LTC1235

13

1235fa

APPLICATIONS INFORMATION

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 mea-

sure 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

comparator trips before the reset threshold is reached.

Adjust R5 such that the PFO output goes low when the

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.

Watchdog Timer

The LTC1235 provides a watchdog timer function to monitor

the activity of the microprocessor. If the microprocessor

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 transi-

tion 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 timer

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 when

Figure 10. Backup Battery Monitor with Optional Test Load

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

t1 = RESET ACTIVE TIME

t2 = WATCHDOG TIME-OUT PERIOD

t1

WDO

WDI

RESET

t2

t1

1235 F11

t1

t2

+3V

OPTIONAL TEST LOAD

+5V

1235 F10

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