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CHAPTER 4
CLOCK GENERATOR
User’s Manual U12697EJ4V1UD
4.5 Clock Generator Operations
The clock generator generates the following types of clocks and controls the CPU operation mode including
the standby mode.
 Main system clock (fXX)
 Subsystem clock (fXT)
 CPU clock (fCPU)
 Clock to peripheral hardware
The following clock generator functions and operations are determined using the standby control register (STBC)
and the oscillation mode selection register (CC).
(a) Upon generation of the RESET signal, the lowest speed mode of the main system clock (1,280 ns: @ 12.5
MHz operation) is selected (STBC = 30H, CC = 00H). Main system clock oscillation stops while a low level
is being applied to the RESET pin.
(b) With the main system clock selected, one of six CPU clock types (80 ns, 160 ns, 320 ns, 640 ns, 1,280 ns:
@ 12.5 MHz operation) can be selected by setting STBC and CC.
(c) With the main system clock selected, two standby modes, the STOP mode and the HALT mode, are available.
To decrease current consumption in the STOP mode, the subsystem clock feedback resistor can be
disconnected to stop the subsystem clock by using bit 7 (SBK) of STBC, when the system does not use the
subsystem clock.
(d) STBC can be used to select the subsystem clock to operate the system with low current consumption (30.5
s: @ 32.768 kHz operation).
(e) With the subsystem clock selected, main system clock oscillation can be stopped using STBC. The HALT
mode can be used. However, the STOP mode cannot be used. (Subsystem clock oscillation cannot be
stopped.)
(f)
The main system clock is divided and supplied to the peripheral hardware. The subsystem clock is supplied
to the 16-bit timer/event counter, the watch timer, and clock output functions only. Thus, the 16-bit timer/event
counter (when watch timer output is selected for the count clock during operation with the subsystem clock),
the watch function, and the clock output function can also be continued in the standby state. However, since
all other peripheral hardware operates with the main system clock, the peripheral hardware (except external
input clock operation) also stops if the main system clock is stopped.