R8C/1A Group, R8C/1B Group
10. Clock Generation Circuit
Rev.1.30
Dec 08, 2006
Page 65 of 315
REJ09B0252-0130
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 a 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
10.4 Power Control
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
