MOTOROLA
Chapter 8. Clocks and Power Control
8-3
System Clock Sources
8.1
System Clock Sources
The system clock can be provided by the main system oscillator, an external clock input, or
the backup clock (BUCLK) on-chip ring oscillator, see
Figure 8-1.The main system oscillator uses either a 4-MHz or 20-MHz crystal to generate the PLL
reference clock. When the main system oscillator output is the timing reference to the
system PLL, skew elimination between the XTAL/EXTAL pins and CLKOUT is not
guaranteed. There is also an on-chip crystal feedback resistor on the MPC561/MPC563;
however, space should be reserved for an off-chip resistor to allow for future
configurations.
Figure 8-2 illustrates the main system oscillator crystal configuration.
The external clock input (EXTCLK pin) can receive a clock signal from an external source.
The clock frequency must be in the range of 3-5 MHz or, for 1:1 mode, at the system
frequency of at least 15 MHz. When the external clock input is the timing reference to the
system PLL, the skew between the EXTCLK pin and the CLKOUT is less than
± 1 ns.
The backup clock on-chip ring oscillator allows the MPC561/MPC563 to function with a
less precise clock. When operating from the backup clock, the MPC561/MPC563 is in limp
mode. This enables the system to continue minimum functionality until the system is fixed.
The BUCLK frequency is approximately 11 MHz for the MPC561/MPC563 (see
For normal operation, at least one clock source (EXTCLK or main system oscillator) must
be active. A configuration with both clock sources active is possible as well. At this
configuration EXTCLK provides the system clock and main system oscillator provides the
PITRTCLK. The input of an unused timing reference (EXTCLK or EXTAL) must be
grounded.
Figure 8-2. Main System Oscillator Crystal Configuration
8.2
System PLL
The PLL allows the processor to operate at a high internal clock frequency using a low
frequency clock input, a feature which offers two benefits: reduces the overall
electromagnetic interference generated by the system, and the ability to oscillate at different
frequencies reduces cost by eliminating the need to add an additional oscillator to a system.
EXTAL
XTAL
CL
1 M
1
1. Resistor is not currently required on the board but space should be available for its addition in the future.