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14.7.4
Switching of External Event Signal and Operation of Edge Detection Circuit
When the external event signal is changed, the edge detection circuit may regard it as a rising or
falling edge. Table 14.4 shows the relationship between the timing at which the external event
signal is switched (bits IS2 to IS0 are rewritten) and operation of the edge detection circuit.
Rising and falling edges of the external event signal are detected by the edge detection circuit.
Thus, if the clock is changed when the old source is low and the new source is high, as in case no.
2 in table 14.4, the changeover is regarded as a rising edge and the TWCNT value is transferred to
TWICR during duty measurement. If the clock is changed when the old source is high and the new
source is low, as in case no. 3 in table 14.4, the changeover is regarded as a falling edge and duty
measurement is started or ended. To prevent incorrect operation, ensure that the IS bit should not
be rewritten during duty measurement.
Table 14.4 Switching of External Event Signal and Operation of Edge Detection Circuit
Timing of Switchover by
No.
Means of IS2 to IS0 Bits
1
Switching from
low to low
2
Switching from
low to high
Rev. 1.00, 09/03, page 414 of 704
Operation of Edge Detection Circuit
External event signal
before switchover
External event signal
after switchover
Rising edge
detection signal
Falling edge
detection signal
External event signal
before switchover
External event signal
after switchover
Rising edge
detection signal
Falling edge
detection signal
Switchover of IS bit
Switchover of IS bit
1
*
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