38C3 Group User’s Manual
HARDWARE
1-41
CLOCK GENERATING CIRCUIT
The 38C3 group has two built-in oscillation circuits. An oscillation
circuit can be formed by connecting a resonator between XIN and
XOUT (XCIN and XCOUT). Use the circuit constants in accordance with
the resonator manufacturer's recommended values. No external re-
sistor is needed between XIN and XOUT since a feedback resistor
exists on-chip. However, an external feedback resistor is needed be-
tween XCIN and XCOUT.
Immediately after power on, only the XIN oscillation circuit starts os-
cillating, and XCIN and XCOUT pins function as I/O ports.
Frequency control
(1) Middle-speed mode
The internal system clock is the frequency of XIN divided by 8. After
reset, this mode is selected.
(2) High-speed mode
The internal system clock is the frequency of XIN divided by 2.
(3) Low-speed mode
The internal system clock is the frequency of XCIN divided by 2.
sNotes on clock generating circuit
If you switch the mode between middle/high-speed and low-speed,
stabilize both XIN and XCIN oscillations. The sufficient time is required
for the sub clock to stabilize, especially immediately after power on
and at returning from stop mode. When switching the mode between
middle/high-speed and low-speed, set the frequency on condition
that f(XIN) > 3f(XCIN).
Oscillation control
(1) Stop mode
If the STP instruction is executed, the internal system clock stops at
an “H” level, and XIN and XCIN oscillators stop. Timer 1 is set to “FF16”
and timer 2 is set to “0116.”
Either XIN divided by 16 or XCIN divided by 16 is input to timer 1 as
count source, and the output of timer 1 is connected to timer 2. The
bits of the timer 12 mode register are cleared to “0.” Set the interrupt
enable bits of the timer 1 and timer 2 to disabled (“0”) before execut-
ing the STP instruction. Oscillator restarts when an external interrupt
is received, but the internal system clock is not supplied to the CPU
until timer 2 underflows. This allows time for the clock circuit oscilla-
tion to stabilize.
(2) Wait mode
If the WIT instruction is executed, the internal system clock stops at
an “H” level. The states of XIN and XCIN are the same as the state
before executing the WIT instruction. The internal system clock re-
starts at reset or when an interrupt is received. Since the oscillator
does not stop, normal operation can be started immediately after the
clock is restarted.
Fig. 42 Ceramic resonator circuit
Fig. 43 External clock input circuit
FUNCTIONAL DESCRIPTION
XCIN
XCOUT
XIN
XOUT
CIN
COUT
CCIN
CCOUT
Rf
Rd
XIN
XOUT
External oscillation circuit
VCC
VSS
open
XCIN
XCOUT
CCIN
CCOUT
Rf
Rd