On 20 Sep 2012 at 20:39, Veronica Merryfield wrote:

> Anyone else tried this? Good/bad?=20
>=20
> I have a project with a PIC24 that is using the self programming of
> flash to store some non-volatile data including a slow count - up
> time in minute increments. The counter is part of a larger structure
> of NV data.=20
>=20
> I want to avoid rewriting the whole block each time the counter is
> updated and came up with the following.=20
>=20
> Flash when erased reads all ones. Writing changes ones to zeros.
> Overwriting can happen but only to change ones to zeros (I assume
> the PIC flash behaves like other NOR flash in this regard). By
> setting aside a byte or two as a counter, each count could be
> represented as a zeroed bit, each bit zeroed being an increment.
> When all are zeroed, a real counter is updated with the number of
> bits at zero and the process can start over. As a further
> enhancement, each time the block is written for an update of
> something else, the counter would be updated from the bit count and
> the bit counter erased to go again.=20

Your idea sounds feasible. Each consecutive bit cleared will represent 1 mi=
nute=20
though so you only store 8 x 1 minute ticks in a byte, vs 256 if stored sim=
ply as an 8=20
bit count.

As the bits are cleared in sequence you can check for an error if a bit is =
cleared out=20
of sequence, but what to do with it then... reset to zero, flag an error? D=
epends on=20
your application.

I have implemented an hours counter for an industrial machine before. It wa=
s=20
surprisingly quite a lot of work to come up with something that worked reli=
ably and=20
covered all the "requirements" of PIC EEPROM usage, ie. endurance, refreshi=
ng,=20
etc.

I used a 4 byte counter incrementing every 0.001 hours (3.6 seconds) of eng=
ine=20
running time, but only update the values in EEPROM every 0.1 hours or whene=
ver=20
power failure is iminent. Also, the EEPROM values are stored in triplicate,=
 using=20
best 2 out of 3 method to ignore and repair errors, plus the data is rotate=
d around=20
the EEPROM to perform wear levelling and make sure it gets refreshed regula=
rly.

Hope this helps~!
--=20
Brent Brown, Electronic Design Solutions
9 Titoki Place, Pukete,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz



--=20
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