NEC mPD780065 Series, Preliminary User'S Manual

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93
CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U13420EJ2V0UM00
The fastest instructions of
µ
PD780065 Subseries are carried out in 2 CPU clocks. The relationship of CPU clock
(f
CPU
) and minimum instruction execution time is shown in Table 5-2.
Table 5-2. Relationship of CPU Clock and Min. Instruction Execution Time
CPU Clock (f
CPU
) Min. Instruction Execution Time: 2/(f
CPU
)
f
X
0.24
µ
s
f
X
2 0.48
µ
s
f
X
2
2
0.95
µ
s
f
X
2
3
1.91
µ
s
f
X
2
4
3.81
µ
s
f
XT
2 122
µ
s
f
X
= 8.38 MHz, f
XT
= 32.768 kHz
f
X
: Main system clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency
5.4 System Clock Oscillator
5.4.1 Main system clock oscillator
The main system clock oscillator oscillates with a crystal resonator or a ceramic resonator (standard: 8.38 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 inverted-phase clock signal to the X2 pin.
Figure 5-4 shows an external circuit of the main system clock oscillator.
Figure 5-4. External Circuit of Main System Clock Oscillator
(a) Crystal and ceramic oscillation (b) External clock
Caution Do not execute the STOP instruction and do not set MCC [bit 7 of processor clock control
register (PCC)] to 1 if an external clock is input. This is because when the STOP instruction or
MCC is set to 1, the main system clock operation stops and the X2 pin is connected to V
DD1
via
a pull-up resistor.
Crystal resonator or
ceramic resonator
X1
X2
µ
PD74HCU04
External
clock
IC
V
SS1
X2
X1