R8C2K datasheet(98/471 Pages) RENESAS

98 / 471 page

R8C/2K Group, R8C/2L Group

10. Clock Generation Circuit

Rev.1.10

Dec 21, 2007

Page 81 of 450

REJ09B0406-0110

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.

Figure 10.10 shows Examples of XIN Clock Connection Circuit.

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.

This MCU has an on-chip feedback resistor and on-chip resistor disable/enable switching is possible by the CM11

bit in the CM1 register.

In stop mode, all clocks including the XIN clock stop. Refer to 10.4 Power Control for details.

Figure 10.10

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)

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