Time Required For Cpu Clock Switchover - NEC 78K0 User Manual

6.7

Time Required for CPU Clock Switchover

The CPU clock can be switched 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 immediately after rewriting to the PCC; operation continues on
the pre-switchover clock for several instructions (see Table 6-6).
Whether the system is operating on the X1 input clock (or Ring-OSC clock) or the subsystem clock can be
ascertained using bit 5 (CLS) of the PCC register.
Table 6-6. Maximum Time Required for CPU Clock Switchover
Set Value 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 0 1 8 clocks
0 1 0 4 clocks
0 1 1 2 clocks
1 0 0 1 clock
× × ×
1 1 clock
Remarks 1. The maximum time is the number of clocks of the CPU clock before switching.
2. Figures in parentheses apply to operation with f
Caution Selection of the CPU clock cycle division factor (PCC0 to PCC2) and switchover from the X1
input clock to the subsystem clock (changing CSS from 0 to 1) should not be set
simultaneously.
Simultaneous setting is possible, however, for selection of the CPU clock cycle division factor
(PCC0 to PCC2) and switchover from the subsystem clock to the X1 input clock (changing CSS
from 1 to 0).
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CHAPTER 6 CLOCK GENERATOR
0 0 0 0 0 1 0
16 clocks
4 clocks
2 clocks
1 clock
1 clock
User's Manual U15947EJ2V0UD
Set Value After Switchover
0 1 0 0 0 1
16 clocks 16 clocks
8 clocks 8 clocks
4 clocks
2 clocks
1 clock 1 clock
*
1 clock 1 clock
= 10 MHz and f
XP
×
1 0 1 0 0 1
16 clocks f /f
XP XT
(306 clocks)
8 clocks f /2f
XP XT
(153 clocks)
4 clocks f /4f
XP XT
(77 clocks)
2 clocks f /8f
XP XT
(39 clocks)
f /16f
XP
(20 clocks)
1 clock
= 32.768 kHz.
XT
× ×
clocks
clocks
clocks
clocks
clocks
XT