Raspberry Pi GPIO Header Pin Protection

The GPIO pins connect directly to the BCM2835 processor, which makes the system extremely sensitive to overvoltage, and overloading. This is a 3.3 V system and the pins are not 5 V tolerant. Feeding the pins 5 V from an external TTL circuit will damage them. In addition, drawing too much current from the pins can also damage the processor.

There is no buffer protection circuitry; hence, chances are that within a school environment some of these boards will fail due to overloading caused through incorrect wiring, incorrect programming, and accidental shorts. For example, if a GPIO pin were fed 5 V due to incorrect wiring, then it would likely fail.

The 5 V pin on the P1 header socket could accidentally connect to one of the nearby GPIO pins causing damage to the BCM2835 processor. It is of course possible to replace the chip but very time consuming and expensive for BGA packages.

Zener Clamping

One way to protect a digital line is to clamp its voltage using a Zener diode. With the correct series resistance, the diode can behave as a shunt regulator, which maintains the voltage to 3.3 V. A BZX79C Zener diode rated at 3.3 V / 0.5 W with a series resistance of 11 Ω would work well. For example, if a student accidentally applied 5 V at the project end, where Vs = 5 V, then the diode brings this voltage down to a safe level (Vz = 3.3 V) at the GPIO end.

There is a limit to how much protection this circuit can provide but it can go a long way to providing some protection. An advantage of this circuit is that the protection is in both directions, for both input and output modes of the pin, because the resistor also prevents the pin from overloading and shorting to ground. One disadvantage is the resistance keeps logic 0 voltage slightly above ground, which on a rare occasion poses an issue with non-Schottky TTL IC.

Buffer Projects

The Cambridge Boffin: Raspberry Pi GPIO Buffer Project is worth building if you want to use buffers to protect the GPIO Pins. It is similar to the Gertboard buffers but you can build this on a prototyping board and is fun to build.

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