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Protecting outputs from freezing

Posted: 03 Dec 2015     Print Version  Bookmark and Share

Keywords:connectors  PCB  printed circuit board  Molex  RFID 

"Intelligent" output drivers enable you to monitor the output current to an external load that your micro is controlling. But what happens if your micro should freeze? What happens to the load when it is permanently on? An instance in my experience was driving the print head of a dot matrix printer, the long gone LRC 7040. Each pin on the 7-pin print head is activated by a solenoid that impels the pin forward causing the tip to compress the inked ribbon against the paper and leaving a small dot on the paper. If the solenoid is energized for longer than a certain period the coil overheats and burns out. The print head is toast! Since the micro is switching a solenoid there are likely to be EMI spikes, and in the prototype stages these may crash the micro, but you don't need a microcontroller malfunction for this to happen—it can actually happen during development if you stop at a breakpoint at an unfortunate stage in the execution of the code.

There could be even more disastrous consequences depending on what you are controlling. Compelled by Murphy's Law, the presumption that a microcontroller can and will malfunction at the worst possible time drives the need for a robust design. Given the above scenario, how can you approach your design so that an output will fail "safe" under a soft failure of the microcontroller?

One approach would be to use a retriggerable monostable to drive the output. In this case the firmware would have to clock the monostable at a rate faster than the timeout of the monostable in a manner reminiscent of a watchdog timer. In its simplest form there is a hang time before the output turns off even when the desired state actively changes from on to off. The monostable could be something like a MC14538, and the number of components would impact the cost, PCB real-estate and reliability of the design (figure 1).

Figure 1: The micro must toggle the input to the MC14538 faster than the monostable timeout period. Obviously there is a hang time of up to the full monostable period when the output is de-activated.

An alternative might be to control both sides of the load—the supply to all loads could be controlled by a separate output, perhaps even linked to an independent monostable or even the output of the watchdog timer, so that if it timed out all outputs would be disabled. The micro could then control the other side of each output load as normal as in figure 2.

Figure 2: Using a single monostable to control the power to the loads spreads the cost across several channels when compared to the approach in Figure 1. Also the load will be deactivated at the same instant that the micro turns it off. An additional benefit (when an enable signal is fed to the monostable CD input) is that the outputs will not be activated until the system is ready. For instance some micros will power up with the outputs high which will activate the outputs until the outputs have been initiated.

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