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

[Bernd Walter]

On Thu, May 22, 2014 at 07:04:14PM +0200, Bernd Walter wrote:
> On Thu, May 22, 2014 at 11:34:50AM -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.
> 
> R/C servos usually are extremly harsh on power sources.
> I know this because decades ago I'd build LED based akku charge displays
> for R/C helicopters and I needed a lot of input filtering to not sweep
> over the full display range when servos had been active.
> 
> > 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.
> 
> It is the noise on the power consumption.
> I don't know how good the 3.3V rail is stabilized, but I wouldn't
> be surrised the the servo would produce a noise level of way more than
> 3.3V, so your rail even can be negative or more than doubled.
> This can even damage components as they are not designed to handle
> such voltages.
> 
> > Connecting the servo to the VDD_5V on the same header makes the problem go away.
> 
> The 5V rail is likely not directly used in any digital circuitry
> beside the volatage regulators input and their output are
> influenced.

uninfluenced...

> The volatage regultaors however are also designed for small input voltage

Doing low level stuff makes one type volatile too often.
Amazing how often I misstype voltage now.
Stupid fingers...

> ranges and can be damaged by invalid input voltages on the 5V rail.
> An inductive load can be a power sink or a power source!
> Effectively you are connecting something to the 3.3V or 5V rail,
> which sometimes act as a high voltage battery.
> 
> > So be careful about what you connect on your boards :)
> 
> Absolutely.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.