[Gb-ccb] forwarded message from John Ford

[John Ford]

Tim Pearson writes:
 > On Wed, 21 Jan 2004, Martin Shepherd wrote:
 > 
 > > This brings up a dump-mode issue that I hadn't thought about yet.
 > > Whereas the time-stamps of the 1ms integrations didn't need to be any
 > > more accurate than about 0.5ms, dump-mode samples will presumably need
 > > much more precise timestamps. So yes, your point about using a 10MHz
 > > signal synchronized to 1PPS may be important.

I guess that it's not necessary to use the 10 MHz reference from the
maser for this.  You might consider using a 10 MHz oscillator and
multiplying it to 40 MHz inside the FPGA, if you want to be really
paranoid about EMI.  Are we planning to use the 1PPS to trigger the
start of data collection?

 > 
 > I do not think that accurate time-stamping of the dump data is a
 > requirement. An approximate time (better than 1s) of the start or end
 > of the dump is all that is needed, assuming the sequence of samples is
 > really a continuous one. Higher accuracy would be nice, but not
 > necessary. This is for diagnostics, not pulsar timing.

Since it is easy to do, I would strongly suggest making a 32 bit 10
MHz counter reset to 0 at the start of the scan (via the 1PPS?) part
of each data dump.  This will allow us to do high-accuracy timestamps
if they are ever deemed to be needed, allow for the detection of
missing/partial frames, and monitor the stability of the data
collection process.  I'm loath to assume that the samples are
continuous using a USB connection and Linux, although that may indeed
be the case (It really should work fine...)

John


 > 
 > - Tim
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