H8SSLASH2218 datasheet(653/750 Pages) RENESAS

653 / 750 page

Section 19 Clock Pulse Generator

Rev.6.00 Jun. 03, 2008 Page 605 of 698

REJ09B0074-0600

19.9

Usage Notes

19.9.1

Note on Crystal Resonator

Since various characteristics related to the crystal resonator are closely linked to the user's board

design, thorough evaluation is necessary on the user's part, using the resonator connection

examples shown in this section as a guide. As the resonator circuit ratings will depend on the

floating capacitance of the resonator and the mounting circuit, the ratings should be determined in

consultation with the resonator manufacturer. The design must ensure that a voltage exceeding the

maximum rating is not applied to the oscillator pin.

19.9.2

Note on Board Design

When designing the board, place the crystal resonator and its load capacitors as close as possible to

the XTAL or OSC1 and EXTAL or OSC2 pins. Other signal lines should be routed away from the

oscillator circuit to prevent induction from interfering with correct oscillation. See figure 19.10.

Prohibit

Signal A Signal B

This LSI

EXTAL or

OSC1

XTAL or

OSC2

Figure 19.10 Note on Board Design of Oscillator Circuit

19.9.3

Note on Switchover of External Clock

When two or more external clocks (e.g. 16 MHz and 13 MHz) are used as the system clock,

switchover of the input clock should be carried out in software standby mode.

An example of an external clock switching circuit is shown in figure 19.11, and an example of the

external clock switchover timing in figure 19.12.

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