NEC 7.4 System Clock Oscillator

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CHAPTER 7 CLOCK GENERATOR
7.4 System Clock Oscillator
7.4.1 Main system clock oscillator
The main system clock oscillator oscillates with a crystal resonator or a ceramic resonator (standard: 5.0 MHz)
connected to the X1 and X2 pins.
External clocks can be input to the main system clock oscillator. In this case, input a clock signal to the X1 pin
and an antiphase clock signal to the X2 pin.
Figure 7-6 shows an external circuit of the main system clock oscillator.
Figure 7-6. External Circuit of Main System Clock Oscillator
(a) Crystal and ceramic oscillation (b) External clock
Caution When an external clock is input, do not execute the STOP instruction or set MCC (bit 7 of the
processor clock control register (PCC)) to 1. If the STOP instruction is executed or MCC is set
to 1, the main system clock will stop operating so that pin X2 can be pulled up to VDD.
Crystal
or
Ceramic Resonator
IC
X1
X2
X1
PD74HCU04
µ
X2
External
Clock