Linux Time Syncrhronization

Many applications in factory automation, test and measurement, and telecommunications require close time synchronization. This is often a scenario where multiple boards are transmitting on the same RF channel and the system clocks must be synchronized precisely. Time Division Multiplex (TDM) is used so the transmitters are not producing collisions, thus maximizing RF throughput.

There are two common solutions to this problem:

IEEE 1588 Precision Time Protocol - uses Ethernet frames GPS Pulse-Per-Seconds - uses GPS information

Time synchronization via IEEE 1588 Precision Time Protocol (PTP) (Ethernet)

The purpose of the IEEE 1588 Precision Time Protocol (PTP) is to synchronize the time between different nodes on an Ethernet network.

The Linux kernel has IEEE 1588 PTP support for a handful of ethernet devices that offer this feature including the Freescale IMX6 FEC which is used as eth0 on the majority of the Gateworks Ventana boards including the GW51xx, GW52xx, GW53xx, and GW54xx (Note GW5520 does not use the IMX6 FEC and instead provides 2x full GigE capable PCIe based MAC/PHY devices so it lacks IEEE 1588 hardware support).

In hardware PTP the ethernet MAC is responsible for getting and setting the timestamps to remove latency of the queuing and network stack which provides the best accuracy.

The ​http://linuxptp.sourceforge.net/ project is an implementation of the IEEE 1588 for Linux featuring:

• Supports hardware and software time stamping via the Linux SO_TIMESTAMPING socket option.
• Supports the Linux PTP Hardware Clock (PHC) subsystem by using the clock_gettime family of calls, including the new clock_adjtimex system call.
• Implements Boundary Clock (BC) and Ordinary Clock (OC).
• Transport over UDP/IPv4, UDP/IPv6, and raw Ethernet (Layer 2).
• Supports IEEE 802.1AS-2011 in the role of end station.
• Modular design allowing painless addition of new transports and clock servos.

Example usage on a GW54xx:

apt-get install linuxptp -y

master:

root@ventana:~# ptp4l -i eth0 -m
ptp4l[112.404]: selected /dev/ptp0 as PTP clock
ptp4l[112.409]: driver changed our HWTSTAMP options
ptp4l[112.409]: tx_type   1 not 1
ptp4l[112.409]: rx_filter 1 not 12
ptp4l[112.409]: port 1: INITIALIZING to LISTENING on INITIALIZE
ptp4l[112.409]: port 0: INITIALIZING to LISTENING on INITIALIZE
ptp4l[120.064]: port 1: LISTENING to MASTER on ANNOUNCE_RECEIPT_TIMEOUT_EXPIRES
ptp4l[120.065]: selected best master clock 00d012.fffe.13f09d
ptp4l[120.065]: assuming the grand master role

• the ptp4l tries to use hardware time stamping by default, otherwise you can use the '-H' to force hardware timestamping, or '-S' to force software timestamping
• the -m parameter tells ptp4l to output messages to the console
• no master was found so this target became the master

slave:

selected /dev/ptp0 as PTP clock
ptp4l[228.239]: driver changed our HWTSTAMP options
ptp4l[228.239]: tx_type   1 not 1
ptp4l[228.239]: rx_filter 1 not 12
ptp4l[228.239]: port 1: INITIALIZING to LISTENING on INITIALIZE
ptp4l[228.239]: port 0: INITIALIZING to LISTENING on INITIALIZE
ptp4l[229.995]: port 1: new foreign master 00d012.fffe.13f09d-1
ptp4l[233.995]: selected best master clock 00d012.fffe.13f09d
ptp4l[233.995]: port 1: LISTENING to UNCALIBRATED on RS_SLAVE
ptp4l[235.995]: master offset 250446314787536 s0 freq      +0 path delay      3768
ptp4l[236.995]: master offset 250446314782408 s1 freq   -5128 path delay      3768
ptp4l[237.995]: master offset      -1635 s2 freq   -6763 path delay      3768
ptp4l[237.995]: port 1: UNCALIBRATED to SLAVE on MASTER_CLOCK_SELECTED
ptp4l[238.995]: master offset        754 s2 freq   -4864 path delay      2996
ptp4l[239.995]: master offset       1083 s2 freq   -4309 path delay      2404
ptp4l[240.995]: master offset        248 s2 freq   -4819 path delay      2404
ptp4l[241.995]: master offset        -53 s2 freq   -5046 path delay      2372
ptp4l[242.995]: master offset       -129 s2 freq   -5138 path delay      2341

• this was started after the master, so it found the master and became a slave

References:

• ​Introduction to linuxPTP
• ​Linux PTP Project
  • ​Linux PTP code
• ​Linux PTP Documentation
  • ​sysfs API
  • ​userspace test program
• ​Configuring PTP Using ptp4l

Time synchronization via PPS (Pulse-Per-Second) (GPS)

One handy feature of an on-board GPS is that it can deliver Pulse-Per-Second (PPS) signal. Most Gateworks SBCs have the option to load an on-board GPS (see individual product pages for specific model numbers and details). The PPS signal can be used to provide a high-precision time reference that an application can use to adjust system clock time or synchronize boards on a network. Common use is to configure the Network Time Protocol Daemon (NTPD) with a PPS source to obtain a wallclock-time with sub-millisecond synchronization to UTC.

The PPS signal is routed to a processor GPIO that can be configured to generate an interrupt for use by the user application. Additionally the PPS signal is optionally routed to one (or several, depending on model) of the Mini-PCIe sites (Pin 49) for use by a radio/peripheral to provide synchronization. Contact support for more information on each board model.

See the GPS page for more info.

Octeon TX IEEE 1588

From CN80xx refrence manual:

The CN80XX hardware supports very accurate timestamping in the PTP, BGX, GTI, and NIC blocks. This timestamping is suitable for use in IEEE 1588 Precision Time Protocol (PTP) or other purposes.

The CN80XX outbound timestamping hardware captures timestamps, but cannot insert the timestamp into any outgoing packets. Using IEEE 1588 vernacular, this means that the CN80XX outbound timestamping hardware is most useful when implementing a two-step clock, not a one-step clock, for PTP event messages sent from CN80XX. After the hardware captures the timestamp for the outgoing 1588 event message and delivers it to software, the 1588 software running on the CN80XX may need to send the timestamp to required recipients in a subsequent 1588 general message as a second step.

NIC has no specific mode supporting PTP timestamping, but contains a number of features to compensate for the PTP timestamp introduced by BGX when it is timestamping. When BGX is in the receive-timestamp mode, NIC receives the timestamp plus packet from the BGX. NIC packet parsing is able to skip over the received timestamps. NIC MAXERR, MINERR, and LENERR checks can compensate for the additional header, since the MAXERR and MINERR byte counts are programmable, and the programmed value can easily be increased by 8. The LENERR check already compensates based on the skip value. NIC makes the timestamp available for software with every packet. The timestamp can be present in the completion-queue entry and/or the receive buffer in L2/DRAM for software, depending on NIC configuration. The software will likely use the timestamp when IEEE 1588 event messages arrive. For other packets, the software may choose to discard or ignore the timestamp that is present with every packet in this mode.