When selecting your RFID tag chip for your next IoT solution take into account that a master or slave SPI or I2C can make or break your design.

I’ll start this post assuming you read our previous post on RFID tag IC selection.

You’re probably interested in creating some sort of solution that requires and RFID tag chip or it can give you interesting advantages over other potential solutions.

You may be interested in the RF energy harvesting capabilities, in the wireless communication protocol or in any other specific part of the RFID tag chip. There are multiple options.

What I’m almost sure is that you will have to connect the RFID tag chip to a different component. A sensor, an actuator, a microcontroller, … it can be anything, but if you are interested in the IoT you’re most probably looking to not just identification but something more.

RFID tag chip with mechanical relay example

Example of RFID tag chip used with a mechanical relay

Communicating with the RFID tag chip

When selecting the RFID chip you will use in your design, you will probably see protocols such as SPI or I2C, due to their popularity in the sensor industry.

I’m not going to go deeper into which one of them would work best for your design because most of the time they both will do the job perfectly. It will basically depend on the component you want to communicate with but take into account that there are not that many RFID chips in the market so you will not have many options available in this side.

Master vs. Slave

What I really would like you to take into account is the role of the RFID tag chip in the communication with your device.

When designing your solution, you will have to decide which of the components in your electronics needs to behave as a master – this component will start the actions. The other one will be the slave – this one will basically reply to the master’s commands.

Example of battery free sensor solution design

I’ll give you an example in which you want an RFID based thermistor to take measurements in an industrial monitoring solution. You want your sensor embedded in hardly accessible places and prefer to avoid changing batteries so you decide to go for a battery free solution.

The thermistor being an analog sensor, you decide to use a microcontroller with an ADC to convert the value. The microcontroller has been thoroughly selected so that it can communicate via SPI.

Now the RFID tag chip needs to be able to communicate via SPI of course but… would you be OK with a slave SPI?

PYROS-03GC schematic - ANDY100 RFID tag chip with a microcontroller and thermistor

PYROS-03GC schematic – ANDY100 RFID tag chip with a microcontroller and thermistor

Most probably not. You’re more likely to be interested in a master SPI due to the fact that you probably want the reader to trigger the measurements. Note that your sensor tag will not have energy to do anything unless the RFID reader is around and generates the RF field to power the RFID tag chip. The RFID chip then transfers part of the energy to the microcontroller.

And the most critical part: passive RFID is commanded by the RFID reader. In general, it doesn’t make sense to be taking measurements on the sensor side without having a request from the reader. You can even go into timing issues – reader requests against microcontroller firmware.

You would be better off by using an RFID tag chip with a master SPI. In this case, the RFID tag chip will start the actions whenever a request from the reader is received.

Please note that this was just an example and you can find solutions that will benefit from a slave SPI. Examples could include BAP sensors where, since you have batteries, you may want the microcontroller, which is probably ‘active’ at all times, drives the logic.

I’m sure you got my point and hope you’re now more concerned about the master vs. slave feature when you are looking for the right RFID tag chip for you. Taking a good decision early on can make a huge difference in your design and development.