NEC PD78F9488 Manuale Utente
CHAPTER 5 CLOCK GENERATOR
106
User’s Manual U15331EJ4V1UD
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; 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