R8C/13 Group
6.1 Main Clock
The following describes the clocks generated by the clock generation circuit.
6.1 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 oscillator circuit is configured by connecting a
resonator between the XIN and XOUT pins. The main clock oscillator circuit contains a feedback resis-
tor, which is disconnected from the oscillator circuit during stop mode in order to reduce the amount of
power consumed in the chip. The main clock oscillator circuit may also be configured by feeding an
externally generated clock to the XIN pin. Figure 6.5 shows examples of main clock connection circuit.
During reset and after reset, the main clock is turned off.
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, set the OCD2 bit in the OCD register to “0” (selecting main
clock) after the main clock becomes oscillating stably.
The power consumption can be reduced by setting the CM05 bit in the CM0 register to “1” (main clock
off) if the OCD2 bit is set to “1” (on-chip oscillator clock selected).
Note that if an externally generated clock is fed into the XIN pin, the main clock cannot be turned off by
setting the CM05 bit to “1”. If necessary, use an external circuit to turn off the clock.
During stop mode, all clocks including the main clock are turned off. Refer to Section 6.4, “Power
Control.”
Microcomputer
(Built-in feedback resistor)
Microcomputer
(Built-in feedback resistor)
X
IN
XOUT
X
IN
XOUT
Open
(1)
d
R
Externally derived clock
Vcc
Vss
C
IN
C
OUT
NOTES:
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 maker 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 external to the chip, insert a feedback resistor between XIN
and XOUT following the instruction.
Figure 6.5 Examples of Main Clock Connection Circuit
Rev.1.20 Jan 27, 2006 page 34 of 205
REJ09B0111-0120
RENESAS [ RENESAS TECHNOLOGY CORP ]
