R8C/18 Group, R8C/19 Group
10. Clock Generation Circuit
Rev.1.30
Apr 14, 2006
Page 60 of 233
REJ09B0222-0130
The clocks generated by the clock generation circuits are described below.
10.1
Main Clock
This clock is supplied by a main clock oscillation circuit. This clock is used as the clock source for the
CPU and peripheral function clocks. The main clock oscillation circuit is configured by connecting
resonator between the XIN and XOUT pins. The main clock oscillation circuit includes an on-chip
feedback resistor, which is disconnected from the oscillation circuit in stop mode in order to reduce the
amount of power consumed by the chip. The main clock oscillation circuit may also be configured by
feeding an externally generated clock to the XIN pin. Figure 10.7 shows Examples of Main Clock
Connection Circuit. During reset and after reset, the main clock stops.
The main clock starts oscillating when the CM05 bit in the CM0 register is set to 0 (main clock on) after
setting the CM13 bit in the CM1 register to 1 (XIN- XOUT pin).
To use the main clock for the CPU clock source, set the OCD2 bit in the OCD register to 0 (selects main
clock) after the main clock is oscillating stably.
The power consumption can be reduced by setting the CM05 bit in the CM0 register to 1 (main clock
stops) if the OCD2 bit is set to 1 (select on-chip oscillator clock).
When an external clock is input to the XIN pin, the main clock does not stop if the CM05 bit is set to 1. If
necessary, use an external circuit to stop the clock.
In stop mode, all clocks including the main clock stop. Refer to
for details.
Figure 10.7
Examples of Main Clock Connection Circuit
XIN
XOUT
MCU
(on-chip feedback resistor)
Rd
(1)
COUT
CIN
XIN
XOUT
MCU
(on-chip feedback resistor)
Externally derived clock
VCC
VSS
NOTE:
1. Insert a damping resistor if required. The resistance will vary depending on the oscillator and the
oscillation drive capacity setting. Use the value recommended by the manufacturer of the oscillator.
When the oscillation drive capacity is set to low, check that oscillation is stable. Also, if the oscillator
manufacturer's data sheet specifies that a feedback resistor be added to the chip externally, insert a
feedback resistor between XIN and XOUT following the instructions.
Open
Ceramic resonator external circuit
External clock input circuit
