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PRELIMINARY
Notice: This is not a final specification.
Some parametric limits are subject to change.
MITSUBISHI MICROCOMPUTERS
M37736MHBXXXGP
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
58
CLOCK GENERATING CIRCUIT
Figures 64 and 66 show the bit configuration of the oscillation circuit
control registers 0, 1 and the clock generating circuit diagram. The
clock generating circuit consists of main- and sub-clock oscillation
circuits, system clock switch circuit, clock dividing circuit, standby
control circuit, and others. The oscillation circuit control registers are
some of the control registers for the clock generating circuit.
Clocks
, f
2
to f
512
, f
C32
, and
peripheral devices or are output from pins, and they are made of the
main or sub clock, as shown in Figure 66.
The system clock and the clock f
2
can be switched to high-speed
clocks or low-speed clocks shown in Table 10. When using the sub
clock, it is possible to select one of 3 types: the main clock divided by
2, the direct main clock (not divided) and the sub clock divided by 2
as the clock f
2
.
When not using the sub clock, it is possible to select one of 4 types:
the main clock divided by 2, divided by 8, divided by 16 and the direct
main clock (not divided) as the clock f
2
.
This function of clocks switch make it possible to adapt power control
to the system operation.
Bits 0 to 4 of the oscillation circuit control register 0 and bit 0 of the
oscillation circuit control register 1 control sub-clock oscillation start,
1
are used in CPU and internal
system clock selection, stop/restart of main-clock oscillation, sub-
clock drivability selection and the main clock division selection.
The method of clocks switch is described bellow.
When selecting the main clock as the system clock, the main clock
division selection bit (bit 0 of the oscillation circuit control register 1)
selects either the main clock divided by 2 or the direct main clock as
the clock f
2
. When this bit is “1”, the clock f
2
is the direct main clock
which is not divided, so that a half external input frequency is enough
to perform the same operation speed. Consequently, power
dissipation could be conserved (refer to Figure 70.) The main clock
division selection bit is valid regardless of either using the sub clock
or not.
Figure 67 shows the system clock state transition when using the
sub clock.
From the time during reset to the time reset is released, only the
main clock, which is selected as the system clock, oscillates.
If the port-X
C
selection bit is set to “1” in this term, the sub-clock
oscillation circuit starts oscillation. When the sub clock is not used,
fix the port-X
C
selection bit to “0” (“0” at reset) and use the P7
7
/AN
7
X
CIN
and P7
6
/AN
6
/X
COUT
pins as I/O ports P7
7
and P7
6
or analog
inputs AN
7
and AN
6
, respectively.
Table 10. Selection of system clock and clock f
2
0
0
0
0
1
1
1
1
Main clock
Main clock
Main clock divided by 8
Main clock divided by 8
Main clock
Main clock
Sub clock
Sub clock
Clock f
2
Main clock divided by 2
Main clock
Main clock divided by 16
Main clock divided by 8
Main clock divided by 2
Main clock
Sub clock divided by 2
Sub clock divided by 2
Port-Xc
selection bit
(CM
4
)
System clock
selection bit
(CM
3
)
0
0
1
1
0
0
1
1
Main clock
division selection
bit (CC
0
)
0
1
0
1
0
1
0
1
System clock
Sub clock
Not used
Used