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<== Date ==> <== Thread ==>

Subject: Re: soft Timestamp fixed point support
From: Andrew Johnson <anj@aps.anl.gov>
To: <tech-talk@aps.anl.gov>
Date: Thu, 18 May 2017 11:11:46 -0500
Hi Will,

On 05/17/2017 08:43 AM, william.smith@stfc.ac.uk wrote:
> I am working on synchronising the timestamp on an IOC with a timestamp
> register on an FPGA. I need access to the 32 bit fixed point nanoseconds
> word that makes up the lower half of the EPICS timestamp. Using the
> “Soft Timestamp” I can only get the value as a floating point number
> (using ai) or a string (using stringin).

> Is there a way of getting it in the fixed point format I want? As I
> understand it “Soft Timestamp” only works with ai and stringin records.
> Ideally I’d like support for longin records as well.
> 
> record(ai,"$(SYS)-EVR:EventTimestamp"){
>           field(DTYP,"Soft Timestamp")
>           field(TSEL,"$(SYS)-EVR:Event-14-SP.TIME")
> }
>  
> The record is processed by a forward link from "$(SYS)-EVR:Event-14-SP”

If you aren't too bothered about performance (i.e. this won't be
happening at high speed) then you can get the nanoseconds value from the
timestamop by using the stringin device support with a "%f" conversion
specifier, then fetch the resulting string as an integer (say with a
longin record).

If you need higher performance (i.e. don't want to round-trip through
the string representation), I would use a subroutine record with say the
INPA link pointing to the $(SYS)-EVR:Event-14-SP record and in the
subroutine do something like this:

    epicsTimeStamp tick;

    if (!dbGetTimeStamp(&prec->lnka, &tick)) {
        prec->val = (double) tick.nsec;
        return 0;
    }
    return -1; /* Raise SOFT_ALARM, BRSV */

This will fetch the timestamp from the record that INPA is pointing to
and set the VAL field of the subroutine to the nanoseconds field of that
timestamp.

HTH,

- Andrew

-- 
Arguing for surveillance because you have nothing to hide is no
different than making the claim, "I don't care about freedom of
speech because I have nothing to say." -- Edward Snowdon

References:
soft Timestamp fixed point support william.smith

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