7470/7471/7477/7478 GROUP USER’S MANUAL
1-156
HARDWARE
1.16 Oscillation circuit
1.16.3 Oscillation operation
(1) Oscillation operation
The microcomputer is put into the ordinary mode at reset release. At this time, only the main clock
oscillates and the P50/XCIN pin and the P51/XCOUT pin function as input ports P50 and P51.
Notes 1: The 7470/7477 group is not provided with XCIN and XCOUT pins.
2: When using the sub-clock f(XCIN) in the 7471/7478 group, set the P50-P53 pull-up control bit
(bit 6) of the P1-P5 pull-up control register to “0” and disconnect the pull-up transistor of the
P50/XCIN pin and P51/XCOUT pin.
2
2 Ordinary mode
The clock resulting from dividing a signal input to the XIN pin by 2 becomes internal clock
φ.
Changing the mode to the low-speed mode (7471/7478 group)
Execute the following procedure.
1 Set the P50, P51/XCIN, XCOUT selection bit (bit 4) of the CPU mode register to “1” (XCIN, XCOUT).
2 Set the XCOUT drive capacity selection bit (bit 5) of the CPU mode register to “1” (High power).
3 Generate oscillation stabilizing wait time of f(XCIN) by software.
4 Set the system clock selection bit (bit 7) of the CPU mode register to “1” f(XCIN). At that time,
set the XCOUT drive capacity selection bit to “0” (low power) as required.
2
2 Low-speed mode (7471/7478 group)
The clock resulting from dividing a signal input to the XCIN pin by 2 becomes internal clock
φ.
In the low-speed mode, a low power dissipation operation can be attained by setting the main clock
(XIN-XOUT) stop bit (bit 6) of the CPU mode register to “1.”
Changing the mode to the ordinary mode
Execute the following procedure.
1 Clear the main clock (XIN-XOUT) stop bit (bit 6) of the CPU mode register to “0” (oscillate).
2 Generate oscillation stabilizing wait time of f(XIN) by software.
3 Clear the system clock selection bit (bit 7) of the CPU mode register to “0” f(XIN).
Notes 1: 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 stablizing time, ask the oscillator maker for
information.
2: Use the low-speed mode in one of the following states.
q Fix the XCOUT drive capacity to the high power mode (set the XCOUT drive capacity selection
bit of the CPU mode register to “1”).
q 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.”
3: When using the sub-clock especially, 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 restored 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.