When a product with a display and some user controls is to be deployed it is really necessary to consider whether external influences can be a factor or not. One thing to consider, even though you may have a nice sturdy metal enclosure, certain types of components that bring conductive leads right up by the user panel may be the path wherein ESD jolts may come right on into the unit, down the leads and into the MCU circuitry.

Buffers such as BJTs to drive your LEDs can go a long way to protect sensitive CMOS micrcontroller pins as opposed to directly driving the LEDs with the MCU.

Common switch pads that use a matrix configuration wherein the MCU is used to scan rows and sense on columns are very commonly connected right to the microcontroller pins. If the product uses a membrane type pad for the key matrix this can lead to a bad situation for the MCU due to ESD. If the membrane material cannot withstand high voltage or if the material cracks (like in many high use applications) there results in a fried MCU on the board. (Note to peer over the cash register at a high volume establishment such as a fast food peddler and you'll often see destroyed membrane keypads!!). In applications where marix keypads must be used consider having special ESD shunting layers added or make the matrix out of discrete tact switches that have metal clips that can be interconnected to the chassis.

Common displays these days are the extremely compact LCD types that use the COG (chip on glass) construction. These are made with the interface cable being a short flex that connects down to the circuit board. It can be important to make sure that the methods used to mount such displays do not place the flex cable at or near the space at the edge of the display opening. If not done properly this can be a wide open invitation for ESD to make its way right to the MCU pins since it is common to attach the flex interface signals directly to the microcontroller. It can be instructive to open up and study the display mounting and protective measures used inside an old cell phone.

Michael Karas