Data Sheet

9

Rev. 1.2, 2009-04-28

TLE7278-2

Block Description and Electrical Characteristics

5.1.2

Watchdog Operation

The watchdog uses a fraction of the charge pump oscillator’s clock signal as timebase. The watchdog timebase

can be adjusted using the pins WM1 and WM2 (see Figure 5). The watchdog can be turned off setting WM1 and

WM2 to high level. The timing values used this text refer to typ. values with WM1 and WM2 connected to GND

(fast watchdog and reset timing).

If the timebase is switched by changing the condition on the WM pins, the new timing is valid from the beginning

of the new period on. From this time on, the frequency on the WDI pin must be adapted.

Figure 5 shows the state diagram of the watchdog (WD) and the mode selection. After power-on, the reset output

to HIGH transition of the signal at RO the device starts the ignore window time

t

the Watchdog Period (time frame within a trigger at WDI must occur). From now on the timing of the signal on WDI

from the micro controller must correspond the WD-Period

t

based on the setting on the WM pins. A Re-Trigger of the WD-Period is done with a HIGH-to-LOW Transient at

the WDI-pin within the set

t

A HIGH to LOW transition of the watchdog trigger signal on pin WDI is taken as a trigger. To avoid wrong triggering

due to parasitic glitches two HIGH samples followed by two LOW samples (sample period

t

decoded as a valid trigger. A reset is generated (RO goes LOW) if there is no trigger pulse during the Watchdog

Period.

Figure 5

Watchdog Timing Diagram, Watchdog and Reset Modes

t

V

Q

Trigger

Window

WDI

T

RD

Ingnore

Wnd

1. Correct Trigger

No Trigger

causing reset

t

rr

t

t

t

t

t

rr

T

RD

Power

Fail

I

Q

t

I

Q, WD_ON

I

Q, WD_OFF

Current Controlled

WD-turn off

No Reset during

Current shut down

Normal operation

Don’t care WDI

during IW

t

sam

t

WD,p

WM1

WM2

Watchdog

Mode

Reset Mode

H

L

Fast

Slow

L

L

Fast

Slow

H

L

Fast

Slow

H

H

Slow

Off

7278 Timing