PC87570 datasheet(111/168 Pages) NSC

111 / 168 page

Multi-Function 16-Bit Timer (MFT16)

111

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Mode 3, Dual Independent Timer

Dual Independent Timer mode can be used for a wide vari-

ety of system tasks such as the generation of period system

interrupts, either based on the prescaled clock or external

events on TB. The timer can also toggle the TA pin on un-

derflow allowing the simple generation of a processor-inde-

pendent 50% duty cycle PWM signal on TA. In this mode

TCNT1 counts down, and reloads from TCRA on underflow

while TCNT2 is reloaded from TCRB on underflow.

In this mode the timer is configured to operate as a dual in-

dependent system timer, or dual external event counter. In

addition, timer/counter 1 can generate a 50% duty cycle

PWM signal on the TA pin. The TB pin can be used as an

external event input, or pulse accumulate input, and forms

the clock source to either counter 1 or counter II, as de-

scribed above. Both counters can also be operated from the

prescaled system clock. Figure 14-6 shows a block diagram

of the timer in mode 3.

Timer/counter 1 (TCNT1) counts down at the rate of the se-

lected clock. (See section “Counter Clock Source Select” on

page 108 for additional details). On underflow TCNT1 is re-

loaded from the TCRA register, and counting proceeds. If

enabled, the TA pin toggles on underflow of TCNT1. Soft-

ware can select the initial value of the TA output as either

high or low. See “Timer I/O Functions” on page 113 for ad-

ditional details. In addition, the TAPND interrupt pending

flag is set, and a timer interrupt 1 generated, if the TAIEN bit

is set to 1. (See Section “Timer Interrupts” on page 113 for

detailed information.) Since TA toggles on every underflow,

a 50% duty cycle PWM signal can be generated on TA with-

out requiring any interaction of the user software, and hence

the CPU.

Timer/counter 2 (TCNT2) counts down at the rate of the se-

lected clock. (See Section “Counter Clock Source Select”

on page 108 additional details). On every underflow of

TCNT2 the value contained in the TCRB register is loaded

into TCNT2, and counting proceeds downwards from that

value. In addition, the TDPND interrupt pending flag is set,

and a timer interrupt 2 is generated if the TDIEN bit is set to

1. (See Section 14.4.2 on page 113 for detailed informa-

tion.)

Reload A

Timer/Counter 1

Reload B

Timer 1

Clock

TAIEN

TAPND

TCNT1

TCRA

TCRB

TDIEN

TDPND

Timer

Interrupt 1

Timer

Interrupt 2

TAEN

Timer/Counter 2

TCNT2

Timer 2

Clock

Selector

Underflow

Figure 14-6. Mode 3, Dual Independent Timer

PC87570 Datasheet(PDF) 111 Page - National Semiconductor (TI)

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