
R8C/2C Group, R8C/2D Group
10. Clock Generation Circuit
Rev.2.00
Dec 05, 2007
REJ09B0339-0200
The clocks generated by the clock generation circuits are described below.
10.1
XIN Clock
This clock is supplied by the XIN clock oscillation circuit. This clock is used as the clock source for the CPU and
peripheral function clocks. The XIN clock oscillation circuit is configured by connecting a resonator between the
XIN and XOUT pins. The XIN 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 XIN clock oscillation circuit may also be configured by feeding an externally generated clock to the XIN
pin.
In reset and after reset, the XIN clock stops.
The XIN clock starts oscillating when the CM05 bit in the CM0 register is set to 0 (XIN clock oscillates) after
setting the CM13 bit in the CM1 register to 1 (XIN- XOUT pin).
To use the XIN clock for the CPU clock source, set the OCD2 bit in the OCD register to 0 (select XIN clock) after
the XIN clock is oscillating stably.
The power consumption can be reduced by setting the CM05 bit in the CM0 register to 1 (XIN 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 are input, the XIN 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 XIN clock stop. Refer to 10.5 Power Control for details.
Figure 10.11
Examples of XIN 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.
Use high drive when oscillation starts and, if it is necessary to switch the oscillation drive capacity, do so
after oscillation stabilizes.
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.
To use this MCU with supply voltage below VCC = 2.7 V, it is recommended to set the CM11 bit in the
CM1 register to 1 (on-chip feedback resistor disabled), the CM15 bit to 1 (high drive capacity), and
connect the feedback resistor to the chip externally.
Open
Ceramic resonator external circuit
External clock input circuit
Rf(1)