13

LTC1235

S

APPLICATI

I FOR ATIO

The 10.7ms allows enough time to execute shut-down

procedure for microprocessor and 831mV of hysteresis

Example 2: The circuit in Figure 9 can be used to measure

the regulated 5V supply to provide early warning of power

failure. Because of variations in the PFI threshold, this

circuit requires adjustment to ensure that the PFI com-

parator trips before the reset threshold is reached. Adjust

reaches the desired level (e.g., 4.85V).

Monitoring the Status of the Battery

C3 can also monitor the status of the memory backup

battery (Figure 10). If desired, the CE OUT can be used to

apply a test load to the battery. Since CE OUT is forced high

in battery backup mode, the test load will not be applied to

the battery while it is in use, even if the microprocessor is

not powered.

Figure 11. Watchdog Time-out Period and Reset Active Time

Watchdog Timer

The LTC1235 provides a watchdog timer function to

monitor the activity of the microprocessor. If the micro-

processor does not toggle the Watchdog Input (WDI)

within the time-out period, the reset outputs are forced to

active states for a minimum of 140ms. The watchdog

time-out period is fixed at 1.0 second minimum on the

LTC1235. This time-out period provides adequate time for

many systems to service the watchdog timer immediately

after a reset. Figure 11 shows the timing diagram of

watchdog time-out period and reset active time. The

watchdog time-out period is restarted as soon as the reset

outputs are inactive. When either a high-to-low or low-to-

high transition occurs at the WDI pin prior to time-out, the

watchdog time is reset and begins to time out again. To

ensure the watchdog time does not time out, either a high-

to-low or low-to-high transition on the WDI pin must

occur at or less than the minimum time-out period. If the

input to the WDI pin remains either high or low, reset

pulses will be issued every 1.6 seconds typically. The

watchdog timer can be deactivated by floating the WDI pin.

The Watchdog Output, WDO, goes low if the watchdog

timer is allowed to time out and remains low until set high

by the next transition on the WDI pin. WDO is also set high

+3V

OPTIONAL TEST LOAD

+5V

LTC1235 F09

GND

PFI

LTC1235

R1

1M

RL 20K

R2

1M

PFI

CE IN

LOW BATTERY SIGNAL

TO µP I/O PIN

TO

µP I/O PIN

PFO

CE OUT

BACKUP

}

Figure 10. Backup Battery Monitor with Optional Test Load

t1 = RESET ACTIVE TIME

t2 = WATCHDOG TIME-OUT PERIOD

t1

WDO

WDI

RESET

t2

t1

LTC1235 F11

t1

t2