
M16C/28 Group
Under development Preliminary specification
Specifications in this manual are tentative and subject to change.
7. Clock Generation Circuit
Rev.0.60 2004.02.01
page 38 of N
REJ09B0047-0060Z
Microcomputer
(Built-in feedback resistor)
XIN
XOUT
Externally derived clock
Open
VCC
Vss
Microcomputer
(Built-in feedback resistor)
XIN
XOUT
Rd
CIN
COUT
(Note)
Note: 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 7.1.1. Examples of Main Clock Connection 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 7.1.1 shows the examples of main clock connection circuit.
The power consumption in the chip can be reduced by setting the CM05 bit of CM0 register to “1” (main
clock oscillator circuit turned off) after switching the clock source for the CPU clock to a sub clock or ring
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 “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.