ISL88705 Просмотр технического описания (PDF) - Renesas Electronics

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ISL88705

µP Supervisor with Watchdog Timer, Power-Fail Comparator, Manual Reset and Adjustable Power-On Reset

Renesas Electronics 

ISL88705, ISL88706, ISL88707, ISL88708, ISL88716, ISL88813

.

CPOR

ISL88707, ISL88708

14

12

NORMALIZED tPOR vs CPOR (pF)

OPEN = 200ms

10

8

6

4

2

0

0 10 20 30 40 50 60 70 80 90 100

CPOR (pF)

FIGURE 3. ADJUSTING tPOR WITH A CAPACITOR

Manual Reset

The manual-reset input (MR) allows the user to trigger a

reset by using a push-button switch. The MR input is an

active low debounced input. By connecting a push-button

directly from MR to ground, the designer adds manual

system reset capability (see Figure 4). Reset is asserted if

the MR pin is pulled low to less than 100mV for the minimum

MR pulse width or longer while the push-button is closed.

After MR is released, the reset outputs remain asserted for

tPOR (200ms) and then released.

Watchdog Timer

The Watchdog Timer circuit checks microprocessor activity

by monitoring the WDI input pin. The microprocessor must

periodically toggle the WDI pin within tWDT (typically ~1.6s),

otherwise the WDO pin pulls low (see Figure 5). The WDO

then signals reset periodically (typically ~1.9s) for ~220ms

until the WDI is again toggled. Internally, the 1.6s timer is

cleared by either a reset or by toggling the WDI input, which

can detect pulses longer than 50ns.

Whenever there is a low-voltage VDD condition, WDO goes

low. Unlike the reset outputs, however, WDO does not have

a minimum reset pulse width (tPOR). WDO goes high as

soon as VDD rises above its voltage trip point (see Figure 5).

With WDI open or connected to a tristated high impedance

input, the Watchdog Timer is disabled and only pulls low

when VDD < VTH1.

20k

MR

PB

ISL8870x

FIGURE 4. CONNECTING A MANUAL RESET PUSH-BUTTON

FN8092 Rev 6.00

November 30, 2015

Page 8 of 16

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