126
CHAPTER 5 CLOCK GENERATOR
User
’
s Manual U14701EJ3V0UD
The fastest instructions of
μ
PD780318, 780328, and 780338 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 Min. Instruction Execution Time
CPU Clock (f
CPU
)
Min. Instruction Execution Time: 2/(f
CPU
)
f
X
0.2
μ
s
f
X
/2
0.4
μ
s
f
X
/2
2
0.8
μ
s
f
X
/2
3
1.6
μ
s
f
X
/2
4
3.2
μ
s
f
XT
/2
122
μ
s
f
X
= 10 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 (10 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
External
clock
X2
X1
IC
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.