
MOTOROLA
Chapter 8. Clocks and Power Control
8-11
Internal Clock Signals
GCLK2_50 is the same as CLKOUT. The general system clock defaults to VCO/2 = 20
MHz (assuming a 20-MHz system frequency) with default power-on reset MF values.
The general system clock frequency can be switched between different values. The highest
operational frequency can be achieved when the system clock frequency is determined by
DFNH (CSRC bit in the PLPRCR is cleared) and DFNH = 0 (division by one). The general
system clock can be operated at a low frequency (gear mode) or a high frequency. The
DFNL bits in SCCR define the low frequency. The DFNH bits in SCCR define the high
frequency.
The frequency of the general system clock can be changed dynamically with the system
Figure 8-5. General System Clocks Select
The frequency of the general system clock can be changed “on the fly” by software. The
user may simply cause the general system clock to switch to its low frequency. However,
in some applications, there is a need for a high frequency during certain periods. Interrupt
routines, for example, may require more performance than the low frequency operation
provides, but must consume less power than in maximum frequency operation. The
MPC565/MPC566 provides a method to automatically switch between low and high
frequency operation whenever one of the following conditions exists:
There is a pending interrupt from the interrupt controller. This option is maskable by
the PRQEN bit in the SCCR.
The (POW) bit in the MSR is clear in normal operation. This option is maskable by
the PRQEN bit in the SCCR.
When neither of these conditions exists and the CSRC bit in PLPRCR is set, the general
system clock switches automatically back to the low frequency.
Abrupt changes in the divide ratio can cause linear changes in the operating currents of the
MPC565/MPC566. Insure that the proper power supply filtering is available to handle this
change instantaneously.
When the general system clock is divided, its duty cycle is changed. One phase remains the
same (for example, 12.5 ns @ 40 MHz) while the other becomes longer.
DFNH Divider
DFNL Divider
VCO/2 (e.g., 40 MHz)
DFNH
Normal
Low Power
General System Clock
DFNL
O