Renesas H8/38024 Series, Hardware Manual

Page: 142 / 661

Rev. 6.00, 08/04, page 112 of 628
4.5 Note on Oscillators
Oscillator characteristics are closely related to board design and should be carefully evaluated by
the user in mask ROM and ZTAT versions, referring to the examples shown in this section.
Oscillator circuit constants will differ depending on the oscillator element, stray capacitance in its
interconnecting circuit, and other factors. Suitable constants should be determined in consultation
with the oscillator element manufacturer. Design the circuit so that the oscillator element never
receives voltages exceeding its maximum rating.
(Vss)
P17
X
1
X
2
Vss
OSC
2
OSC
1
TEST
Figure 4.12 Example of Crystal and Ceramic Oscillator Element Arrangement
Figure 4.13 (1) shows an example measuring circuit with the negative resistance suggested by the
resonator manufacturer. Note that if the negative resistance of the circuit is less than that suggested
by the resonator manufacturer, it may be difficult to start the main oscillator.
If it is determined that oscillation is not occurring because the negative resistance is lower than the
level suggested by the resonator manufacturer, the circuit may be modified as shown in figure 4.13
(2) through (4). Which of the modification suggestions to use and the capacitor capacitance should
be decided based upon an evaluation of factors such as the negative resistance and the frequency
deviation.