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Rev. 1.00, 05/04, page 218 of 544
10.10.5 Switching of Internal Clocks and TCNT Operation
TCNT may increment erroneously when the internal clock is switched over. Table 10.9 shows the
relationship between the timing at which the internal clock is switched (by writing to the CKS1
and CKS0 bits) and the TCNT operation.
When the TCNT clock is generated from an internal clock, the falling edge of the internal clock
pulse is detected. If clock switching causes a change from high to low level, as shown in no. 3 in
table 10.9, a TCNT clock pulse is generated on the assumption that the switchover is a falling
edge, and TCNT is incremented.
Erroneous incrementation can also happen when switching between internal and external clocks.
Table 10.9 Switching of Internal Clocks and TCNT Operation
No.
Timing of Switchover
by Means of CKS1
and CKS0 Bits
TCNT Clock Operation
1 Clock switching from low to
low level*
1
Clock before
switchover
Clock after
switchover
TCNT
clock
TCNT
CKS bit rewrite
N N + 1
2 Clock switching from low to
high level∗
2
Clock before
switchover
Clock after
switchover
TCNT
clock
TCNT
CKS bit rewrite
N N + 1 N + 2