Datasheet

Rev.6.00 Oct.28.2004 page 451 of 1016
REJ09B0138-0600H
12.6.5 Switching of Internal Clocks and TCNT Operation
TCNT may increment erroneously when the internal clock is switched over. Table 12-5 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 case 3 in table 12-5, a TCNT clock pulse is generated on
the assumption that the switchover is a falling edge. This increments TCNT.
The erroneous incrementation can also happen when switching between internal and external clocks.
Table 12-5 Switching of Internal Clock and TCNT Operation
No.
Timing of Switchover
by Means of CKS1
and CKS0 Bits TCNT Clock Operation
1 Switching from
low to low*
1
Clock before
switchover
Clock after
switchover
TCNT clock
TCNT
CKS bit write
N N+1
2 Switching from
low to high*
2
Clock before
switchover
Clock after
switchover
TCNT clock
TCNT
CKS bit write
N N+1 N+2
3 Switching from
high to low*
3
Clock before
switchover
Clock after
switchover
TCNT clock
TCNT
CKS bit write
N N+1 N+2
*
4