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14.7.3
Switching of Internal Clock and TWCNT Operation
The changeover may cause TWCNT to be incremented depending on the timing at which the
internal clock is switched (bits CKS2 to CKS0 are rewritten). Table 14.3 shows the relationship
between the timing and TCNT operation.
The TWCNT clock is generated on detection of the falling edge of the internal clock. If the clock
is changed when the old source is high and the new source is low, as in case no. 3 in table 14.3, the
changeover is regarded as a falling edge that generates the TWCNT clock, and TWCNT is
incremented.
Switching between an internal clock and external clock may also cause TWCNT to be
incremented. To prevent incorrect operation, ensure that TWCNT should be stopped before an
internal clock is changed.
Table 14.3 Switching of Internal Clock and TWCNT Operation
No.
Timing of Switchover by
Means of CKS2 to CKS0
Bits
TWCNT Operation
1
Switching from
low to low
*
1
Clock
before switchover
Clock
after switchover
TWCNT clock
TWCNT
Switchover of CKS bit
N
N+1
2
Switching from
low to high
*
2
N
N+1
N+2
Clock
before switchover
Clock
after switchover
TWCNT clock
TWCNT
Switchover of CKS bit
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