CHAPTER 5 CLOCK GENERATOR
User’s Manual U16899EJ2V0UD
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5.6 Time Required to Switch Between Ring-OSC Clock and High-Speed System Clock
Bit 0 (MCM0) of the main clock mode register (MCM) is used to switch between the Ring-OSC clock and high-
speed system clock.
In the actual switching operation, switching does not occur immediately after MCM0 rewrite; several instructions
are executed using the pre-switch clock after switching MCM0 (see Table 5-5).
Bit 1 (MCS) of MCM is used to judge that operation is performed using either the Ring-OSC clock or high-speed
system clock.
To stop the original clock after switching the clock, wait for the number of clocks shown in Table 5-5 before
stopping.
Table 5-5. Time Required to Switch Between Ring-OSC Clock and High-Speed System Clock
PCC
Time Required for Switching
PCC2
PCC1
PCC0
High-Speed System Clock
→
Ring-OSC
Ring-OSC
→
High-Speed System Clock
0 0 0
f
XP
/f
R
+ 1 clock
0 0 1
f
XP
/2f
R
+ 1 clock
2 clocks
Caution To calculate the maximum time, set f
R
= 120 kHz.
Remarks 1. PCC: Processor clock control register
2. f
XP
: High-speed system clock oscillation frequency
3. f
R
: Ring-OSC clock oscillation frequency
4. The maximum time is the number of clocks of the CPU clock before switching.