PC87570-ICCSLASHVUL datasheet(112/168 Pages) NSC

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Multi-Function 16-Bit Timer (MFT16)

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Mode 4, Input Capture and Timer

It is also possible to operate in a mode which offers a com-

bination of a single timer with automatic reload and a single

capture timer. In this mode TCNT1 operates as a PWM-tim-

er which is reloaded from TCRA on underflow while TCNT2

forms the time base of the capture timer. The value on

TCNT2 is transferred to TCRB on detection of a valid event

on TB. It is possible to toggle TA on every underflow of

TCNT1 and thus generate a 50% duty cycle PWM signal on

TA.

This mode is a combination of mode 3 and mode 2, and al-

lows you to operate the timer/counter 2 as a single input

capture timer, while timer/counter 1 can be used as a sys-

tem timer as described above. Figure 14-7 shows a block

diagram of the timer in mode 4.

TCNT1 starts counting down once a clock has been en-

abled. On underflow TCNT1 is reloaded from the TCRA reg-

ister, and counting proceeds downwards from that value. If

enabled the TA pin toggles on every underflow of TCNT1.

Software can select the initial value of the TA output signal

as either high or low. See “Timer I/O Functions” on page

113 for additional details. In addition, the TAPND interrupt

pending flag is set, and a timer interrupt 1 is generated, if

the TAIEN bit is set to 1. (See Section “Timer Interrupts” on

page 113 for detailed information). Since TA toggles on ev-

ery underflow a 50% duty cycle PWM signal can be gener-

ated on TA without requiring any interaction of the user

software and thus the CPU.

TCNT2 starts counting down once a clock has been en-

abled. When a transition is received on TB the value con-

tained in TCNT2 is transferred to TCRB, and the interrupt

pending flag TBPND is set. A timer interrupt 2 is generated,

if it is enabled. You can enable a preset of the counter to

FFFFh on detection of a transition on TB. In this case the

current value of TCNT2 is transferred to TCRB, followed by

a preset of the counter to FFFFh. TCNT2 starts counting

downwards from FFFFh, until the next transition is received

on TB, which causes the procedure of capture and preset to

be repeated. Underflow of TCNT2 sets the TDPND interrupt

pending flag, and can also generate a timer interrupt II, if

enabled. (See Section “Timer Interrupts” for detailed infor-

mation.) The input signal on TB must have a pulse width

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

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

can be configured to sense either positive-going or nega-

tive-going transitions.

Note that TCNT2 can not 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

TCNT2 causes TCNT2 to stop.

However, all available clock source modes may be selected

for TCNT1. Thus it is possible to determine the number of

capture events on TB or the elapsed time between capture

events on TB by using TCNT1.

Reload A

Timer/Counter 1

Capture B

Timer 1

Clock

TAIEN

TAPND

TCNT1

TCRA

TCRB

Timer

Interrupt 1

TATEN

Timer/Counter 2

TCNT2

Timer 2

Clock

Underflow

TDIEN

TDPND

Timer

Interrupt 2

TBIEN

TBPND

Timer

Interrupt 1

TBEN

Preset

Figure 14-7. Mode 4, Input Capture and Timer

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