94
CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U16035EJ1V0UM
The fastest instructions of
µ
PD780024AS, 780034AS Subseries are carried out in two 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 Minimum Instruction Execution Time
CPU Clock (f
CPU
)
Minimum 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 (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
(b) External clock
Crystal resonator or
ceramic resonator
X2
X1
PD74HCU04
External
clock
X2
X1
IC
µ
V
SS1
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.
