Datasheet

Section 5 Clock Pulse Generator
REJ09B0465-0300 Rev. 3.00 Page 131 of 982
Sep 17, 2010
H8S/20103, H8S/20203, H8S/20223, H8S/20115, H8S/20215, H8S/20235 Group
5.3.3 Backup Operation
If the operating clock for the system is φosc and the backup function is enabled, when the main
oscillator detects an oscillation halt condition, the system clock automatically switches to φlow,
according to BAKCKSEL in BAKCR. The period from the stopping of the main oscillator to the
time the system clock is operating from φlow will be clock halt detection time + oscillation
stabilization wait time for the backup clock. Time to wait for oscillation stabilization is 0 ms if the
target backup clock is already oscillating when stopping of the main oscillator is detected.
To reduce power consumption, it is also possible to set the target backup clock to a stopped state
when the backup function is enabled. In that case, the target backup clock is automatically
activated when stopping of the main clock oscillator is detected. The system clock is then changed
after a certain amount of oscillation settling time. This LSI circuit may malfunction during
operation with the back-up function. Accordingly, usage with the watchdog timer is
recommended.
OSCHLT
BAKCLK
φ OSC
φ base
Operation at main oscillator clock
φOSC stops
Clock switching
time
Operation at backup destination clock
[Legend]
φOSC: Main oscillator clock
OSCHLT: Main oscillator clock stop detection signal
BAKCLK: Backup destination clock
φbase: System base clock
Figure 5.8 Timing of Backup Operation When Main Oscillator Stops at High Level
(When the backup destination clock is active)