active. In this interval, two active clock edges are produced. Here, T1 is the previous clock period, and T2 is the period corresponding to the new prescaler setting.

To avoid unintentional changes of clock frequency, a special write procedure must be followed to change the CLKPS bits:

1. Write the Clock Prescaler Change Enable (CLKPCE) bit to '1' and all other bits in CLKPR to zero: CLKPR=0x80.
2. Within four cycles, write the desired value to CLKPS[3:0] while writing a zero to CLKPCE: CLKPR=0x0N.

Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted.