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Section 12 8-Bit Timer (TMR)
12.9.5
Switching of Internal Clocks and TCNT Operation
TCNT may increment erroneously when the internal clock is switched over. Table 12.8 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 12.8, 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 12.8 Switching of Internal Clocks and TCNT Operation
Timing of Switchover
by Means of CKS1
No.
and CKS0 Bits
1
Clock switching from low
1
to low level*
2
Clock switching from low
2
to high level*
Rev. 1.00 May 09, 2008 Page 346 of 954
REJ09B0462-0100
TCNT Clock Operation
Clock before
switchover
Clock after
switchover
TCNT
clock
TCNT
N
Clock before
switchover
Clock after
switchover
TCNT
clock
TCNT
N
N + 1
CKS bit rewrite
N + 1
N + 2
CKS bit rewrite
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