On Fri, Jun 22, 2018 at 04:10:21PM +0100, Martyn Welch wrote: > On Wed, 2018-06-20 at 19:41 +0000, Karoly Pados wrote: > > Here I argue the following multiple ways. First, I say that claiming > > that a pin which is used as an input is actually an output is not > > only confusing, but also much less correct than thinking of it as an > > input pin with a weak pullup to prevent floating signals. Second, > > the pullups - while not explicitly listed in the datasheet - can be > > calculated from what is there, and for the cp2105 are typically > > 132k, for the cp2102n even higher around 165k. These are pretty > > weak pullups, so weak that they won't matter for the vast majority > > of applications as people rarely use pull-ups or pull- downs higher > > than 100k (not never, but rarely). So claiming that it can result in > > false expectation, while not completely wrong, is favoring the > > needs of a few instead of the much more common practice. > > > > Lastly, and maybe most importantly, I argue that calling everything > > an "output" pin only in name does not actually avoid any design > > errors, as the same circuit that would case a false reading in one > > case would also cause the same false reading in the other, and the > > circuits are usually developed before the software. So it'll be too > > late anyway by the time somebody realizes such a mistake. But on > > the contrary, it opens up more opportunities for errors, because > > now you are open to software bugs that ignore a pin's direction > > because everything's an output either way even when it really > > isn't, and think that they can treat it as as open-drain while for > > some reason it is in push-pull mode. Worse, even if it is in > > open-drain mode, it will only work with a specific output values - > > it must be high, which is not the default. With my proposal, > > setting a pin's direction to "input" will make sure it cannot be > > actively driven by the chip, avoiding such "misunderstandings" and > > errors, and similar measures are also in place for the push-pull > > pins. > > Yeah, I'll go with that. :-) Sounds good to me too. Thanks to both of you for spelling this out. > > The only problem I can see is if there isn't a way for the cp2105 to > > query the reset values of the pins (maybe there is, I just haven't > > looked into it). Then I don't know how the direction could be > > determined for an open-drain pin during initialization. But this is > > solved for the cp2102n, and then it is a device-specific issue for > > the cp2105, which shouldn't be forced onto other devices if we > > otherwise decide the approach to be inferior. > > I'm pretty sure there is a way to determine the pin state, though > unfortunately I no longer have access to the HW to be able to test... If this isn't (yet) possible, it's never wrong to continue treating the cp2105 pins as (open-drain) outputs. Thanks, Johan -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
