
MOTOROLA
Chapter 8. Clocks and Power Control
8-7
Low-Power Divider
NOTE
When the VCO is the system clock source, chip operation is
unpredictable while the PLL is unlocked. Note further that a
switch to the backup clock is possible only if the LME bit in the
SCCR is set.
8.4
Low-Power Divider
The output of the PLL is sent to a low-power divider block. (In limp mode the BUCLK is
sent to a low-power divider block.) This block generates all other clocks in normal
operation, but has the ability to divide the output frequency of the VCO before it generates
the general system clocks sent to the rest of the MPC565/MPC566. The PLL VCOOUT is
always divided by at least two.
The purpose of the low-power divider block is to allow reduction and restoration of the
operating frequencies of different sections of the MPC565/MPC566 without losing the PLL
lock. Using the low-power divider block, full chip operation can still be obtained, but at a
lower frequency. This is called gear mode. The selection and speed of gear mode can be
changed at any time, with changes occurring immediately.
The low-power divider block is controlled in the system clock control register (SCCR). The
default state of the low-power divider is to divide all clocks by one. Thus, for a 40-MHz
system, the general system clocks are each 40 MHz. Whenever power-on reset is asserted,
the MF bits are set according to
Table 8-1, and the division factor high frequency (DFNH)
and division factor low frequency (DFNL) bits in SCCR are set to the value of 0 (
÷1for
DFNH and
÷2 for DFNL).
8.5
Internal Clock Signals
The internal clocks generated by the clocks module are shown in
Figure 8-4. The clocks
module also generates the CLKOUT and ENGCLK external clock signals. The PLL
synchronizes these signals to each other. The PITRTCLK frequency and source are
specified by the RTDIV and RTSEL bits in the SCCR. When the backup clock is
functioning as the system clock, the backup clock is automatically selected as the time base
clock source and is twice the MPC565/MPC566 system clock.