Re: Linux on CF-Cards
Jérôme Warnier wrote:
I was looking for some inexpensive IDE to CF converters on ebay,
ended up buying some IDE with CF modules attached:
CF cards in embedded systems are very touchy if you have write access
enabled. Basically if your power supply goes south during a write the
meta-data on the CF card can become corrupted and it's trashed. I've
had personal experience with this and read of many other reports of this.
To make a CF robust under write conditions there are specific signals
that _must_ retain power during a write. A normal CF->IDE adapter will
not do this for you.
Here's a piece I received outlineing the issue further:
We have information from a Compact Flash manufacturer that says you
should have hardware to solve this problem - I do not believe there is a
software solution. The problem is caused by the fact (as you state)
that CF stores metadata in the flash blocks along with the disk block
data. If the metadata is corrupted by an aborted write, it can affect
all of the disk blocks stored in the flash block, and the metadata can
even be so corrupt that you lose unrelated blocks. Our most common
scenario for failure is to lose disk block #0, which, of course, is the
boot block for our OS.
We solve this using hardware. You need to use buffers to disconnect the
CF's ATA reset, output-enable and write-enable signals from their source
on the bus, and hold up DC power to the pull-ups, the buffers and the CF
itself for at least 10ms-20ms after detecting the imminent loss of
power.
This is being implemented in our new hardware designs, and we have not
yet gotten one back from fabrication to test. But it sure looks like it
will work to solve the problem.
Alan Mimms, Senior Architect
F5 Networks, Inc. Spokane Development Center
Liberty Lake, Washington
v: 509-343-3524 f: 509-343-3501
--
Richard A. Smith
rsmith@bitworks.com
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