Anatomy of a TPMS SoC: what lives inside a tyre

A direct tyre-pressure monitoring system puts a small electronic module inside every tyre, usually on the valve stem. The module measures the air in the tyre and radios the result to a receiver in the vehicle. It sounds simple. The hard part is doing it for years, inside a spinning wheel, on a coin cell that cannot be recharged.

The five jobs of the chip

1. Sense pressure. A pressure-sensing element converts the force of the air into a small electrical signal. The range depends on the vehicle: a scooter tyre and a truck tyre sit at very different pressures.

2. Sense temperature. Pressure rises as a tyre heats up. Without a temperature reading, a hot, correctly inflated tyre and a cold, over-inflated one look the same. Temperature is also a safety signal in its own right.

3. Condition the signal. The raw sensor output is tiny and noisy. An analog front end amplifies it and turns it into numbers the digital logic can use.

4. Decide. A small controller works out whether anything has changed enough to matter and builds the data frame to send. In VAAYU this is a RISC-V digital controller.

5. Transmit. An RF transmitter sends the frame to the vehicle. In India and Europe, TPMS links commonly use the 433.92 MHz ISM band.

A sixth block ties the others together: power management, which decides when each part of the chip is awake.

Why power drives every decision

The sensor cannot be plugged in, and nobody wants to dismount a tyre to change a battery. So the chip sleeps almost all of the time. It wakes briefly, takes a measurement, transmits only when it needs to, and goes back to sleep.

That shapes the whole design:

• Sleep current matters more than active current, because sleep is where the chip spends its life.
• The radio is the most expensive thing to switch on, so frames are short and sent sparingly.
• Detection logic has to be good enough to raise an alert quickly without waking the radio for every small change.

Why one chip

A module can be built from separate parts: a pressure sensor, a microcontroller and an RF chip. Integrating them on one die removes parts from the board, shrinks the module and cuts the number of things that can fail. For a two-wheeler rim or a cost-sensitive passenger car, size and bill of materials decide whether TPMS gets fitted at all.

In later posts we will go block by block through the design choices in VAAYU.