The purpose of the IEEE 1588 Precision Time Protocol (PTP) is to synchronize the time between different nodes on an Ethernet network. Many applications in factory automation, test and measurement, and telecommunications require very close time synchronization. This requirement is often well beyond what can be provided by a standard software solution.

Table of Contents

Introduction to PTP

The IEEE 1588-2002 standard defines a protocol, Precision Time Protocol (a.k.a PTP Version 1), which enables precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication, local computing, and distributed objects. The PTP applies to systems communicating by local area networks supporting multicast messaging, including (but not limited to) Ethernet. This protocol enables heterogeneous systems that include clocks of varying inherent precision, resolution, and stability to synchronize. The protocol supports system-wide synchronization accuracy in the sub-microsecond range with minimal network and local clock computing resources.

The PTP is transported over UDP/IP. The system or network is classified into master and slave nodes for distributing the timing and clock information. Figure 1 shows the process that PTP uses for synchronizinga slave node to a master node by exchanging PTP messages.

Figure 1. 《Synopsys DesignWare Cores Ethernet MAC Universal Databook》4.1 IEEE 1588-2002 Timestamps

Most of the PTP implementation is done in the software above the UDP layer. However, the hardwaresupport is required to capture the exact time when specific PTP packets enter or leave the Ethernet port at the GMII/MII. This timing information must be captured and returned to the software for the proper implementation of PTP with high accuracy.

Check if PTPv2 is supported?

To check whether support IEEE 1588-2008 standard (a.k.a PTP Version 2), read HW feature register bit 13 and bit 24.

SP7350 does not support PTPv2.

Compare with NTP

	NTP	IEEE 1588

	NTP	IEEE 1588
Communication	Internet	LAN
Accuracy	Millisecond	< Microsecond
H/W Support	No	Usually required

Network Time Protocol (NTP) has been the traditional way to synchronization time over Ethernet networks. NTP allows time synchronization up to 100 milliseconds. The IEEE 1588 PTP is required to achieve tighter synchronization. In software PTP applications, single link synchronization in the range of 100 microseconds can be reached. Hardware assistance is required to achieve time synchronization in the nanosecond region.

Hardware Timestamping of PTP

Hardware time stamp unit (TSU) incorporate a time of day (TOD) accumulator that measures the passage of time by counting the cycles of a reference clock signal from a PTP hardware clock (PHC). The PHC is steered by the PTP clock servo that issues corrections (clock operations) to the PHC. The IEEE 1588 protocol stack obtains the PTP timestamp information from the hardware TSU and provides it to the clock servo.

Check whether your NIC(Network Interface Card) supports PTP with the following command.

$ ethtool -T eno1
Time stamping parameters for eno1:
Capabilities:
        hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
        software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
        hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
        software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
        software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
        hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
        off                   (HWTSTAMP_TX_OFF)
        on                    (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
        none                  (HWTSTAMP_FILTER_NONE)
        ptpv1-l4-event        (HWTSTAMP_FILTER_PTP_V1_L4_EVENT)
        ptpv2-l4-event        (HWTSTAMP_FILTER_PTP_V2_L4_EVENT)
        ptpv2-l2-event        (HWTSTAMP_FILTER_PTP_V2_L2_EVENT)

The above example output indicates support for hardware time stamping.

Run PTP App on SP7350

We use linuxptp project to test PTP flow on SP7350, it contains several tools to use, like ptp4l, phc_ctl etc.

Basically, just follow these steps:

1. Run PTP Grandmaster within LAN

PTP grandmaster has accurate time source, such as GPS.

We run ptp4l on an PC as a PTP grandmaster to test.

$ sudo ./ptp4l -i enp2s0 -m -S

ptp4l[34476.707]: port 1 (enp2s0): INITIALIZING to LISTENING on INIT_COMPLETE
ptp4l[34476.707]: port 0 (/var/run/ptp4l): INITIALIZING to LISTENING on INIT_COMPLETE
ptp4l[34476.707]: port 0 (/var/run/ptp4lro): INITIALIZING to LISTENING on INIT_COMPLETE
ptp4l[34483.780]: port 1 (enp2s0): LISTENING to MASTER on ANNOUNCE_RECEIPT_TIMEOUT_EXPIRES
ptp4l[34483.781]: selected local clock 94c691.fffe.fc9abf as best master
ptp4l[34483.781]: port 1 (enp2s0): assuming the grand master role

2. Run PTP Slave

To run ptp4l on SP7350, we need cross compile linuxptp, and install it on SP7350 Linux Roofs.

Check the master offset value, if its absolute value is getting smaller and smaller, it means that the time difference between slave and master is getting smaller and smaller, i.e., time synchronization is successful.

Check NIC at promise mode?

If not, you can switch over manually with this command.

Thus, SP7350 can receive all the broadcast packets.

3. Check Synchronization Result

Use tool phc_ctl to check PHC time.

Before synchronization.

After synchronization.

C3V Documentation

Synchronizing Clocks Using PTP Over Ethernet