M16C28 Datasheet, PDF (71/423 Pages) Renesas Technology Corp – 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/Tiny SERIES

M16C/28 Group (M16C/28, M16C/28B)
7. Clock Generation Circuit
The following describes the clocks generated by the clock generation circuit.
7.1 Main Clock
The main clock is generated by the 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 resistor,
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 7.8 shows the examples of main clock connection circuit.
The main clock oscillates after reset. The power consumption in the chip can be reduced by setting the
CM05 bit in the CM0 register to “1” (main clock oscillator circuit turned off) after switching the clock source
for the CPU clock to a sub clock or on-chip oscillator clock. In this case, XOUT goes “H”. Furthermore,
because the internal feedback resistor remains on, XIN is pulled “H” to XOUT via the feedback resistor.
During stop mode, all clocks including the main clock are turned off. Refer to 7.6 power control.
If the main clock is not used, it is recommended to connect the XIN pin to VCC to reduce power consump-
tion during reset.
Microcomputer
(Built-in Feedback Resistor)
XIN
Oscillator
XOUT
VSS
Rd⁽¹⁾
Microcomputer
(Built-in Feedback Resistor)
CIN
XIN
External Clock
VCC
VSS
COUT
XOUT Open
NOTE:
1. Insert a damping resistor if required. Resistance value varies depending on the oscillator setting.
Use resistance value recommended by the oscillator manufacturer. If the oscillator manufacturer
recommends that a feedback resistor be added to the chip externally, insert a feedback resistor
between XIN and XOUT.
2. The external clock should not be stopped when it is connected to the XIN pin and the main clock is
selected as the CPU clock.
Figure 7.8 Examples of Main Clock Connection Circuit
Rev. 2.00 Jan. 31, 2007 ^{page 51} of 385
REJ09B0047-0200