NEC mPD789425, User Manual, Page: 113 / 321

CHAPTER 5 CLOCK GENERATOR
User’s Manual U15075EJ1V0UM00
113
5.6 Changing Setting of System Clock and CPU Clock
5.6.1 Time required for switching between system clock and CPU clock
The CPU clock can be selected by using bit 1 (PCC1) of the processor clock control register (PCC) and bit 4
(CSS0) of the subclock control register (CSS).
Actually, the specified clock is not selected immediately after the setting of PCC has been changed, and the old
clock is used for the duration of several instructions after that (see Table 5-2).
Table 5-2. Maximum Time Required for Switching CPU Clock
Set Value Before Switching Set Value After Switching
CSS0 PCC1 CSS0 PCC1 CSS0 PCC1 CSS0 PCC1
0 0 0 1 1 x
0 0 4 clocks 2f
X
/f
XT
clocks
(306 clocks)
1 2 clocks f
X
/2f
XT
clocks
(76 clocks)
1 x 2 clocks 2 clocks
Remarks 1. Two clocks are the minimum instruction execution time of the CPU clock before switching.
2. The parenthesized values apply to operation at f
X
= 5.0 MHz or f
XT
= 32.768 kHz.
3. x: don’t care