Running a happy chip

Running a happy chipHarry Yeates
Likebabies, microcontrollers, microprocessors, and memory need looking after. Unlike babies,this care regime involves reset ICs, voltage regulators and monitors, power failurewarning circuits, and watchdog timers. Babies only need a woolly hat and a bottle of milk.
However, similarities re-emerge when you consider that babies inevitably grow up andbecome more complex. While the traditional approach to power management in microcontrollerdevices involves relatively simple protection circuits, adding intelligence is adeveloping trend, with some applications using one microcontroller to supervise another.
At the standalone part level, the individual resets and regulators are subject to thehunger for higher levels of integration. In particular, integration’s the name of thegame for power management inside handheld devices such as cellphones and PDAs. As thesedwindle in size, saving board space is a top priority. Lower power, smaller packages andcombination parts continue the trend.
For example, companies might offer a number of voltage monitors together in the samepack, in multiple rails. Maxim’s new MAX6709/MAX6714 quad voltage monitors offerfactory-fixed thresholds for monitoring from 1.8V to 5.0V, in fifteen differentcombinations. The ZXCM209 series resets from UK analogue specialist Zetex offers amodified take on the popular MAX809 reset, reducing the supply current to 2?A, from 17?Ain the 809. Parts using SC70 packages save space over the already tiny SOT23.
The huge range of these supervisory parts, available from many different companies,covers every conceivable supply voltage, drop-out voltage, threshold and timingcombination. You simply look through the catalogues until you find what you need. Thereis, however, an alternative. You could use a microcontroller to implement the requiredfeatures.
“The traditional approach was you had the voltage regulator or supervisor circuitthat had a fixed trip or output voltage level,” says Volker Soffel of NationalSemiconductor’s microcontroller division. “For every different voltage level youbasically had to design a new device. That was pretty design intensive.
“The new thinking is if you put some intelligence on the same chip, in the form ofa microcontroller, and some non-virtual memory, you can make many of those thingsconfigurable easily.”
This means you can load different parameters, via some external interface, givingdifferent trip levels or different output voltages. “The microcontroller is primarilyused to get a larger degree of flexibility, for making the device configurable fordifferent voltage levels,” says Soffel.
An example system could be an airbag application, in which one microcontrollersupervises another, which is performing the actual function. In this case a watchdog timerfunction, and an integrated brownout circuit to detect voltage drops, are included to makethe system failsafe. These features are included in National’s COB8CBR32 controller.Soffel also points out that using a microcontroller as the supervisor means recoveriesfrom any illegal operating conditions can be performed in software.
Finally, a perspective on supervisors comes from Michael Bairanzade at ONSemiconductor, whose preoccupations are with interfaces. “There is a need for asmartcard interface, because a micro [in a card reader] can be powered from 3.3 or 1.8V,but the card itself can be anything from 1.8V to 3.0V and 5V,” he explains. “Youand I don’t know which card we have, so we need a clever interface, capable ofdetecting card type and adapting voltage according to I/O differences.” After all,you don’t want to fry your smart banking card whenever you want some cash.
The means of looking after your logic are expanding. Whether it’s systemflexibility, safety, power, size or cost you need to optimise, you can do it. Designersare only too aware that microprocessors and controllers, just like babies, need a bit ofTLC to keep them happy.