uPD789074 Subseries 8-Bit Single-Chip Microcontrollers UD - Home

CHAPTER 5 CLOCK GENERATOR5.6 Changing Setting of CPU Clock5.6.1 Time required for switching CPU clockThe CPU clock can be switched by using bit 1 (PCC1) of the processor clock control register (PCC).Actually, the specified clock is not switched immediately after the setting of PCC has been changed; old clock isused for the duration of several instructions after that (see Table 5-2).Table 5-2. Maximum Time Required for Switching CPU ClockSet Value Before SwitchingSet Value After SwitchingPCC1PCC1PCC10 10 4 clocks1 2 clocksRemarkTwo clocks is the minimum instruction execution time of the CPU clock before switching.5.6.2 Switching CPU clockThe following figure illustrates how the CPU clock is switched.Figure 5-5. Switching Between System Clock and CPU ClockVDDRESETCPU ClockfXSlowoperationfXFast operationWait (3.28 ms: @ 10.0 MHz operation)Internal reset operation 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 (2 15 /fX) is automatically secured.After that, the CPU starts instruction execution at the slow speed of the system clock (0.8 µs: @10.0 MHzoperation/1.6 µs: @5.0 MHz operation). After the time required for the VDD voltage to rise to the level at which the CPU can operate at the highspeed has elapsed, the processor clock control register (PCC) is rewritten so that the high-speed operationcan be selected.User’s Manual U14801EJ3V1UD 81