7477 datasheet(176/354 Pages) RENESAS

176 / 354 page

1-159

7470/7471/7477/7478 GROUP USER’S MANUAL

HARDWARE

1.16 Oscillation circuit

1.16.5 Notes on use

1. When inputting the external clock to the XIN pin or the XCIN pin, use a pulse signal with a duty ratio

of 50 % as an input signal. At this time, make the XOUT pin and the XCOUT pin open.

Refer to a example of circuit in “Chapter 2 application, 2.7 Oscillation circuit.”

2. When using the sub-clock f(XCIN) in the 7471/7478 group, set f(XCIN)

< 50 kHz < f(XIN)/3.

3. In the 7471/7478 group, switch between the ordinary mode and the low-speed mode after the oscillation

of the main clock and the sub-clock becomes stable. For the oscillation stabilizing time, ask the

oscillator maker for information.

4. Use the low-speed mode in one of the following states.

• Fix the XCOUT drive capacity to the high power mode (Set the XCOUT drive capacity selection bit

of the CPU mode register to “1”).

• When fixing the XCOUT drive capacity to the Low-power mode (clear the XCOUT drive capacity

selection bit of the CPU mode register to “0”), lower the value of the resistor Rd+ in the sub-clock

oscillation circuit to a level at which the oscillation of f(XCIN) does not stop.

+ Resister Rd: Refer to a example of circuit in “Chapter 2 application, 2.7 Oscillation circuit.”

5. When using the sub-clock f(XCIN) in the 7471/7478 group, it takes a long time until the oscillation

becomes stable.

When the ordinary mode is changed to the stop mode while the sub-clock is in the oscillation state and

then the ordinary mode is recovered from the stop mode, the oscillation of the sub-clock is not stabilized

even if the main clock becomes stable and the CPU is restored.

Note: In the 7470/7477 group, the sub-clock f(XCIN) is not available because neither XCIN pin nor XCOUT

pin is provided.

7477 Datasheet(PDF) 176 Page - Renesas Technology Corp