Hardware manual

Rev. 3.0, 09/98, page 169 of 361
8.3 Operation
8.3.1 TCNT Incrementation Timing
The timer counter increments on a pulse generated once for each period of the selected (internal or
external) clock source.
Internal Clock: Internal clock sources are created from the system clock by a prescaler. The
counter increments on an internal TCNT clock pulse generated from the falling edge of the
prescaler output, as shown in figure 8.2. Bits CKS2 to CKS0 of the TCR and bits ICKS1 and
ICKS0 of the STCR can select one of the six internal clocks.
N
N - 1
N + 1
Internal clock
TCNT clock pulse
Ø
TCNT
Figure 8.2 Count Timing for Internal Clock Input
External Clock: If external clock input (TMCI) is selected, the timer counter can increment on
the rising edge, the falling edge, or both edges of the external clock signal. Figure 8.3 shows
incrementation on both edges of the external clock signal.
The external clock pulse width must be at least 1.5 system clock periods for incrementation on a
single edge, and at least 2.5 system clock periods for incrementation on both edges. See
figure 8.4. The counter will not increment correctly if the pulse width is shorter than these values.