System Clock Oscillator; Main System Clock Oscillator - NEC mPD780024AS Series Preliminary User's Manual

The fastest instructions of µ PD780024AS, 780034AS Subseries are carried out in two CPU clocks. The relationship
of CPU clock (f ) and minimum instruction execution time is shown in Table 5-2.
CPU
Table 5-2. Relationship of CPU Clock and Minimum Instruction Execution Time
CPU Clock (f
f
X
f /2
X
2
f /2
X
3
f /2
X
4
f /2
X
f /2
XT
f = 8.38 MHz, f
X
f : Main system clock oscillation frequency
X
f : Subsystem clock oscillation frequency
XT

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 (8.38 MHz TYP.)
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
V
SS1
Crystal resonator or
ceramic resonator
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
a pull-up resistor.
94
CHAPTER 5 CLOCK GENERATOR
) Minimum Instruction Execution Time: 2/f
CPU
0.24 µ s
0.48 µ s
0.95 µ s
1.91 µ s
3.81 µ s
122 µ s
= 32.768 kHz
XT
IC
X2
X1
Preliminary User's Manual U16035EJ1V0UM
CPU
(b) External clock
X2
External
X1
clock
µ
PD74HCU04
via
DD1