On 13 May 2015 at 13:09, Mark Brown <broonie@xxxxxxxxxx> wrote: > On Tue, May 12, 2015 at 02:46:11PM -0700, Doug Anderson wrote: >> Since the regulator used for the SDMMC IO voltage is not expected to >> draw a lot of current, most systems will probably use an inexpensive >> LDO for it. LDO regulators apparently have the feature that they >> don't actively drive the voltage down--they wait for other components >> in the system to drag the voltage down. Thus they will transition >> faster under heavy loads and slower under light loads. > > What a LDO is doing is basically just charging up a capacitor - the > regulation consists of monitoring the voltage on the capacitor and > opening a transistor to charge the capacitor when the voltage droops too > much. > >> From experimental evidence, we've seen the voltage change fail if the >> card doesn't detect that the voltage fell to less than about 2.3V when >> we turn on the clock. On one device (that admittedly had a 47K CMD >> pullup instead of a 10K CMD pullup) we saw that the voltage was just >> about 2.3V after 5ms and thus the voltage change would sometimes fail. >> Doubling the delay gave margin and made the voltage change work 100% >> of the time, despite the slightly weaker CMD pull. > >> At the moment submitting this as an RFC patch since my problem _could_ >> be fixed by increasing the pull strength (or using a smaller >> capacitor). However being a little bit more lenient to strange >> hardware could also be a good thing. > > Right, and this is probably going beyond the delays that the regulator > API is handling since it's not something the regulator hardware is > actively managing. Thanks for elaborating from the regulator perspective. So may I apply your ack for this one? Kind regards Uffe -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
