Hacker News Reader

Comment by slasaus | Hacker News Reader

It's even a bit worse when looking at it from a fail-safe point of view. To turn the bluetooth and wifi off, 3.3 Volt has to be put on the pins. Giving no power on the pins turns both devices on.

View on HN

No, that's probably just by convention. If it's strictly an input then both directions are just the same in terms of safety. It's just sometimes more convenient to only pull things down or up and have a resistor provide the opposite pullup/pulldown current.

Maybe it was chosen because the "high" voltage isn't specified and the standard might say: "Pull down to GND to activate, leave open to keep card off." Then you don't have to think about the internal logic voltage of the circuit, you might fry the card if you pull up to 3v3 if the logic input is only 1.8V tolerant.

This would work better if the switch worked as you described - close a circuit to GND to turn the card on, leave the circuit open to turn the card off.

If the wires to the switch fail then the card fails on.

Not necessarily. An open wire doesn't mean 0 volts -- it is whatever the input of the gate floats to. There is no standard -- it depends if the manufacturer put a weak pull-up or pull-down into the chip, or if they have an active termination that will pull to either end. Or if the gate isn't designed any particular way, it could be subject to how well doped the transistors of that particular chip turned out. Being open also means it is subject to noise from adjacent signals, so it could be random that way, too.

It depends on what you're looking for. If you're worried about security you probably want it to fail off instead of on.

Devices that function when the power has been disconnected would be wonderful energy-saving devices.

These are signal pins, not power pins. The device is still receiving power when the switch is "off", all the switch does is provide a signal to the pins that tell it to disable itself.

Also signal input can in certain cases power devices[1].

[1] https://youtu.be/1jOJl8gRPyQ?t=502

Which still allows potential side-channel attacks in circuitry.

Reply on news.ycombinator.com