Main System Clock Operations - NEC mPD780024AS Series Preliminary User's Manual

5.5.1 Main system clock operations

When operated with the main system clock (with bit 5 (CLS) of the processor clock control register (PCC) set to
0), the following operations are carried out by PCC setting.
(a) Because the operation guarantee instruction execution speed depends on the power supply voltage, the minimum
instruction execution time can be changed by bits 0 to 2 (PCC0 to PCC2) of the PCC.
(b) If bit 7 (MCC) of the PCC is set to 1 when operated with the main system clock, the main system clock oscillation
does not stop. When bit 4 (CSS) of the PCC is set to 1 and the operation is switched to subsystem clock operation
(CLS = 1) after that, the main system clock oscillation stops (see Figure 5-7).
(a) Operation when MCC is set after setting CSS with main system clock operation
Main system clock oscillation
Subsystem clock oscillation
(b) Operation when MCC is set in case of main system clock operation
Main system clock oscillation
Subsystem clock oscillation
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CHAPTER 5 CLOCK GENERATOR
Figure 5-7. Main System Clock Stop Function (1/2)
MCC
CSS
CLS
CPU clock
MCC
CSS L
CLS L
CPU clock
Preliminary User's Manual U16035EJ1V0UM
Oscillation does not stop.