On 12 Dec 2017 at 9:44, Isaac M. Bavaresco wrote:

> > For my previously mentioned requirement of the check value not equallin=
g 0xFF, I=20
> > think I can live with testing for and replacing that occurence with a d=
ifferent code.
>=20
> Did you see my other reply about spreading the CRC or checksum over two
> bytes? That way you can ensure you will never have a 0xff.
> But by using the CRC you don't have to worry about having a 0xff value
> for CRC or even for the data, because CRC will catch virtually any data
> corruption.
> If you use BCD for your data you will use half the storage space, which
> equals to half EEPROM writing time too.

I could spread the CRC8 value over 2 bytes, similar to what I've done with =
the digits.=20
Though I don't see any major problem with sticking to 1 byte and translatin=
g a CRC=20
of 0xFF into say 0x5A. It does feel a little cheeky, but it's not like I'm =
transmitting the=20
data between devices... the CRC function will be literally the same code on=
 the=20
same device for both reading and writing.

Yes I could have packed 2 BCD digits into one byte, but have opted to sprea=
d the=20
data wider to give me an extra method of corruption detection. I can test 3=
 ways=20
now: check the high/low nibbles to see if a symbol for a single digit is va=
lid, check=20
the CRC over all 6 digits, and I can also check against the previous record=
 to=20
ensure it has incremented by only 1 count.

> And much better than using just a "dirty" flag, adding a sequence count
> to each record allows you to find the latest valid one.
> You could use a record with 2-byte sequence counter, 3-byte BCD value
> plus 1-byte CRC. The CRC calculated over the sequence counter and data,
> so it would be easy to spot corrupted records.

I'm not sure that I'll use a dirty flag... if need be I can change data so =
the checks fail=20
and that way mark the record as bad/dirty/deleted.

A sequence count would increase just like the hours count does, and at some=
 point=20
it will roll over as well. So if I have to handle a rollover eventually, th=
en I may as well=20
just handle that for the hours count alone. The thought is that marking old=
 records=20
"dirty" on roll-over would be one way of handling it.

Thanks.

--=20
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