When reset is the only option, what do you do if there is no button?

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It is widely believed that code can never be bug-free, and although embedded software developers will do everything they can to find and eliminate bugs, the best we can say is that the software has passed every test condition we could think of. Conversely, hardware is seen as being much more robust and, in general, can be relied on to work 'properly' all of the time.

electronicspecifier.com, Jul. 23, 2019 – 

By: Paul Hill, Director, Product Marketing at Adesto Technologies.

The truth, which many design engineers will already know, is that hardware crashes too, and while perhaps not quite so error-prone as embedded software, it is by no means out of the ordinary for hardware to get into a condition where it simply stops working.

Under these conditions the only recourse is to hit reset. But what happens if there is no reset button to hit? Embedded designs are typically buried away from the user, intended to operate flawlessly and repetitively, this is essentially what embedded control is all about. But as design complexity continues to increase, the surface area for failure also increases, or to put it another way as functional density goes up, the probability of a small glitch rippling through the entire system to become a fully-fledged fault goes up with it.

Most embedded systems still follow a hierarchical design, using a host processor or, perhaps more likely, a microcontroller (MCU) to manage everything. It's important for the MCU to have the ability to hit the rest button, but self-resetting isn't necessarily a comprehensive approach, which is why it's commonplace for embedded designs to use a WatchDog Timer (WDT) implemented using either software or (probably preferable) an external device.

The WDT's job is to issue a hard reset of the MCU if it doesn't service the timer within a defined amount of time. The problem is that resetting the MCU isn't the same as resetting the whole system.

Although the WDT will most likely only reset the MCU, power-cycling is completely indiscriminate and will result in the loss of all operational data. Neither is ideal, which is why some critical devices that require synchronicity, such as SPI-based Flash memory, also feature a hard-wired reset pin. At least, that used to be the case.

The drive for greater density and smaller packages, incident with the move to Quad SPI and Quad Peripheral Interface (QPI) ports means some manufacturers have opted to leave the reset pin out of newer designs, in order to use the smallest possible package size. Embedded developers may welcome these smaller outlines, but the loss of the reset function is a major drawback to system stability and reliable operation.

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