Hacker News Reader

Comment by jabot | Hacker News Reader

The state of the pin is encoded in the type of the variable that is used to access the pin.

This kind of encoding prevents you from making the programming error of writing to an input pin - which is technically possible, just useless.

View on HN

Not even remotely useless, it's often a feature.

Well, _if_ you need that feature, you can wrap the trait into another trait that allows a write to the pin, but discards it.

However, by default, nonsensical operations should not be possible.

AVR microcontrollers use the same register to define the pin value when it's set as an output, and define the state of the internal pullups when it's set as an input.

I haven't seen that in ARM microcontrollers, but as peripherals vary across manufacturers I don't think you can assume that it's not done this way.

Wouldn't you then write two sets of functions? set_high, set_low, enable_pullup, disable_pullup. Now internally set_high and enable_pullup might do the exact same thing, but externally they signal intent and prevent you from trying to set a pullup on an output for example.

Oh? When would you use this? I'm sure there must be some reason, but I can't think of one.

To prevent glitches it rarely problem but sometimes happens: imagine that default pin state must be high (on reset pin are floating) and moment you set pin mode to output it goes low, because data register is 0

It can be used to implement capacitive touch.

As mentioned above, in some uC's the output register defines the pull direction when the pin is an input.

In other cases, you want to avoid glitching when the pin switches to an output on startup.

Consider a I/O pin set to open-drain drive. It either actively pulls the output pin to the ground rail, or the I/O floats. Externally, you have a pull-up resistor.

If there are multiple chips connected in open-drain, you write the I/O pin to 0 or 1 to either ground or float the pin. You read the I/O pin to see if anyone else has grounded it if you haven't. This was called "wired-OR" in the old days. (It's really AND, but was commonly used in DeMorgan equivalent form for communication buses before tri-state drivers became a thing. I'll get my cane and hobble back to my rocking chair now..)

I think in the old days, before CMOS, this was called open-collector. I had learned about it in first semester digital logic not too long ago playing with the "high speed" 7400 series. If I remember correctly, the latch needs to support the open collector capability, but the reason escapes me.

Open collector if you have a bipolar technology. Modern microcontrollers are CMOS, so the pull-down transistor has a drain, not a collector. TTL is bipolar, so the transistor has a collector.

Reply on news.ycombinator.com