TC1232 datasheet(9/24 Pages) MICROCHIP

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DS20001370E-page 9

TC1232

4.3

Watchdog Timer

When the ST input is not stimulated for a preset time

period, the Watchdog Timer function forces RST and

RST signals to the active state. The preset time period

is determined by the TD inputs to be 150 ms with TD

connected to ground, 600 ms with TD floating or

1200 ms with TD connected to VCC (typ.). The

Watchdog Timer starts timing-out from the set time

period as soon as RST and RST are inactive. If a high-

to-low transition occurs on the ST input pin prior to

time-out, the Watchdog Timer is reset and begins to

time-out again. If the Watchdog Timer is allowed to

time-out, the RST and RST signals are driven to the

active state for 250 ms, minimum (Figure 4-7).

The software routine that strobes ST is critical. The

code must be in a section of software that is executed

frequently enough so the time between toggles is less

than the Watchdog Time-out period. One common

technique controls the microprocessor I/O line from two

sections of the program. The software might set the I/O

line high while operating in the Foreground mode and

set it low while in the Background or Interrupt modes. If

both modes do not execute correctly, the Watchdog

Timer issues Reset pulses.

tTD is the maximum elapsed time between ST high-to-

low transitions (ST is activated by falling edges only),

which will keep the Watchdog Timer from forcing the

Reset outputs active for a time of tRST. tTD is a function

of the voltage at the TD pin, as tabulated below:

TABLE 4-1:

WATCHDOG TIMER

PERIODS

Figure 4-7 shows a block diagram for using the

TC1232 with a PIC® MCU and the Watchdog input.

FIGURE 4-7:

Watchdog Timer.

Figure 4-8 shows the expected Reset output pin wave-

forms depending on the period of the ST pin falling

edge and the state of the TD input pin.

FIGURE 4-8:

Strobe Input.

4.4

Supply Monitor Noise Sensitivity

The TC1232 is optimized for fast response to negative-

going changes in VDD. Systems with an inordinate

amount of electrical noise on VDD (such as systems

using relays) may require a 0.01 µF or 0.1 µF bypass

capacitor to reduce detection sensitivity. This capacitor

should be installed as close to the TC1232 as possible

to keep the capacitor lead length short.

Condition

tTD

Min.

Typ.

Max.

TD pin = 0V

62.5 ms

150 ms

250 ms

TD pin = Open

250 ms

600 ms

1000 ms

TD pin = VCC

500 ms

1200 ms

2000 ms

VCC

TOL GND

RST

RESET

+5V

TC1232

0.1

10 k



I/O

PIC®

3-Terminal

Regulator

+5V

µF

MCU

(example:

MCP1700)

tST

tTD

RST (when tTD  tTD (min))

“H”

RST

(when tTD(min)  tTD  tTD(max))

(when tTD  tTD(max))

RST

RST (when tTD  tTD(min))

“L”

RST

(when tTD(min)  tTD  tTD(max))

(when tTD  tTD(max))

RST

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