The Tech Blog

Pullup & Pulldown resistors are used in logic circuits to ensure that a particular pin(or logic gate) is put at a well-defined state ie) at high & low respectively. If it’s not connected in a pullup or pulldown mode, the particular pin can be in a floating state and it can be read as either a high or a low, at any instant of time, in the absence of a signal. Refer to the image on how you would wire up in these modes.

The selection of these resistors is not that tricky for normal operations. The things to consider are the amount of current which can be sourced or sunk from a particular pin. A strong pullup means that the resistor value is small(Means larger current, more power dissipation/wastage) making the “pull” to the supply stronger. Nomenclature is reversed on that one. Strong means lower resistance and a weak pull-up means a larger resistance. The ideal chosen pullup resistance value is the one that takes the minimum current to maintain its high logic state. Usually, most people randomly choose a resistance in the 1k-10k ohm range and it works for most use cases. But ideally, you need to calculate it based on the minimum current required and minimum recognisable logic gate voltage. I will get to that in a future post. This becomes really critical in some use cases like high-speed I2C where the communication bus capacitance also starts playing a role in selection.

Electro Static Discharge (ESD) is one of the most critical things to guard against a mass-produced product. They can easily damage your product, especially in areas where someone can potentially touch it like exposed pads, USB connectors, screens etc. To protect against this issue, you use a Transient Voltage Suppressor(TVS) or ESD diode on the input side of your circuit. TVS diodes and ESD diodes are technically more or less the same as TVS diodes capable of protecting against larger surge currents too. If you are only worried about ESD, then use the smaller ESD diodes.

You connect ESD diodes right at the input(In parallel, across the rest of the input), usually very close to the part which is exposed. There is a common misconception that the ESD diode will absorb all the transient energy just because you put it somewhere. Well, that’s not how it works. ESD diodes help shunt away the high voltage back to the source, hence it’s imperative that you choose a diode that is fast responding and is placed as close to the input as possible so that the energy put in can return back faster thereby protecting circuits downstream. Never use Zener or other types of diodes for this as it’s usually slow to react to the pulse of an ESD.

IEC 61000-4-2 is the testing standard applicable for ESD before releasing your product on the market. Your product will be tested with an ESD gun capable of injecting a voltage of the order of kilovolts.

They are usually of 2 types, Air Discharge(That is when you bring your hand close to a device but not touch it. Think sparks from fingertips due to charge build-up) and Contact discharges(Direct touch). There are different levels from Level 1 to Level 4 increasing the amount of protection your device is ensured against. For example, Level 4 protects against +-15kV(Air Discharge ) and +-8kV(Contact Discharge) There are lots of considerations in choosing diodes and types of them. Can’t fit them on a single post. If there is some interest, will probably do a detailed series later on.