Power supply problems and software lockups are amongst the most common issues…
Power supply problems and software lockups are amongst the most common issues plaguing embedded systems. Watchdog timers alleviate the latter – they are now embedded in almost all microcontrollers. Some standards, however, demand the use of dedicated IC – in this case, Texas Instruments TPS7B68x is just the part you need.
Coming from a background of voltage regulator design, it is obvious that Texas Instruments integrates all kinds of advanced features into the IC. In particular, the part presents itself as shown in figure one.
…The TPS7B68x family combines a variety of useful features.
First of all, automotive applications are known for their voltage ranges. TI takes this into account by allowing an input range of up to 40 V. Output voltage is set by the device type, and both 3V3 and 5V parts are available. Furthermore, with sufficient cooling, all of the parts are able to supply to 500 mA of current – this, of course, requires the presence of a large ground plane for heat dissipation.
In addition to acting as a voltage regulator, the part provides a power good threshold pin. The pin will be enabled when the voltage output has reached the defined threshold, thereby preventing BOR issues.
And watch over it…
The main feature, however, is the watchdog timer. Texas Instruments allows developers to choose between normal and window-based watchdog operation by setting the voltage level present at a specific pin. Window-based watchdog timers are especially interesting in that they control both over- and underflow scenarios in a fashion similar to the one outlined in figure two.
Fret not -the TPS family can be used without a dedicated crystal. TI implements an RC oscillator on the part, whose frequency can be tuned with a single external resistor – as controllers get faster, the watchdog window can be narrowed to reduce downtime.