[gpsd-dev] Baud barf problem on Pi3 SOLVED

[Eric S. Raymond]

It gives me great pleasure to report that applying Frank Nicholas's
recipe for disabling Bluetooth worked perfectly on the Pi 3. I now
have consistent good data from ttyAMA0.

Here is my writeup on the instructions I followed:

------------------------------------------------------------------------------
== SemPiTernal: A Raspberry Pi Stratum 1 timeserver build ==
:description: Building and configuring a tiny Stratum 1 timeserver with NTPsec
:keywords: Raspberry Pi, NTP, NTPsec, time service
by Eric S. Raymond
version 0.1

== Introduction ==

This HOWTO gives complete instructions for building a headless Stratum
1 timeserver using a Raspberry Pi, the Adafruit GPS hat, and NTPsec.
Total parts cost should be about $80.  Light soldering is required.

Why do it?  It's cheap, fun, and because your NTP server is running
on a dedicated machine you won't have so much jitter due to variable load.

The Adafruit GPS hat is a better idea than an external GPS because it
also supplies a battery-backed-up RTC, which NTP can use for holdover.

== Parts list and hardware assembly ==

You will need:

* One Raspberry Pi.  I used a Raspberry Pi 3: these instructions cover
  older variants as well.

* One
  https://learn.adafruit.com/adafruit-ultimate-gps-hat-for-raspberry-pi[Adafruit
  GPS HAT]

* One CR1220 3-volt lithium coin cell.  They're cheapest ordered from
  Adafruit along with the HAT, but you can buy them at most places
  that carry hearing-aid batteries.

* (Optional) A Raspberry Pi clear case to protect your hardware from
  dust and curious felines.

The HAT is shipped as two parts, a circuit board and a 40-pin header.
Your first step will be to solder the header to the bottom of the
board, on the opposite side from the GPS module (under what would be
the north edge if the writing on the board were map legends). Poke the
header pins upwards through the double row of through holes at the
north edge of the board and go.  This is the last bit of hardware
hacking absolutely required.

(The device is shipped as parts almost certainly to evade a regulatory
requirement for FCC conformance testing to RF emission standards,
which is expensive and would have added to the unit cost.)

The header on the assembled HAT fits down over the double row of pins
on the north edge of the Pi, such that the two boards make a neat
stack.

== Case and board modding == 

As shipped, the Pi + Hat assembly will *not* fit in a stock Adafruit
clear case.  The HAT is slightly too large for that.

As an optional step, you can modify the board and case so the HAT
fits in.  You'll do this in two steps:

First, modify the flanges that are engaged by the five clips on the
bottom of the case top. Unaltered, the flanges will physically
interfere with the HAT.

Ideally, you want to cut a rectangular piece off each flange about an
eighth of an inch horizontal dimension and about a quarter-inch
vertical dimension.  You do _not_ want to shave the flanges all the
way back to the case walls if you can avoid it: if you do that, the
clips won't have anything to engage and the top will sit slightly
loose. (Mine is modded that way.  It's not worth re-doing with a new
case.)

Your other problem is that an unused portion of the board (consisting of
an array of through holes for mounting ICs) collides with the case
shield for the Ethernet.  To fix this, cut a rectangular piece off
the south-east corner with a small coping saw.  The piece needs to be two
through-holes wide (east-west) and nine holes high (north-south).

== Beheading the Pi ==

You first step is to configure the Pi to run headless attached to an
Ethernet cable. You'll do all subsequent configuration via ssh, and
your other machines will take time corrections via TCP/IP over the
Ethernet.

Plug in a monitor, mouse, and keyboard.  Power up the Pi.  Invoke
a terminal emulator.

Normally the Pi configures itself using DHCP when you connect it to
your Ethernet, acquiring a TCP/IP address that may vary depending on
what other devices are hooked to your network. For our use case,
giving it a static address is more convenient.  Here's how to do that:

. Just in case, make a backup copy of your /etc/network/interfaces file:

------------------------------------------------------------------------
sudo cp /etc/network/interfaces /etc/network/interfaces.backup
------------------------------------------------------------------------

. Edit that file:

------------------------------------------------------------------------
sudo vi /etc/network/interfaces
------------------------------------------------------------------------

. Change the contents to look something like this:

------------------------------------------------------------------------
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.23
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 192.168.1.1
------------------------------------------------------------------------

Once the file has been updated, use :wq to save and exit.

Note: This assumes that (a) You're using the usual 192.168.1.x convention
for private addresses, (b) your router lives at .1, and (c) you have
DNS running on your router.  If any of these assumptions are false, you
probably knew what you were doing when you set up your network and
know how to modify the above.

The "23" should change to whatever low quad you want to use to make
the Pi's address unique on your network.  I like to follow up by
adding a line like this to the /etc/hosts files of the other machines
on my network, allowing me to refer to it by name: 

------------------------------------------------------------------------
192.168.1.23 timekeeper
------------------------------------------------------------------------

. Reboot the Pi.

