Hitachi H8/329 Series Hardware Manual page 150

(4) Incrementation Caused by Changing of Internal Clock Source: When an internal clock
source is changed, the changeover may cause the FRC to increment. This depends on the time at
which the clock select bits (CKS1 and CKS0) are rewritten, as shown in table 6-5.
The pulse that increments the FRC is generated at the falling edge of the internal clock source. If
clock sources are changed when the old source is High and the new source is Low, as in case No. 3
in table 6-5, the changeover generates a falling edge that triggers the FRC increment clock pulse.
Switching between an internal and external clock source can also cause the FRC to increment.
Table 6-5. Effect of Changing Internal Clock Sources
No. Description
Low → Low:
CKS1 and CKS0 are
1 rewritten while both
clock sources are Low.
Low → High:
CKS1 and CKS0 are
2 rewritten while old
clock source is Low and
new clock source is High.
Timing chart
Old clock
source
New clock
source
FRC clock
pulse
N
FRC
CKS rewrite
Old clock
source
New clock
source
FRC clock
pulse
N
FRC
141
N + 1
N + 1 N + 2
CKS rewrite