88
CHAPTER 4 CLOCKS
MB90620 series
4.4
Clock Mode
Two clock modes are provided: main clock mode and PLL clock mode.
I
Main clock mode and PLL clock mode
G
Main clock mode
In main clock mode, the main clock, which is the oscillation clock divided by 2 and is used as the
operating clock for the CPU and peripheral resources. Meanwhile, the PLL clocks are disabled.
G
PLL clock mode
In PLL clock mode, a PLL clock is used as the operating clock for the CPU and peripheral
resources. A PLL clock multiplier is selected with the clock selection register (CKSCR: CS1 and
CS0).
I
Clock mode transition
Switching between main clock mode and PLL clock mode is done by writing to the MCS bit of
the clock selection register (CKSCR).
G
Switching from main clock mode to PLL clock mode
When the MCS bit of CKSCR is “1”, writing “0” to it will switch the operating clock from the main
clock to a PLL clock after the PLL clock oscillation stabilization wait period (2
13
/HCLK).
G
Switching from PLL clock mode to main clock mode
When the MCS bit of CKSCR is “0”, writing “1” to it will switch the operating clock from the PLL
clock to the main clock when the edges of the PLL clock and the main clock coincide (after 1 to 8
PLL clocks).
<Check>
Even though the MCS bit of CKSCR is rewritten, machine clock switching does not occur
immediately. When operating a peripheral function that depends on the machine clock, make
sure that machine clock switching has been done by referring to the MCM bit of CKSCR
before operating the peripheral function.
I
Selection of a PLL clock multiplier
Writing a value from “00
B
” to “11
B
” to the CS1 and CS0 bits of CKSCR selects one to the four
PLL clock multipliers.
I
Machine clock
The machine clock may be either a PLL clock output from the PLL multiplier circuit or the clock
that is the source oscillation clock divided by 2. This machine clock is supplied to the CPU and
peripheral functions.
Either the main clock or a PLL clock can be selected by writing to the MCS bit of CKSCR.