![\"Preview](\"\/blog\/wp-content\/uploads\/2014\/05\/ntp.png\")
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Introduction<\/strong> Using NTP can be very important for many applications. When you visit an “https:\/\/” site with your browser, the connection is encrypted with SSL. SSL requires that the time on your clock is synchronized with the time on the server that you are connecting to with your browser. Having accurate time is imporant when evaluating server logs, such as in troubleshooting. When logging into work remotely with a VPN client, the time on your laptop needs to be accurate, and in synch with the time on the VPN server. Even simple things, such as logging into an Active Directory domain or LDAP, require an accurate clock. Many, many, many online transactions, in one form or another, will require accurate clocks.<\/p>\n A Raspberry Pi is a small “system on a chip” board, where all the standard components of a computer (networking, video, audio, CPU, RAM, etc) are on a single board. All that is required is external power, and storage for an operating system, which in the case of the Raspberry Pi is an external SD card. These little devices are cheap, and have been used for everything from powering 3D printers to home automation, such as turning on and off automatic sprinkler zones.<\/p>\n In this post, well setup a stratum 1 NTP server that uses GPS satellites as the time source, and it will deliver that time to the Internet.<\/p>\n Materials<\/strong><\/p>\n So, for about $130 plus tax and shipping, you can have a very accurate stratum 1 NTP server.<\/p>\n One note, however: the Raspberry Pi does not come with a hardware clock, so NTP is required to keep the clock ticking. You may want to install a real time clock (RTC) as well, such as the ChronoDot<\/a>. The GPS board has an RTC, which is battery backed, so that’s good enough for me. Unfortunately, the kernel can’t see the GPS RTC as a separate hardware device. This isn’t necessarily a problem; if the antenna loses connection to the orbiting satellites, then the GPS RTC will keep ticking away, giving the Pi its time, and NTP will continue to keep the clock corrected, so long as you keep an Internet connection running.<\/p>\n The Hardware Setup<\/strong> You should note that you will need to solder the both battery house and the small breadboard breakout pins to the GPS breakout board. This isn’t too difficult, but if you have zero experience soldering, you will want to get familiar with how to tin and solder first, before attempting this build.<\/p>\n Operating System Setup<\/strong> I’m not a big fan of setting up the HDMI and keyboard on the physical Raspberry Pi. So if you’re like me, you’ll want to SSH to it immediately. I wrote a blog post on my personal blog about masquerading ethernet interfaces<\/a>, so that can help here. Mount the SD card, edit the “\/etc\/network\/interfaces” file, and setup the virtual interface as described in that blog post. Then, when booting up the Raspberry Pi, you can insert an ethernet cable from your laptop to your Pi, and SSH directly.<\/p>\n Install Custom Kernel<\/strong> Open up a terminal, and as root, install Git then clone his respository:<\/p>\n Now copy over the new kernel and kernel modules:<\/p>\n Make sure that the “pps-gpio” module is loaded on each boot by making it persistent. Add it to the “\/etc\/modules” config file by running the following command (make sure you’re appending STDOUT):<\/p>\n Modify the “\/boot\/config.txt” to reflect the new kernel. Add the following line at the end of that file:<\/p>\n Reboot, and verify that you are in the right kernel, and that the “pps-gpio” module is loaded:<\/p>\n Compile NTP<\/strong> Now, in your terminal as root, type the following commands:<\/p>\n
\nI wanted to build my own NTP stratum 1 time server. An NTP server is a server that connects to the Internet to serve time to clients who can use that time to adjust the clock on their computer. NTP uses stratum levels<\/a> to determine the distance from the source clock. A stratum 0 NTP server is actually the source clock. This could be something like a GPS satellite, a CDMA or GSM cellular broadcast tower, a cesium fountain, and many other sources. A stratum 1 NTP server is a server that receives time from a stratum 0 source. A stratum 2 NTP server is a server that connects to a stratum 1 NTP server, and so forth. A stratum 16 NTP server is considered as “unsynchronized”.<\/p>\n\n
\nThe completed setup will look something like this, including the pin breakout. Note that the GPS PPS output connects to pin #23 on the Raspberry Pi, and the GPS TXD connects to the Raspberry PI RXD. 5V power to VIN, and GND to GND. If you want, you can connect another cable from the GPS RXD to the Raspberry PI TXD, for troubleshooting (not shown). The breadboard back has a sticky adhesive backing that I applied to the case, so it remains permanently fixed.<\/p>\n![\"Completed](\"\/blog\/wp-content\/uploads\/2014\/05\/finished-product-300x225.jpg\")
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\n![\"Breadboard](\"\/blog\/wp-content\/uploads\/2014\/05\/rpi-breakout-300x225.png\")
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\nRather than using the stock Raspbian, we will be using a fork from Adafruit called “Occidentalis<\/a>“. The latest version as of this post is “v0.2”. It still uses the Raspbian repositories, but comes with some additional packages pre-installed. Download the image, and write the image to an SD card:<\/p>\n# dd if=\/path\/to\/occidentalis.img of=\/dev\/sdz<\/pre>\n
\nUnfortunately, and this royally sucks, the default kernel shipped with Raspbian does not support PPS over any of the hardware pins. Further, the Raspbian kernel does not ship with a PPS module at all. So, we need to use a custom kernel. It sucks, because the kernel is old. Rolling your own custom kernel, means you’re a super villian<\/a>. However, someone has already done the heavy lifting for us, and it shouldn’t be too difficult to apply the same patches to more modern kernels. However, we’ll just stick with his 3.1 kernel for this post.<\/p>\n# apt-get install git\r\n# git clone https:\/\/github.com\/davidk\/adafruit-raspberrypi-linux-pps.git<\/pre>\n
# cd adafruit-raspberrypi-linux-pps\r\n# cp kernel.img \/boot\/kernel.img.pps\r\n# cp -a modules\/* \/lib\/modules<\/pre>\n
# echo 'pps-gpio' >> \/etc\/modules<\/pre>\n
kernel=kernel.img.pps\r\ngpu_mem=16<\/pre>\n
# uname -a\r\nLinux ntp.example.com 3.1.9adafruit-pps+ #21 PREEMPT Sun Sep 2 10:57:58 PDT 2012 armv6l GNU\/Linux\r\n# lsmod | grep pps-gpio\r\npps_gpio 2314 0 \r\npps_core 7808 2 pps_gpio,pps_ldisc<\/pre>\n
\nThe default Raspbian NTP package does not enable ATOM support, which we’ll need to take advantage of. As such, we’ll need to custom-compile NTP. Thankfully, Debian makes this easy. Pull up a terminal, and as root, edit the “\/etc\/apt\/sources.list” file, and add the following line:<\/p>\ndeb-src http:\/\/mirrordirector.raspbian.org\/raspbian\/ wheezy main contrib non-free rpi<\/pre>\n
# apt-get update\r\n# apt-get build-dep ntp\r\n# apt-get source ntp\r\n# cd ntp-4.2.6.p5+dfsg<\/pre>\n