HD64F3437TF16 Datasheet, PDF (238/752 Pages) Hitachi Semiconductor – 12 V must not be applied to the S-mask model (single-power-supply specification), as this may permanently damage the device.

Table 9.5 Effect of Changing Internal Clock Sources
No. Description
1
Low → low^*1
Timing
Old clock
source
New clock
source
TCNT clock
pulse
2
Low → high^*2
TCNT
Old clock
source
New clock
source
TCNT clock
pulse
N
CKS rewrite
N+1
TCNT
N
N+1
N+2
CKS rewrite
209