[ntp:questions] Re: TAC-2 internal delay

Tim Shoppa shoppa at trailing-edge.com
Tue Oct 21 12:50:04 UTC 2003

shoppa at trailing-edge.com (Tim Shoppa) wrote in message news:<bec993c8.0310200757.594ff855 at posting.google.com>...
> test <test at iinet.net.au> wrote in message news:<ge92pv0ta14fj5u9o7djco6pp4ga5jl9tg at 4ax.com>...
> > Looking at the TAC-2 schematic, the 1PPS signal from the GPS module
> > gets to the DCD serial pin via one 74AC04 and one MAX232.  The
> > datasheets I have for these chips give their propagation delays as 8ns
> > for the 74AC04 and 1.3us for the transmit section of the MAX232.
> > 
> > So, the question I have is about the relevance of the 8ns receiver
> > delay.  Shouldn't it be closer to 1.3us, or am I just missing the
> > point completely?
> The 1.3 us number for the MAX232 includes the effect of driving a
> long capacitive line.  When driving a short line, the actual delays that
> I've measured are much shorter.

I went back and re-measured, and it's not all that much shorter without
a capacitive load.  It seems that the MAX232 is internally slew-rate
limited, not surprising considering that the RS-232 standard specifically
limits the slew rate and traditionally this was done by just putting
extra capacitance on driver outputs until the slew rate condition was
met.  This seems to be a feature of the MAX232 that doesn't exist
with older RS-232 drivers.

Looking inside my gadget box, I see that I used a MC1488 driver, and with
the slew-rate-limiting-capacitor removed I measure a propagation delay of
about 60ns to "assert" and about 200ns to "de-assert" on DCD (I may have the
polarities mixed up... only one matters for NTP!).  I should
also note that my motherboards' (ECS K7S5A and Via Epia) DCD input will
not reliably interrupt on pulses as short as 1 microsecond as put out by some
GPS receivers... I used a one-shot to stretch it out real long.  In
retrospect, with slew rate limiting it's likely that 1 microsecond is
just too short to be reliable.

I might try noodling around with MAX232 vs MC1488 for delivering PPS to
a commodity motherboard's serial port and see if there's any
measurable difference in jitter measured by the nanokernel.


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