Setting up the Raspberry Pi to talk to the GPS/RTC board / 2020-07-02

2020-07-02 Setting up the Raspberry Pi to talk to the GPS/RTC board 9 months ago
With most of the hardware in, it is time to configure the Raspberry Pi to allow the GPS/RTC board to be installed. One tip was to do this before installing the board to avoid serial conflicts.

First steps based on Building a GPS Time Server with the Raspberry Pi 3 which uses a different GPS board.

Disabling tty service on the UART:
# systemctl stop serial-getty@ttyAMA0.service
# systemctl disable serial-getty@ttyAMA0.service
And make changes to /boot/cmdline.txt to disable serial console, removing the console=serial0,115200 part.

Also needed is to disable the use of the hardware uart for bluetooth. This device does not need to do bluetooth at all, so I disable the software.
sudo systemctl disable hciuart
And add the lines to disable the bluetooth uart to /boot/config.txt:
dtoverlay=pi3-disable-bt
And with that the UART is completely free to use for GPS and PPS messages. I made all these changes and only added the GPS/RTC hat to the Pi after these changes were done.

Next steps were to add the i2c settings according to the GPS/RTC manual. For this I added
dtoverlay=i2c-rtc,rv3028
dtoverlay=pps-gpio
And indeed the i2c bus appears as the manual says:
# apt-get install python-smbus i2c-tools
[..]
# i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- 42 -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- UU -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         
I removed the fake-hwclock package and tested operation. On the commandline it works, but in a reboot I still see weird times in the log.

After that I did the changes to /lib/udev/hwclock-set, now it looks like:
dev=$1

#if [ -e /run/systemd/system ] ; then
#    exit 0
#fi
if [ -e /run/udev/hwclock-set ]; then
    exit 0
fi

if [ -f /etc/default/rcS ] ; then
    . /etc/default/rcS
fi

# These defaults are user-overridable in /etc/default/hwclock
BADYEAR=no
HWCLOCKACCESS=yes
HWCLOCKPARS=
HCTOSYS_DEVICE=rtc0
if [ -f /etc/default/hwclock ] ; then
    . /etc/default/hwclock
fi

if [ yes = "$BADYEAR" ] ; then
#    /sbin/hwclock --rtc=$dev --systz --badyear
    /sbin/hwclock --rtc=$dev --hctosys --badyear
else
#    /sbin/hwclock --rtc=$dev --systz
    /sbin/hwclock --rtc=$dev --hctosys
fi

# Note 'touch' may not be available in initramfs
> /run/udev/hwclock-set
The rtc has to be configured correctly, I used information from A Raspberry Pi Stratum 1 NTP Server - Phil's Occasional Blog to configure the rv3028 chip. Get the gpsctl tool and use configure-rv3208.sh to set up the chip. Now the rtc is correct and used at boot time.

I'm seeing NMEA messages when I run gpsd or ask the serial port for data. The NMEA messages are very limited because there is no GPS antenna connected yet.

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