[time-nuts] PPS over USB (Was SiLabs)

Robert Atkinson robert.atkinson at genetix.com
Mon Aug 14 03:14:52 EDT 2006


Hi,
USB is no use for any time critical application. We put considerable
effort into using it for an industrial automation type I/O application a
few years ago and it was a waste of time and effort. The protocols are
non-deterministic and the data is packetised so timing is inconsistent.
There are problems with even "simple" USB to RS232 applications (I have
experience of problems on motion control, Development systems and
instrumentation (Gamma Ray Spectrometer). Some adaptors work better than
others (The FTDI chipsets work better than most), but even if you find
one that works, small changes in the computer configuration can cause
havoc. It only takes someone to plug a USB device with poor drivers into
the system to mess everything up. When you come to fault-find, the
offending device has often gone, but the damage has been done.

Robert G8RPI.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hal Murray
Sent: 11 August 2006 21:05
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] PPS over USB (Was SiLabs)


> Is the USB bus really a good way to get 1 PPS into a machine? I would
> think it would have quite a bit of jitter in it. 

I'm not a USB wizard, but I work with people who are.

There is no interrupt mechanism on a USB cable.  The host/master polls
the 
devices/slaves.  Typical numbers are 10 ms or 1 ms.

At setup time (I forget what USB calls it), the device can tell the host
how 
often it wants to be polled.  (I assume the host can ignore that or
round up 
to what it can handle.)


For a homebrew project, it might be possible to turn things inside out.
Have 
the host flap a signal, say from the printer port, and the GPS device
records 
the time.  Then the device reports that time via USB.

One of the popular GPS over serial port devices works that way.  (I
forget 
which one.)

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