CHAPTER 5 CLOCK GENERATOR
93
5.5 Clock Generator Operations
The clock generator generates the following various types of clocks and controls the CPU operating mode
including the standby mode.
Main system clock f
X
Subsystem clock f
XT
CPU clock f
CPU
Clock to peripheral hardware
The function and operation of the clock generator circuit are determined by the processor clock control register
(PCC) as follows:
(a) Upon generation of RESET signal, the lowest speed mode of the main system clock (6.4
μ
s when operated
at 5.0 MHz) is selected (PCC = 04H). Main system clock oscillation stops while low level is applied to the RESET
pin.
(b) With the main system clock selected, one of the five (0.4
μ
s, 0.8
μ
s, 1.6
μ
s, 3.2
μ
s and 6.4
μ
s: when operated
at 5.0 MHz) CPU clock stages can be selected by setting the PCC.
(c) With the main system clock selected, two standby modes, the STOP and HALT modes, are available. In a
system where the subsystem clock is not used, the current consumption in the STOP mode can be further
reduced by not using the internal feedback resistor if so specified by bit 6 (FRC) of the PCC.
(d) The PCC can be used to select the subsystem clock and to operate the system with low current consumption
(122 ms when operated at 32.768 kHz).
(e) With the subsystem clock selected, main system clock oscillation can be stopped with the PCC. 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 watch timer and clock output functions only. Thus, the watch function and the clock output function can
also be continued in the standby state. However, since all other peripheral hardware operate with the main
system clock, the peripheral hardware also stops if the main system clock is stopped (except external input
clock operation).