On 06/25/2013 08:57 AM, James Hogan wrote: > On 25/06/13 14:22, Linus Walleij wrote: >> Can't we just try to come up with a patch that nails down the meaning of >> slew rate in some meaningful manner then? >> >> So according to: >> http://en.wikipedia.org/wiki/Slew_rate >> a proper expression for slew rate would be dV/dt i.e. >> something like microvolts per microsecond (which then just >> becomes volts/second). >> >> What we need to figure out is what range will be applicable within >> reasonable doubt for current scenarios and the next few years. >> >> What are your datasheets specifying here, and what would be >> a proper measure? > > My datasheet says: > > 0: slow (half frequency) > 1: fast > > I just got a reply back from a hardware engineer, who said that the > relationship with the actual volts/usec will depend on both the drive > strength and the load on the pad, and that a definite answer probably > requires running a simulation.
Tegra is similar here. The docs just say (for a 2-bit field expressed in binary) "Code 11 is the least slewing of the signal, code 00 is the highest slewing of the signal". I'm not sure that a generic parameter actually needs specific units. Why can't we simply specify the units as HW-defined, even while using a standardized DT property name and kernel-internal enum to represent the concept of slew rate? Even the order of whether 0 or 3 is highest or lowest need not be mandated by the spec? Note also that Tegra has separate rising and falling slew-rate configuration. And the slew rate is influenced by a "low-power mode" setting. And as for James, I imagine the actual dV/dT is influenced by the voltage on the IO rail for a particular board, since I'm pretty sure we have some IOs that can operate at multiple different voltages, simply based on whatever voltage is supplied for that pin/block's VDD. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
