Changing Setting Of System Clock And Cpu Clock; Time Required For Switching Between System Clock And Cpu Clock - NEC mPD789426 Series User Manual

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).
Set Value Before Switching
CSS0 PCC1
0 0
1
1 x
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
3. x: don't care
CHAPTER 5 CLOCK GENERATOR
Table 5-2. Maximum Time Required for Switching CPU Clock
CSS0 PCC1
0
2 clocks
2 clocks
User's Manual U15075EJ1V0UM00
Set Value After Switching
CSS0
0 0
4 clocks
2 clocks
= 5.0 MHz or f
X
PCC1 CSS0
1 1
2f /f clocks
X XT
(306 clocks)
f /2f clocks
X XT
(76 clocks)
= 32.768 kHz.
XT
PCC1
x
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