Date: Wed, 7 Jul 1999 11:58:44 -0700 From: "David Schwartz" <davids@webmaster.com> To: "John Hay" <jhay@mikom.csir.co.za> Cc: <stable@FreeBSD.ORG> Subject: RE: PPS API change assert/clear Message-ID: <000f01bec8aa$bc9bb870$021d85d1@youwant.to> In-Reply-To: <000a01bec8a9$8237dd40$021d85d1@youwant.to>
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Wow. It seems you're right. This still makes no sense to me, but it does
seem to be the standard!
When DCD is asserted, you will get a clear event. When DCD is cleared, you
will get an assert event. Go figure:
A PPS signal consists of a series of pulses, each with an
``asserted'' (logical true) phase, and a ``clear'' (logical false)
phase. The two phases may be of different lengths. The API may
capture an ``assert timestamp'' at the moment of the transition into
the asserted phase, and a ``clear timestamp'' at the moment of the
transition into the clear phase.
The specific assignment of the logical values ``true'' and ``false''
with specific voltages of a PPS signal, if applicable, is outside the
scope of this specification. However, these assignments SHOULD be
consistent with applicable standards. Implementors of PPS sources
SHOULD document these assignments.
---------
Reminder to implementors of DCD-based PPS support: TTL and
RS-232C (V.24/V.28) interfaces both define the "true" state as
the one having the highest positive voltage. TTL defines a
nominal absence of voltage as the "false" state, but RS-232C
(V.24/V.28) defines the "false" state by the presence of a
negative voltage.
---------
DS
> > >
> > > It seems that the logic of assert and clear have changed.
> > Now an assertion
> > > of DCD is considered a clear, and vice-versa. Why was the
> logic broken?
> > >
> >
> > The PPSAPI draft was clarified and we were the odd ones out, so
> we changed
> > to be compatable to the rest.
>
> That makes no sense. I know Linux is currently the same way
> FreeBSD was.
> Can you send me a copy of this 'clarification'?
>
> I can't understand how this can possibly make sense. I now
> get a 'CLEAR'
> event whenever I assert DCD and an 'ASSERT' event whenever I clear DCD.
>
> DS
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