Hardware manual

Rev. 3.0, 09/98, page 146 of 361

7.4 Operation

7.4.1 FRC Incrementation Timing

The FRC increments on a pulse generated once for each period of the selected (internal or

external) clock source. The clock source is selected by bits CKS0 and CKS1 in the TCR.

Internal Clock: The internal clock sources (φ/2, φ/8, φ/32) are created from the system clock (φ)

by a prescaler. The FRC increments on a pulse generated from the falling edge of the prescaler

output. See figure 7.4.

N - 1

N + 1

Internal

clock

FRC clock

pulse

FRC

Figure 7.4 Increment Timing for Internal Clock Source

External Clock: If external clock input is selected, the FRC increments on the rising edge of the

FTCI clock signal. Figure 7.5 shows the increment timing.

The pulse width of the external clock signal must be at least 1.5 system clock (φ) periods. The

counter will not increment correctly if the pulse width is shorter than 1.5 system clock periods.