Changing Setting Of System Clock And Cpu Clock; Time Required For Switching Between System Clock And Cpu Clock - NEC UPD789426 Series User Manual

5.6 Changing Setting of System Clock and CPU Clock

5.6.1 Time required for switching between system clock and CPU clock

The CPU clock can be selected by using bit 1 (PCC1) of the processor clock control register (PCC) and bit 4
(CSS0) of the subclock control register (CSS).
Actually, the specified clock is not selected immediately after the setting of PCC has been changed, and the old
clock is used for the duration of several instructions after that (see Tables 5-2 and 5-3).
Table 5-2. Maximum Time Required for Switching CPU Clock (When Crystal/Ceramic Oscillation Is Selected)
Set Value Before Switching
CSS0 PCC1
0 0
1
1 x
Remarks 1. Two clocks are the minimum instruction execution time of the CPU clock before switching.
2. The parenthesized values apply to operation at f
3. x: don't care
Table 5-3. Maximum Time Required for Switching CPU Clock (When RC Oscillation Is Selected)
Set Value Before Switching
CSS0 PCC1
0 0
1
×
1
Remarks 1. Two clocks are the minimum instruction execution time of the CPU clock before switching.
2. The parenthesized values apply to operation at f
3. ×: Don't care
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CHAPTER 5 CLOCK GENERATOR
CSS0 PCC1
0 0
2 clocks
2 clocks
CSS0 PCC1
0 0
2 clocks
2 clocks
User's Manual U15075EJ2V1UD
Set Value After Switching
CSS0 PCC1
0 1
4 clocks
2 clocks
= 5.0 MHz or f = 32.768 kHz.
X XT
Set Value After Switching
CSS0 PCC1
0 1
4 clocks
2 clocks
= 4.0 MHz or f = 32.768 kHz.
CC XT
CSS0 PCC1
1 x
2f /f clocks
X XT
(306 clocks)
f /2f clocks
X XT
(76 clocks)
CSS0 PCC1
×
1
2f /f clocks
CC XT
(244 clocks)
f /2f clocks
CC XT
(61 clocks)