PC87570-ICCSLASHVUL datasheet(110/168 Pages) NSC

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Multi-Function 16-Bit Timer (MFT16)

110

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Mode 2, Dual Input Capture

Dual capture mode can be used to precisely measure the

frequency of an external clock that is slower than the select-

ed clock source frequency, or to measure the elapsed time

between external events. A transition received on the TA or

TB pin causes a transfer of the timer/counter 1 contents to

the TCRA or TCRB register, respectively. In this mode tim-

er/counter 2 can be utilized as a system timer which is pre-

loaded by the user software and will generate an interrupt

on underflow.

Figure 14-5 shows a block diagram of the timer operating in

mode 2. In this mode of operation the timebase of the cap-

ture timer is formed by counter I, which counts down with

the clock selected via the counter 1 clock selector. In the

dual-input capture mode, the TA and TB pins function as

capture inputs. A transition received on the TA pin causes a

transfer of the timer contents to the TCRA register. Similar-

ly, a transition received on the TB pin causes a transfer of

the timer contents to the TCRB register.

The TA and TB inputs can be configured to perform a

counter preset to FFFFh on reception of a valid capture

event. In this case the current value of the counter is trans-

ferred to the corresponding capture register and then the

counter is preset to FFFFh. Using this approach allows you

to determine directly the on-time, off-time, or period of an

external signal while reducing CPU overhead.

The pulse width of the input signal on TA and TB must be

equal to or greater than one system clock cycle. (See the

AC Electrical Specs in Chapter 3 for additional details.) The

values captured in the TCRA register at different times re-

flect the elapsed time between transitions on the TA pin.

The same is true for the TCRB register and TB pin. Each in-

put pin can be configured to sense either positive-going or

negative-going transitions.

The timer can be configured to generate interrupts on re-

ception of a transition on either TA or TB, which can en-

abled or disabled separately by the TAIEN and TBIEN bits.

An underflow of TCNT1 can also generate an interrupt, if

enabled by the TCIEN bit. All three interrupts have individ-

ual pending flags associated with them. (See Section 14.4.2

on page 113 for detailed information.)

The second timer/counter 2 can be used as a simple system

timer in this mode of operation. The counter TCNT2 counts

down with the clock selected via the counter 2 clock selec-

tor, and can be configured to generate an interrupt on un-

derflow, if enabled by the TDIEN bit. (See Section 14.4.2 on

page 113 for detailed information.)

Note that TCNT1 cannot operate in the “Pulse Accumulate”

or “External Event Counter” modes of operations since the

TB input is used as a capture input. Selecting either “Pulse

Accumulate” mode or “External Event Counter” mode for

TCNT1 causes TCNT1 to stop.

However, all available clock source modes may be selected

for TCNT2. Thus it is possible to determine the number of

capture events on TB, or the elapsed time between capture

events on TB by using TCNT2.

Capture A

Timer/Counter 1

Capture B

TCNT1

TCRA

TCRB

Timer/Counter 2

TCNT2

Timer1

Clock

Timer 2

Clock

TAIEN

TAPND

Timer

Interrupt 1

TBIEN

TBPND

Timer

Interrupt 1

TCIEN

TCPND

Timer

Interrupt 1

Underflow

TDIEN

TDPND

Timer

Interrupt 2

Underflow

TAEN

Preset

TBEN

Figure 14-5. Mode 2, Dual Input Capture

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