
585
(3) Note on Switchover of External Clock
When two or more external clocks (e.g. 10 MHz and 2 MHz) are used as the system clock,
switchover of the input clock should be carried out in software standby mode.
An example of an external clock switching circuit is shown in figure 16-7, and an example of the
external clock switchover timing in figure 16-8.
External clock 1
External clock 2
External clock
switchover request
External interrupt signal
External clock
switchover signal
Control
circuit
H8S/2214
Port output
External
interrupt
EXTAL
Selector
Figure 16-7 Example of External Clock Switching Circuit
200 ns or more
(2)
(1)
(2)
(3)
(4)
(5)
Port setting (clock switchover)
Software standby mode transition
External clock switchover
External interrupt generation
(Input interrupt at least 200 ns after transition to software standby mode.)
Interrupt exception handling
(5)
SLEEP instruction
execution
Interrupt exception handling
Operation
External
clock 1
External
clock 2
(1)
Port setting
(3)
External clock
switchover
signal
EXTAL
Internal
clock
(4)
Wait time
External
interrupt
Active (external clock 2)
Software standby mode
Active (external clock 1)
Clock switchover
request
Figure 16-8 Example of External Clock Switchover Timing