The maximum CPU clock frequency depends on the Vdd applied to the device.
For a Vdd greater than 4.75V, the maximum CPUClk Frequency is 24MHz. However, if running under 4.75V, the maximum CPU frequency is 12 MHz (though the sysclk can still be set as 24MHz). Note that the SysClk is the IMO output frequency while CPUClk is the frequency at which CPU is clocked and is derived from SysClk. CPU clock is adjustable, and must be adjusted for lower voltages.
A common mistake is to set the CPU to run at 24 MHz, while the chip is powered at 3.3V. Unexpected results may occur in this configuration. Make sure the maximum CPU speed is 12 MHz when running under 4.75V.