Date: Thu, 30 Oct 2014 20:10:08 +0100 From: Bernd Walter <ticso@cicely7.cicely.de> To: Ian Lepore <ian@freebsd.org> Cc: Andreas Schwarz <Andreas.Schwarz@schwarzes.net>, George Rosamond <george@ceetonetechnology.com>, Tim Kientzle <tim@kientzle.com>, "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org>, ticso@cicely.de Subject: Re: sd card probing (was: FreeBSD 10.0 on Raspberry PI B+ no network devices Message-ID: <20141030191008.GE59614@cicely7.cicely.de> In-Reply-To: <1414677395.17308.155.camel@revolution.hippie.lan> References: <CAB=2f8xHEeF8DtP1eCkpp3Y0rZu3w0Phi_gzMSByGJ74xaFchg@mail.gmail.com> <20141023022244.GB16490@cicely7.cicely.de> <20141029172937.GB59614@cicely7.cicely.de> <1414605501.17308.97.camel@revolution.hippie.lan> <20141029200403.GC59614@cicely7.cicely.de> <1414613786.17308.124.camel@revolution.hippie.lan> <6CC5D29F-C3F7-4913-9D77-D275EEDDC1DD@bsdimp.com> <20141030021857.GD59614@cicely7.cicely.de> <CAB=2f8zamdWrSv15h4Zq0%2BdD-nc_QSUdT3NKJ%2BpbrvY81w0MLQ@mail.gmail.com> <1414677395.17308.155.camel@revolution.hippie.lan>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thu, Oct 30, 2014 at 07:56:35AM -0600, Ian Lepore wrote: > On Thu, 2014-10-30 at 10:54 -0200, Luiz Otavio O Souza wrote: > > > > I tried to check the SD clock frequency with the scope and just by > > connecting the probe on the SD clock line the card that was previously > > working flawless, start to exhibit some i/o errors, so i guess it is > > pretty sensible to capacitance load on that line (that may explain why > > they removed the series resistor). > > Is your probe set for 10x mode? When I first looked at the clock and > data signals on the rpi sdcard they looked horrible, the waveforms were > anything but square. I thought I was on to something at first, maybe > the board was just too underpowered. But eventually I realized I had > the probe set to 1x mode and it was pulling too much current off the > lines. When I switched it to 10x everything looked good. (Maybe all of > this is a no-brainer to a hardware person. I'm a software person with > just enough hardware knowledge to confuse himself.) Yes - rule of thumb: always use the divider. In 1:1 mode you just have an unterminated cable into the scope requiring a 50 Ohm termination at scope side. Many modern scopes have a 50 Ohm termination feature, with older your need a 50 Ohm BNC inline terminator. You can only use it unterminated (and therefor high impedance) with very low frequencies below the reflection time. With divider the cable unleashes all it's black magic. Dave Jones did a nice video to explain the details: http://www.eevblog.com/2013/04/13/eevblog-453-mysteries-of-x1-oscilloscope-probes-revealed/ He also did many other very interesting videos on using scopes. -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20141030191008.GE59614>