CHAPTER 8 CLOCK GENERATOR (μPD789861 SUBSERIES)User’s Manual U14826EJ5V0UD828.6 Changing Setting of CPU Clock8.6.1 Time required for switching CPU clockThe CPU clock can be selected by using bit 1 (PCC0) of the processor clock control register (PCC).Actually, the specified clock is not selected immediately after the setting of PCC has been changed, and the oldclock is used for the duration of several instructions after that (see Table 8-2).Table 8-2. Maximum Time Required for Switching CPU ClockSet Value Before Switching Set Value After SwitchingPCC0 PCC0PCC00 10 4 clocks1 2 clocksRemark Two clocks are the minimum instruction execution time of the CPU clock before switching.8.6.2 Switching CPU clockThe following figure illustrates how the CPU clock is switched.Figure 8-5. Switching Between System Clock and CPU ClockCPU ClockRESETVDDSlowoperationFast operationWait (128 s: @1.0 MHz operation)Internal reset operationμ<1> The CPU is reset when the RESET pin is made low on power application. The effect of resetting is releasedwhen the RESET pin is later made high, and the system clock starts oscillating. At this time, the oscillationstabilization time (27 /f CC) is automatically secured.After that, the CPU starts instruction execution at the slow speed of the system clock (8.0μs: @1.0 MHzoperation).<2> After the time required for the V DD voltage to rise to the level at which the CPU can operate at the high speedhas elapsed, the processor clock control register (PCC) is rewritten so that the high-speed operation can beselected.
