Datasheet

Table Of Contents
Section 21 Clock Pulse Generator
Page 678 of 846 REJ09B0140-0900 Rev. 9.00
Sep 16, 2010
H8S/2215 Group
21.8 Usage Notes
21.8.1 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.
21.8.2 Note on Board Design
When designing the board, place the crystal resonator and its load capacitors as close as possible
to the XTAL or XTAL48 and EXTAL or EXTAL48 pins. Other signal lines should be routed
away from the oscillator circuit to prevent induction from interfering with correct oscillation. See
figure 21.12.
Prohibit
Signal A Signal B
H8S/2215 Group
XTAL or
XTAL48
EXTAL or
EXTAL48
C
L2
C
L1
Figure 21.12 Note on Board Design of Oscillator Circuit
21.8.3 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 21.13, and an example of the
external clock switchover timing in figure 21.14.