Changing System Clock And Cpu Clock Settings; Time Required For Switchover Between System Clock And Cpu Clock; Maximum Time Required For Cpu Clock Switchover - NEC mPD780208 Subseries User Manual

5.6 Changing System Clock and CPU Clock Settings

5.6.1 Time required for switchover between system clock and CPU clock

The system clock and CPU clock can be switched over by using bits 0 to 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; operation continues on the
pre-switchover clock for several instructions (see Table 5-3).
It can be judged by bit 5 (CLS) of PCC whether the system is operating on the main system clock or the subsystem
clock.
Table 5-3. Maximum Time Required for CPU Clock Switchover
Set Values
Before Switchover
CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0
CSS PCC2 PCC1 PCC0
0 0
0 0 0
0 0 1 8 instructions
4 instructions
0 0 1 0
0 1 1 2 instructions
1 0 0
1 instruction
x x x
1 1 instruction
Caution Selection of the CPU clock cycle division ratio (PCC0 to PCC2) and switchover from the main
system clock to the subsystem clock (changing CSS from 0 to 1) should not be specified
simultaneously.
Simultaneous setting is possible, however, for selection of the CPU clock cycle division ratio
(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.
2. Figures in parentheses apply to operation with f
CHAPTER 5 CLOCK GENERATOR
0 0 0 0 0
1 0
16 instructions 16 instructions
8 instructions
4 instructions
2 instructions 2 instructions
1 instruction 1 instruction
1 instruction 1 instruction
User's Manual U11302EJ4V0UM
Set Values After Switchover
0 1 0 0 0 1 1
16 instructions
8 instructions
4 instructions
1 instruction
1 instruction
= 5.0 MHz or f
X
x x
0 1 0 0 1
f /2f
X XT
16 instructions
instructions
(64 instructions)
f /4f
X XT
8 instructions instructions
(32 instructions)
f /8f
X XT
4 instructions
instructions
(16 instructions)
f /16f
X XT
2 instructions instructions
(8 instructions)
f /32f
X XT
instructions
(4 instructions)
1 instruction
= 32.768 kHz.
XT
x
117