
119
μPD780948, μPD78F0948, μPD780949, μPD78F0949
6.6
Changing System Clock and CPU Clock Settings
6.6.1 Time required for switchover between system clock and CPU clock
The system clock and CPU clock can be switched over by means of bit 0 to bit 2 (PCC0 to PCC2) and
bit 4 (CSS) of the processor clock control register (PCC).
The actual switchover operation is not performed directly after writing to the PCC, but operation continues
on the pre-switchover clock for several instructions (see Table 6-2).
Determination as to whether the system is operating on the main system clock or the subsystem clock
is performed by bit 5 (CLS) of the PCC register.
Table 6-2: Maximum Time Required for CPU Clock Switchover
Caution:
Selection of the CPU clock cycle scaling factor (PCC0 to PCC2) and switchover from
the main system clock to the subsystem clock (changing CSS from 0 to 1) should not
be performed simultaneously. Simultaneous setting is possible, however, for selection
of the CPU clock cycle scaling factor (PCC0 to PCC2) and switchover from the subsystem
clock to the main system clock (changing CSS from 1 to 0).
Remarks:
1.
One instruction is the minimum instruction execution time with the pre-switchover CPU clock.
Set Values after Switchover
Set Values before Switchover
MCS CSS
PCC2 PCC1 PCC0
CSS PCC2PCC1PCC0 CSS PCC2PCC1PCC0 CSS PCC2PCC1PCC0 CSS PCC2PCC1PCC0 CSS PCC2PCC1PCC0 CSS PCC2PCC1PCC0
0
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
1
X
X
X
X
0
0
0
0
8 instructions
4 instructions 2 instructions 1 instruction
1 instruction
0
0
1
16 instructions
4 instructions 2 instructions 1 instruction
1 instruction
0
1
0
16 instructions
8 instructions
2 instructions 1 instruction
1 instruction
0
1
1
16 instructions
8 instructions
4 instructions
1 instruction
1 instruction
1
0
0
16 instructions
8 instructions
4 instructions 2 instructions
1 instruction
1
1
X
X
X
f
X
/2f
XT
instruction
f
X
/4f
XT
instruction
f
X
/8f
XT
instruction
f
X
/16f
XT
instruction
f
X
/32f
XT
instruction
(77 instructions)
(39 instructions)
(20 instructions)
(10 instructions)
(5 instructions)
0
f
X
/4f
XT
instruction
f
X
/8f
XT
instruction
f
X
/16f
XT
instruction
f
X
/32f
XT
instruction
f
X
/64f
XT
instruction
(39 instructions)
(20 instructions)
(10 instructions)
(5 instructions)
(3 instructions)