[LinuxPPS] Re: PPS driver for ISA bus

[Cirilo Bernardo]

>> it's easy to make a simple PCB for ISA which only connects Ground (perhaps
>> XClock and +5VDC) and _one_interrupt_line_.
>
> Ah! Ok, now I see.
>
Yes, the wiring is something like that.

>> So the question is, how to add some PPS-detection into the ISA-bus ISR.
>
> Do you mean how to auto-magically detect the board? If so with ISA
> board you may only use heuristic methods.

I wouldn't use automagic detection; I believe if the ISA bus is being used it is best for an
administrator to configure things by hand. I will make the module very simple - only 1 such
PPS device can exist and the only parameter needed is the IRQ number. If anyone says later on
that they want 2 or more such devices (unlikely because you run out of IRQs so quickly), it
is not difficult to extend the driver.

I have a Winsystems PPM-GPS:
(http://pc104.winsystems.com/products/pc104/pcmgps.html)

I am using this for an embedded project.  The PPS signal of the GPS is connected
directly to an interrupt line on the ISA bus, rather than connected to the DCD line.

Give me a few days; I will make 2 drivers - one which mindlessly uses the PPS interrupt
and another more sensible one which only calls pps_event when the GPS is locked to a
satellite signal. The problem with using only the PPS interrupt without checking for a
satellite lock is that the PPS can jump when a satellite signal is acquired.  However there
are PPS sources which do not behave like that, so even the simple driver may have its
uses. The simple driver is also generic - any PPS signal on the ISA bus can be used;
the other driver will be specifically for the PPM-GPS device because I have to control the
serial communications channel directly to get the behavior I want.

I have no idea how many people actually use this device I have - but some people may
find the drivers useful.

- Cirilo
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