Lux, James P skrev:
On 5/17/09 9:24 AM, "Hal Murray" <[email protected]> wrote:
In which case, if you're saddled with 32 bit (or 8 bit!) reads, you
have to do multiple reads, so that by the end of the process, you can
assure yourself it's consistent.
E.g read high, read low, read high, read low So you can check low #1
against low #2, and figure out if you had a roll over.
Why read the low twice?
I though the normal recipe was to read high, low, high. If the two highs are
the same you know a carry didn't happen so a high/low pair is a valid reading.
Yes, but then, if it did happen, then you need to read low again. If you do
the 4 reads as a block (say, with interrupts disabled), then you get a nice
deterministic timing for the code. In practice, it's just a design decision
which way one does it.
No, if the high parts differs, then the MSB of the lower part indicates
which of the higher parts to go with it, MSB low is the late high read
and MSB high is the early high read. Infact, by always look at the MSB
then it will always be correct. With a second lower read will the low
read point in time be modulated and hence jitter is added.
Cheers,
Magnus
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