Explanation

One of the most powerful features of the Propeller chip is the ability to change the clock behavior at run time. An application can choose to toggle back and forth between a slow clock speed (for low-power consumption) and a fast clock speed (for high-bandwidth operations), for example. CLKSET is used to change the clock mode and frequency during run time. It is the run-time equivalent of the _CLKMODE and _CLKFREQ constants defined by the application at compile time. For example:

clkset(%01101100, 4_000_000)   'Set to XTAL1 + PLL2x

This sets the clock mode to a low-speed external crystal and a Clock PLL multiplier of 2, and indicates the resulting System Clock frequency (CLKFREQ) is 4 MHz. After executing this command, the CLKMODE and CLKFREQ commands will report the updated settings for objects that use them.

In general, it is safe to switch between clock modes by using a single CLKSET command, however, if enabling the Crystal Oscillator circuit (CLK register's OSCENA bit) it is important to perform the clock mode switch as a three-stage process:

First set the CLK register's PLLENA, OSCENA, OSCM1 and OSCM0 bits as necessary. See CLK Register for more information.
Wait for 10 ms to give the external crystal time to stabilize.
Set the CLK register's CLKSELx bits as necessary to switch the System Clock to the new source.

The above process is only necessary when switching the Crystal Oscillator circuit on. No other clock mode changes require this process if the Crystal Oscillator circuit is left in its current state, either off or on. See the Clock object in the Propeller Library for clock modification and timing methods.

NOTE: It takes approximately 75 µs for the Propeller Chip to perform the clock source switching action.