Sunday, September 22, 2013

DCF77 via GPIO on the Raspberry Pi (patched radioclkd2)

Update 2015-02-15, see below.
Update 2016-02-14, see even farther below.

After replacing my old linux-PC based router with a Telco-supplied plastic-box, I no longer have a usable NTP server at home. As a first measure, I’ve put a small DCF77 module on a raspberry-pi. It seems that most people add a serial to usb converter for that, but the raspberry has perfectly fine GPIOs that are capable of generating an interrupt for low CPU load -- and that’s all one needs. You can find my very lightly patched radioclkd2 for parsing the pulses on github.
I’ve added an additional filtering capacitor to the back of the module (be careful, inrush current will crash your Pi when connecting the leads!) a long time ago when it was still connected to the old PC, and there’s a pullup from the pulse output (which typically is open-drain/open-collector) to Vcc (3.3V). In my case, on a Rev. A Raspberry, I use GPIO0 (probably a bad choice, because it’s also one of the two accessible i2c pins, but easily changed).
Unfortunately it turned out that at its current location, reception is pretty bad, but easily solved by repositioning the Pi.

Update:I got a few emails regarding this project (which I hadn't had running for quite some time) recently, so I'll try to add this information on the old blog-post.

First, if you get a lot of pulses with bad, and very short, lengths (<10ms)
  • Reposition the DCF77 receiver with a long cable, away from all your computer stuff and switch-mode power supplies.
  • Wrap a huge ferrite core around this cable, to attenuate wire-conducted noise to the DCF77 receiver.
  • Add additional filtering caps at both sides of the cable (near the Rpi and DCF77 receiver module).
That way, I could get reliable reception again.

Second, if your ntpd doesn't seem to register any time from radioclkd2, even if it seems to receive properly: If you enable debug mode in radioclkd2, it will *not* update the shared memory. So after you've verified proper operation, put radioclkd2 in the background without -d.

Update 2016-02-14: Here are two pictures showing radioclkd2 acquiring time from a DCF77 module.

From Chris’ Miscellanea

From Chris’ Miscellanea

Monday, September 02, 2013

Webmail Notifer, Linux Kernel Module

This is an update on the Webmail Notifier, a cheap chinese gadget that's nothing more than a RGB LED connected via USB. In this case, it's an especially cheap version that can only display 7 distinct colours, plus black (off).

Finally I was sufficiently annoyed to put the code into a “proper” kernel module (original module on Here's my patch that adds this particular LED gadget.

Here's the thing cycling through random colours: