Sunday, January 20, 2013

1PPS Support with GPIO For Raspberry Pi in Fedora 17 Remix

One of my projects as of the last 5 months (although slow going) is making a full-out stratum-1 time server appliance out of the Raspberry Pi.  I'm going to be using an EM408 GPS module + generating my own 1PPS signal with an AVR attiny84 microcontroller inputting into GPIO on the Pi, and if i have enough GPIO left, will be driving a 16x4 LCD parallel output-only display for fun as well.

When it comes to just using the Raspberry Pi as a NTP stratum-1 server, there's quite a few documented artifacts out there on the internets already being done, most notably from David Taylor's excellent documentation on his website.  

It's worthy to note that most, if not all, Raspberry Pi Linux distribution spins DO NOT come with CONFIG_PPS or CONFIG_PPS_CLIENT_GPIO compiled into the kernel or as modules.  

I'm not totally down with the amount of hackery floating around to insert dirty modules into the kernel.  Ya, it works.  Yes, if you have  CONFIG_MODVERSIONS set, then you should be able to install modules not compiled with your kernel into it.  However, I'm rolling with Fedora 17 remix and option isn't compiled in with the stock kernel.  So either way, I had to result to building my own RPMs for this.  

If you're interested in getting 1PPS supprt on the Raspberry Pi via GPIO (like I and many other time enthusiasts are), then here's some steps to get you in the right direction.  Below are some options you have:

Download 1PPS Packaged RPM for Fedora 17 Remix

Here's a link to my binary RPM package for 1PPS, which includes support for:
  • PPS Support (CONFIG_PPS=m)
  • PPS Source using GPIO pin (CONFIG_PPS_CLIENT_GPIO=m)

NOTE:  Yes, I am using the FC 18 source rpm, but they will install on FC 17.  I have tested.

Build your own Kernel + RPMs

There's quite a few step(s) I'll omit since it's clearly beyond the scope of this blog post, but here's a quick and dirty run-down on how to do it on an RPM-based Linux distribution:

1)  Build yourself a soft-float ARM cross compiler (at least for Fedora 17 remix) using crosstool-ng

2)  Set up your RPM build environment using `rpmdev-setuptree`

3)  Download the raspberry-pi-kernel source RPM from here

4)  Patch the kernel config ( in SOURCES folder) and .spec file (in SPECS folder) using `patch` with my patches that are posted on the Fedora Remix as an 'enhancement' option:

5)  Rebuild kernel package + sub-packages with:  

# rpmbuild -bb --clean --target=armv5tel raspberrypi-kernel.spec

6)  Copy 'at least' the kernel-pps RPM package to you RPi and install with:

# rpm -ivh raspberrypi-kernel-pps-3.2.27-1.20120926git9245b4c.rpfr18.armv5tel.rpm

7)  On the RPi, run:

# modprobe pps-gpio

     ...then verify the module loaded correctly with:

# dmesg

     ...and look for some output like:
[ 1404.141098] pps_core: LinuxPPS API ver. 1 registered
[ 1404.141123] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti 

Do note that applying my patches in step #4 will also apply some of my other .spec file changes that change the way the kernel image is built and installed on the FC remix distro.  Do take a look at the changes and don't apply blindly if you can help it.  This only matters if you plan to install my kernel RPM.


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