------------------------------------------------------------------------
sudo shutdown -r now
------------------------------------------------------------------------

.  From some machine on your network, do:

------------------------------------------------------------------------
ping timekeeper
------------------------------------------------------------------------

If you don't get a ping response, troubleshoot (unplugged or loose
cables are always the first thing to check for). If you do, you can
log out and then disconnect the keyboard/screen/mouse, which you
should do to reduce power draw and heating of the board (graphics
processing for the GUI is expensive that way).  You'll do the
remaining steps over ssh.

. Remove network-manager, because it's now just dead weight eating your
cycles.

------------------------------------------------------------------------
apt-get remove network-manager
------------------------------------------------------------------------

. Now uninstall systemd.  It is a CPU hog (not just on the Pi; similar
problems have been reported across many architectures) and a
complexity sinkhole. The things that have it as a dependency are
things you don't want on a headless NTP server.

------------------------------------------------------------------------
apt-get install sysvinit
apt-get remove systemd
reboot
------------------------------------------------------------------------

. Ensure that any /etc/inittab entries referring to ttyAMA0 or ttyS0 are
commented out. You do not want a getty to spawn on these. To check
this, you can reboot and do

------------------------------------------------------------------------
ps ax | grep getty
------------------------------------------------------------------------

If neither ttyS0 or ttyAMA0 appears in any of the selected lines,
you're in good shape.

== Pi 3 only: disable Bluetooth and remap console device ==

The Pi 3 Raspbian implementors made a design decision on the Raspberry
Pi 3 that ties the serial baud rate to the CPU clock rate (by
default).  This was done because the normal lines that fed the serial
port were used for the built-in Bluetooth. This does not affect any
other Pi variant.

Your headless timeserver is not going to need Bluetooth, so you should
disable it and remap the devices.  Our instructions come from
http://www.briandorey.com/post/Raspberry-Pi-3-UART-Overlay-Workaround[this
original], which explains the problem in more detail.  We don't use
some of their steps because this build is designed to run headless 

. Copy the
http://www.briandorey.com/docs/Raspberry-Pi-3-UART-to-gpio-workaround/pi3-miniuart-bt-overlay.zip[boot
overlay] to the root directory of your Pi 3, and unzip it.

. Edit the /boot/config.txt file to append these lines:

------------------------------------------------------------------------
# Disable Bluetooth so serial-tty speed is no longer tied to CPU speed
dtoverlay=pi3-miniuart-bt-overlay
force_turbo=1
------------------------------------------------------------------------

== Release the console pins ==

. Next, edit the boot command line. You need to be root to do this.
This is what I saw on an unmodified Pi 3:

------------------------------------------------------------------------
address@hidden:/home/pi# cat /boot/cmdline.txt
dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p7 
rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait
------------------------------------------------------------------------

You may want to save a copy of the original
/boot/cmdline.txt before modifying it.

To disable the serial console, remove all "console" options.  The
Adafruit
https://learn.adafruit.com/adafruit-ultimate-gps-hat-for-raspberry-pi/pi-setup[Pi
GPS setup] page mentions a "kgdboc" option; if present, it should be
removed.

You must reboot the Pi for these changes to take effect.

== Testing the GPS ==

Next thing you want to do is verify that the GPS works.  Put your
micro-server on someplace like a windowsill with a good sky view outside.
The MKT3339 on the HAT has very good weak-signal discrimination and
is much less fussy about siting than older GPS receivers.

On first (cold) boot, the device may take 20-30 minutes to download a
satellite ephemeris. After that, time to get a fix should be much faster
unless you live in a canyon (including the urban kind) or dense forest.
I, living in a suburb with the front of my house half-screened by tall
trees, typically get lock about 30 seconds from power up.  It will
seem longer than it is first time: have patience.

(Some material in this section is from Adafruit's
https://learn.adafruit.com/adafruit-ultimate-gps-hat-for-raspberry-pi/basic-test[basic
test] page for the HAT.)

When the red LED on the HAT blinks once per second, you don't have a
satellite fix. When it achieves lock it will blink with much lower
frequency.

To test that you can read data from the device, do this:

------------------------------------------------------------------------
stty -F /dev/ttyS0 raw 9600 cs8 clocal -cstopb
cat /dev/ttyS0
------------------------------------------------------------------------

You should see NMEA0183 sentences issuing in bursts once per second.

Now install GPSD.  You won't use this for production, as we plan to
direct-connect NTPsec to the GPS, but its diagnostic tools (especially
gpsmon) are useful for sanity-checking the output of the HAT.

------------------------------------------------------------------------
apt-get install gpsd gpsd-clients python-gps
------------------------------------------------------------------------

Run 

------------------------------------------------------------------------
gpsmon /dev/ttyAMA0
------------------------------------------------------------------------

You should see a curses panel display that shows you both raw GPS
data and the results of data reduction on it.  Later, this tool
will be useful for checking that you can see the 1PPS signal from
your device.
------------------------------------------------------------------------------

My next step will be to strip out as many Raspbian packages as I can relating
to the desktop GUI.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>