NEC uPD78P078 - Manual (Page 171)

171
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 Do not execute the STOP instruction nor set MCC (bit 7 of the processor clock control register
(PCC)) to 1 while an external clock is input. This is because the operation of main system clock
is stopped and X2 pin is pulled-up to V DD when a STOP instruction is executed and MCC is set
to 1.
Crystal or
Ceramic Resonator
IC
X1
X2
X1
PD74HCU04 µ
X2
External
Clock