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CHAPTER 5 CLOCK GENERATION CIRCUIT
96
5.5 Clock Generation Circuit Operation
The clock generation circuit generates the following clocks and controls operation modes of the CPU, such as
standby mode:
Main system clock f
X
Subsystem clock f
XT
CPU clock f
CPU
Clock to peripheral hardware
The operation of the clock generation circuit is determined by the processor clock control register (PCC),
suboscillation mode register (SCKM), and subclock control register (CSS), as follows:
(a)
The slow mode 2 f
CPU
(1.6
μ
s: at 5.0-MHz operation) of the main system clock is selected when the RESET
signal is generated (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
CPU
(0.2
μ
s and 0.8
μ
s: main system clock (at 5.0-MHz operation), 61
μ
s:
subsystem clock (at 32.768-kHz operation)) 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 SCKM so that the built-in feedback resistor cannot
be used reduces current drain during 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 drain operation is used
(122
μ
s: at 32.768-kHz operation).
(e)
With the subsystem clock selected, it is possible to cause the main system clock to stop oscillating by using
bit 7 (MCC) of PCC. HALT mode can be used, but STOP mode cannot.
(f)
The clock for the peripheral hardware is generated by dividing the frequency of the main system clock. The
subsystem clock is supplied to 16-bit timer counter 90, 8-bit timer counter 82, and the watch timer only. So,
even in standby mode, 16-bit timer counter 90, 8-bit timer counter 82, and watch function can keep running.
The other hardware stops when the main system clock stops, because it runs based on the main system
clock (except for an external clock).