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5.5 Clock Generator Operation
The clock generator generates the following clocks and controls the operation modes of the CPU, such as the
standby mode.
• Main system clock
• Subsystem clock
f
XT
• CPU clock
f
CPU
• Clock to peripheral hardware
The operation and function of the clock generator is determined by the processor clock control register (PCC),
suboscillation mode register (SCKM), and subclock control register (CSS), as follows.
(a)
The low-speed mode 2f
(PCC = 02H). While a low level is input to the RESET pin, oscillation of the main system clock is
stopped.
(b)
Three types of CPU clocks f
Register) can be selected by the PCC, SCKM, and CSS settings.
(c)
Two standby modes, STOP and HALT, can be used with the main system clock selected. In a system
where no subsystem clock is used, setting bit 1 (FRC) of the SCKM so that the on-chip feedback
resistor cannot be used reduces power consumption in STOP mode. In a system where a subsystem
clock is used, setting SCKM bit 0 to 1 can cause the subsystem clock to stop oscillation.
(d)
CSS bit 4 (CSS0) can be used to select the subsystem clock so that low current consumption operation
is used (122
(e)
With the subsystem clock selected, it is possible to cause the main system clock to stop oscillating
using bit 7 (MCC) of PCC. The HALT mode can be used, but the STOP mode cannot.
(f)
The clock pulse for the peripheral hardware is generated by dividing the frequency of the main system
clock, but the subsystem clock pulse is only supplied to the 16-bit timer, 8-bit timer, watch timer, and
LCD controller/driver. The 16-bit timer, 8-bit timer, watch timer, and LCD controller/driver can therefore
keep running even during standby. The other hardware stops when the main system clock stops
because it runs based on the main system clock (except for external input clock operations).
CHAPTER 5 CLOCK GENERATOR
f
or f
X
CC
of the main system clock is selected when the RESET signal is generated
CPU
(for details, see Figure 5-2 Format of Processor Clock Control
CPU
µ
s: at 32.768 kHz operation).
User's Manual U15075EJ2V1UD
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