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SubjectRe: Serial port blues
Joe Barr wrote:
> I'm forwarding this post by the author of a great little program for
> digital amateur radio on Linux, because I'm curious whether or not the
> problem he is seeing can be resolved outside the kernel.
>
> All comments welcome on/off list.
>
> Thanks,
> Joe Barr
> K1GPL

[...]

>
> I've spent the last day staring at the oscilloscope and pins RTS and DTR
> on the serial output for 4 different computers running 4 different
> versions of Linux. Also have exhausted the search on the internet for
> information regarding both the latency and jitter associated with ioctl
> calls to the serial driver (both ttyS and ttyUSB). I'm sure it is out
> there somewhere, I just cannot find it.
>
> I am now convinced that the current serial port drivers available to us
> on the Linux platform WILL NOT support CW and/or RTTY that is software
> generated in a satisfactory manner.
>
> To test the latency and jitter of the ioctl calls to set or clear RTS
> and / or DTR I built a basic square wave generator with microsecond
> timing precision. The timing could be derived either from the select
> system call or by controlled i/o to the sound card. Both provide very
> precise timing of the program loop. Each time through the loop either
> the RTS/DTR was set or cleared. The timing jitter for each 1/2 cycle
> was from 0 to +4 msec. This varied between systems as each had
> different cpu clock rates. The jitter is caused by the asynchronous
> response of the kernel to the request to control the port. ioctl
> requests apparantly do not have a very high priority for the kernel.
> They are probably just serviced by a first-in first-out interrupt
> service request loop. That type of jitter is tolerable up to about 20
> wpm CW. It totally wipes out the ability to generate an FSK signal on
> the DTR or RTS pin.

Okay, here he's using bit-banging of the DTR and RTS pins to generate a
fairly high precision output wave. This is not really the

> Direct access to the serial port(s) is a kernel perogative in Linux.
> Only kernel level drivers are allowed such port access.

So write a kernel driver. It's not like we're locking anybody out.
There is certainly enough Amateur Radio/Linux crossover that a kernel
enhancement to support Amateur Radio is going to get frowned upon.

> So ... bottom line is that all of my attempts over the past couple of
> months to provide CW and / or FSK output signal have been to fraught
> with pitfalls. The CW seems OK for slow speed keying, but the FSK seems
> impossible to achieve.
>
> The FSK using the UART is also limited by the Linux operating system and
> the current drivers. That limitation excludes the use of 45 or 56 baud
> BAUDOT.

That is true at the moment (due to unfortunate design choices made early
on), but this is already in the process of being changed:

http://lkml.org/lkml/2006/10/18/280

> Until such time as new information becomes available I am going to
> comment out all references to CW and / or FSK via RTS/DTR. I also
> question how useful the FSK on TxD (UART derived) might be to most users
> since the 45.45 baudrate is not available in the serial port driver.
> That function will also be commented out.
>
> All this should not really come as a surprise since Linux is not a
> real-time operating system. By the way, I did try the tests with the
> test program running with nice -20. Not much difference.

See again how he should be using real-time priority rather than nice -20.

> Sorry folks, but we win some and lose some.
>
> 73, Dave, W1HKJ

-hpa

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