NEC PD750008 User Manual
Page 108
88
µPD750008 USER'S MANUAL
(2) System clock control register (SCC)
The SCC is a 4-bit register for selecting CPU clock
F
with the least significant bit and for controlling the
termination of main system clock generation with the most significant bit (see Figure 5-13).
Bits 0 and 3 of the SCC are located at the same data memory address, but both bits cannot be changed
at the same time. Accordingly, bits 0 and 3 of the SCC are set using bit manipulation instructions. Bits
0 and 3 of the SCC can be manipulated regardless of MBE setting.
Main system clock generation can be terminated by setting bit 3 of the SCC only when the subsystem
clock is used for operation. The STOP instruction must be used to terminate main system clock generation.
A RESET signal clears the SCC to 0.
Figure 5-13. Format of the System Clock Control Register
Cautions 1. A time period of up to 1/f
XT
is needed to change the system clock. This means
that to terminate main system clock generation, bit 3 of the SCC must be set to 1
when the machine cycles indicated in Table 5-4 or more have elapsed after the
clock is switched from the main system clock to the subsystem clock.
2. When the main system clock is used for operation, setting bit 3 of the SCC to
stop clock generation does not enter the normal STOP mode.
3. When the PCC is set to 0001B (
F
F
F
F
F
= f
X
/16), do not set SCC.0 to 1. Before switch-
ing the main system clock to the subsystem clock, be sure to manipulate the
PCC so other than 0001B is set. When the system operates on the subsystem
clock, the PCC must also be other than 0001B.
4. When SCC.3 is set to 1, the X1 input pin is connected to V
SS
(ground electric
potential) to prevent leakage in the crystal oscillator. When an external clock is
used as the main system clock, never set SCC.3 to 1.
Address
FB7H
SCC3
—
—
SCC0
Symbol
SCC
CPU clock frequency
Main system clock
Main system clock operation
Subsystem clock
Can oscillate
Subsystem clock
0
0
1
0
0
1
1
1
Oscillation stopped
SCC0
SCC3
Not to be set
3
2
1
0