On Wed, 2018-06-20 at 19:41 +0000, Karoly Pados wrote: > Hi Martyn, > > > The rationale for the pins being permanently configured as output > > pins > > is that these pins (at least on the cp2105) do not appear to > > provide a > > true input mode. They offer a "push-pull" mode (where the voltage > > is > > pulled directly to ground or supply rail) and an "open-drain" mode > > (where the pin is weakly pulled high by an internal resistor and > > can be > > pulled to ground). Unless I missed something, there is no > > tristate/high > > impedance mode typically associated with a pin being used as input. > > You didn't miss anything. It is the same for many (maybe all) SiLabs > devices, in particular also for the cp2102n, which spawned this > conversation. > I sent in patches for its GPIO support, but in contrast to the > cp2105, > I try to emulate input mode by making sure that a pin is never pulled > low > when it is set as an input, and that you cannot set push-pull pins as > inputs. > The reason why you were invited is because we are trying to figure > out which > approach should be taken. > > > Sure, you can use the open-drain mode as input as long as you > > understand that the permanent pull up in the cp2015 might have an > > impact on what you are reading. For example, if you have a signal > > that > > is externally weakly pulled down, it's going to depend on the > > relative > > resistances of the internal and external resistors as to what > > voltage > > the line rests at and therefore what state the line is considered > > to be > > in. This could stop things working if you naively think the cp2105 > > is > > acting as a typical high-impedance input. > > 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. :-) > 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... Martyn > Karoly -- 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
