NTP

From https://en.wikipedia.org/wiki/Network_Time_Protocol, "Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks." Practically, it allows you to have current time/date on your APF even if your don't have an RTC (you will need an Internet connection or a GPS ;-)).

Get current Time from Internet

# ntpd -nqp 217.147.208.1

Time synchronisation from GPS

• You will need the "real" NTP daemon, not the Busybox one.
• Edit config file (/etc/ntp.conf):

# Allow only time queries, at a limited rate, sending KoD when in excess.
# Allow all local queries (IPv4, IPv6)
restrict default nomodify nopeer noquery limited kod
restrict 127.0.0.1
restrict [::1]

server 127.127.20.0 mode 17 minpoll 4 maxpoll 4 prefer
fudge 127.127.20.0 flag3 1 flag2 0 flag1 1 time1 0.0

• 127.127.20.0 means: "use /dev/gps0 and parse it as NMEA to get time" (See [1]).
• mode 17 is for 9600 bauds GPS providing GPRMC frames

• To create /dev/gpsX (here GPS is connected to UART2):

# ln -sf /dev/ttymxc1 /dev/gps0

• To launch ntpd:

# /usr/sbin/ntpd -g -c /etc/ntp.conf

Links

• https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html