[ntp:questions] Atom PPS with parallel port

William Unruh unruh at invalid.ca
Sun Feb 23 20:00:26 UTC 2014

On 2014-02-23, Rob <nomail at example.com> wrote:
> I would like to use the Atom driver (22) on a Linux system with a
> parallel port.  It is not clear to me from the scattered info I have
> found on internet if this is going to work.
> Using a modern Linux kernel with the PPS module, is it possible to
> symlink /dev/pps0 to a parallel port device and then connect the PPS
> signal to the ACK input (pin 10)?


> If not, what else is required to get this working?

modules pps_core and ppd_parport

> Examples always refer to the use of a serial port DCD input, but for
> best accuracy (in the microsecond range) I think the parallel port
> is better.
> (no RS232 drivers/receivers, no funny UART that may delay interrupts)

Probably true. But it depends on how the the interrupt servicing
routines with the drivers are designed. It could be faster it could be
slower. I suspect that on modern machines, the hardware is not the
problem. They run at GHz ( which is nano second range)

> Any other suggestions for an accurate PPS input?

You could run a wire from one of the parallel output ports to the
parallel or serial interrupt line, and time when the signal goes out the
parallel port and comes into the ports to see the time difference. 

One problem is that in a heavily loaded system, interrupt processing
could be slow, and in any case will be in the few usec region. 

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