Datasheet

78K0/Ix2 CHAPTER 5 CLOCK GENERATOR
R01UH0010EJ0500 Rev.5.00 184
Feb 28, 2012
5.6.6 Time required for switchover of CPU clock and main system clock
By setting bits 0 to 2 (PCC0 to PCC2) and bit 4 (CSS) of the processor clock control register (PCC), the division ratio of
the main system clock can be changed.
The actual switchover operation is not performed immediately after rewriting to PCC; operation continues on the pre-
switchover clock for several clocks (refer to Table 5-6).
Table 5-6. Time Required for Switchover of CPU Clock and Main System Clock Cycle Division Factor
Set Value Before
Switchover
Set Value After Switchover
PCC2 PCC1 PCC0 PCC2 PCC1 PCC0 PCC2 PCC1 PCC0 PCC2 PCC1 PCC0 PCC2 PCC1 PCC0PCC2 PCC1 PCC0
0 0 0 0 0 1 0 1 0 0 1 1 1 0 0
0 0 0 16 clocks 16 clocks 16 clocks 16 clocks
0 0 1 8 clocks 8 clocks 8 clocks 8 clocks
0 1 0 4 clocks 4 clocks 4 clocks 4 clocks
0 1 1 2 clocks 2 clocks 2 clocks 2 clocks
1 0 0 1 clock 1 clock 1 clock 1 clock
Remark The number of clocks listed in Table 5-6 is the number of CPU clocks before switchover.
By setting bit 0 (MCM0) of the main clock mode register (MCM), the main system clock can be switched (between the
internal high-speed oscillation clock and the high-speed system clock).
The actual switchover operation is not performed immediately after rewriting to MCM0; operation continues on the pre-
switchover clock for several clocks (refer to Table 5-7).
Whether the CPU is operating on the internal high-speed oscillation clock or the high-speed system clock can be
ascertained using bit 1 (MCS) of MCM.
Caution Be sure to stop the operation of the PLL when switching the main system clock.
Table 5-7. Maximum Time Required for Main System Clock Switchover
Set Value Before Switchover Set Value After Switchover
MCM0 MCM0
0 1
0 1 + 2fIH/fXH clock
1 1 + 2fXH/fIH clock
Caution When switching the internal high-speed oscillation clock to the high-speed system clock, bit 2
(XSEL) of MCM must be set to 1 in advance. The value of XSEL can be changed only once after a
reset release.
Remarks 1. The number of clocks listed in Table 5-7 is the number of main system clocks before switchover.
2. Calculate the number of clocks in Table 5-7 by removing the decimal portion.
Example When switching the main system clock from the internal high-speed oscillation clock to the high-speed
system clock (@ oscillation with f
IH = 4 MHz, fXH = 10 MHz)
1 + 2f
IH/fXH = 1 + 2  4/10 = 1 + 2  0.4 = 1 + 0.8 = 1.8  1 clock