BBB MMC / SD detection instability with U-Boot 2014.04 (CPU 1GHz)

[Sulev-Madis Silber (ketas)]

On 2014-05-22 17:53, Ian Lepore wrote:
> On Thu, 2014-05-22 at 11:34 -0300, Luiz Otavio O Souza wrote:
>> Just picking one email to reply...
>>
>> This is probably unrelated, but i can consistently corrupt my SD Card
>> data if i connect (and use) a very small R/C servo (the 9g ones, used
>> on foam planes) to the 3.3v on the expansion headers.
>>
>> I'm not sure if it is because of noise or the power consumption, but
>> looks like the BBB is really picky about the power conditions here.
>>
>> Connecting the servo to the VDD_5V on the same header makes the problem go away.
>>
>> So be careful about what you connect on your boards :)
>>
> 
> It looks like the 3v3b rail comes off a 500mA LDO and is already
> powering several of the onboard chips (including eMMC and at least part
> of the ethernet phy), so it may be power consumption.
> 
> -- Ian
> 
> 
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It's somehow related... Why does the ADC driver discard the input #8
(measures VDD_3V3B)? I don't know why exactly board designers decided to
connect it there (instead of PMIC ADC mux or just header), but I have
some ideas why and what to do with it. I can patch it locally too, if
absolutely required. Maybe you could measure LDO status that way a bit.
That of course doesn't mean you should run motors off IC power rails.
Voltage measurements can't really help there. Even shared GND can
sometimes be problem...