I tried to connect a reichelt usb module to a linux machine running a simple interface with one opamp for rs232 conversion. That worked on a test machine but not on the server i tried it for. I dont know why but i assume the serial interface in the server had some problems with the 50 baud used by ntp.
After that i found a working dcf77 software for an atmega cpu on Ulrichs Page. I tried his software on a stk500 board and it worked fine. To connect it to ntp, i modified the software to send data supported by ntp. I used the Meinberg Protocol that was simple and supported by ntp.
The Software will wait for a valid dcf signal before the first time packet will be sent to ntp. After that, up to 4 failed dcf signals will not interrupt sending of valid time packets. In the meantime the clock will run internally in the avr. In case more than 4 invalid dcf time date was received, the failure will be reported to ntp.
Of cause, there will be some delay between the beginning of a second and the time ntp will receive the packet. They are:
3. could be reduced if the result package would be build in the 1 second interrupt routine
The hardware is really simple, one atmega8 running at 4.194304 or 16 MHz (both frequencies allow a 1 second interrupt using the 16 bit timer) and a ft232bm to handle the usb stuff.
The ft232 is supported by current linux usb stacks (i'm using fedora core 4)
First you have to create a link to the ttyUSB device because ntp wants to see /dev/refclock-n where n is a number beginning with 0.
Connect the dcf77usb interface and you will see something like
Sep 1 20:27:48 lnx kernel: ftdi_sio 1-1:1.0: FTDI FT232BM Compatible converter detected
Sep 1 20:27:48 lnx kernel: usb 1-1: FTDI FT232BM Compatible converter now attached to ttyUSB0
Sep 1 20:27:48 lnx kernel: usbcore: registered new driver ftdi_sio
Sep 1 20:27:48 lnx kernel: drivers/usb/serial/ftdi_sio.c: v1.4.2:USB FTDI Serial Converters Driver
As you can see, the new device is ttyUSB0, so
ln -sf /dev/ttyUSB0 /dev/refclock-0
will create the needed link. On newer systems where /dev is a filesystem maintained by the kernel, the link has to be created on system start. I have added the ln into the /etc/rc.d/rc.local on my fc4 system. (May be that would also work via some Linux USB Magic but i'm not aware of)
Next you need to change your /etc/ntp.conf, add the following lines
server 127.127.8.0 mode 2 prefer burst minpoll 4 maxpoll 4
fudge 127.127.8.0 stratum 1
I added the burst, minpoll and maxpoll to decrease the time until ntp is synced
You can check it with ntpq:
remote refid st t when poll reach delay offset jitter
LOCAL(0) LOCAL(0) 10 l 18 64 377 0.000 0.000 0.001
*GENERIC(2) GENERIC(2) 1 l - 16 377 0.000 -1.331 11.247
and check the dcf clock:
ind assID status conf reach auth condition last_event cnt
1 29748 9014 yes yes none reject reachable 1
2 29749 9624 yes yes none sys.peer reachable 2
ntpq> cv 29749
assID=29749 status=0000 clk_okay, last_clk_okay,
device="Meinberg DCF77 C51 or compatible",
timecode="\x02D:01.09.05;T:4;U:21.20.23; S \x03", poll=1033, noreply=0,
badformat=0, baddata=0, fudgetime1=10.000, fudgetime2=0.000, stratum=1,
refclock_time="c6c1d2f7.00000000 Thu, Sep 1 2005 19:20:23.000",
refclock_status="DST; TIME CODE; (LEAP INDICATION)",
refclock_states="*NOMINAL: 00:47:31 (100.00%); running time: 00:47:31"