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Precision Time Protocol IEEE1588

Klaus Mählert

Product Management Group Manager

Konstanz – April 17, 2012

Precision Time Protocol IEEE1588 with AVT GigE cameras

Presentation by courtesy of Allied Vision Technologies

What is Precision Time Protocol?

• An IEEE1588 networking standard

• Used to manage clock synchronization of multiple devices across an Ethernet network

• Cameras can utilize the same time base and relevance for the timestamp

• Adopted by machine vision with GigE Vision 2.0

Competing clock synchronization schemes

• Network time protocol (NTP)• Offers poor accuracy ~ 10 ms• Low cost, most common clock synchronization scheme in networking

• Global Positioning System (GPS) • Provides excellent precision ~ ns• An expensive component to incorporate into every

device• Precision Time Protocol (PTP)

• Provides excellent precision ~ µs• Low cost - can be incorporated into cameras using a

firmware change and without additional hardware

TimeStampValue represents camera clock

• The heart beat of the camera• Drives signal timing including exposure start time• Attached to every frame and used by the driver to

calculate frame rate

Enabling PTP Synchronization

• PtpMode = Off• TimeStampValue is not synchronized with any other device.

• PtpMode = Master• All Slave devices synchronize their clock to this camera's

TimeStampValue.

• PtpMode = Slave• TimeStampValue is altered to align with a Master

TimeStampValue.

• PtpMode = Auto• Uses the IEEE1588 best master clock algorithm to determine

which device is master.

Monitoring PTP Synchronization

• PtpStatus = Off (Read only)• The device PtpMode is set to off.

• PtpStatus = Master (Read only)• The device is configured and working as Master PTP device.

• PtpStatus = Error (Read only)• The slave device is in the process of synchronizing with the master.

• PtpStatus = Slave (Read only)• The device is acting as a slave and synchronized to within several

microseconds of the master.

How is PTP synchronization realized?

• Master PTP clock transmits a “Sync” telegram using multicast messaging.

• Slave PTP clock cameras receive multicast and calculate time difference between its own clock and the Master PTP clock.

• Clock synchronization, using follow up and delay messages is used to align the Slave PTP device clocks to the Master PTP clock.

How is PTP synchronization realized?

• Once a Slave clock is within 2 µs of the master, its PtpStatus is changed from Error to Slave.

• If the timestamps are 2 seconds or more apart, the Slave camera overwrites its own timestamp with the Master camera timestamp.

Implementing a scheduled software triggerThe PtpTriggerTime attribute is used to define a future TimeStampValue for start of exposure.

1. Achieve clock synchronization. Set PtpMode = Master, Slave or Auto.

2. Set camera FrameRate, FrameStartTriggerMode, and AcquisitionMode as desired. Start an acquisition stream using AcquisitionStart.

3. Determine the current camera TimeStampValue using TimeStampValueLatch.

4. Set PtpTriggerTime to a value which sufficiently exceeds the current camera TimeStampValue. Allow several seconds for the camera to receive PtpTriggerTime

PtpTriggerTime Notes

• Setting PtpTriggerTime will suspend all frame triggers in a camera’s current AcquisitionStart stream while PtpTriggerTime >= TimeStampValue

• When TimeStampValue increases to reach PtpTriggerTime, frame triggers resume

• To implement a scheduled (future) software trigger set

PtpTriggerTime = TimeStampValue + = Time required to upload

PtpTriggerTime to the camera

The PtpTriggerTime event does not start a new acquisition stream, it simply resumes frame triggers on an existing stream.

Suggested PTP Camera Networks

• A switch is required for implementing a PTP network

• Cannot currently implement PTP synchronization across multiple adapters

• LAG between host and switch has been tested and shown poor results

• 10 GigE Uplink ports are being tested and have shown good initial results

Comparing TimeStampValues between camera events and frames

• Camera EventControls allow the host to receive a TimeStampValue for a selected event, e.g. EventSyncIn1Rise. Likewise all frames returned from camera have a TimeStampValue associated with them.

• Comparison of these TimeStampValues can give valuable information, and with PTP synchronization, allow for a common time base between cameras and event sensors.

Thank you for your attention.

STEMMER IMAGING GmbHGutenbergstr. 9-1382178 PuchheimGermany

Phone: +49 89 80902-236Fax: +49 89 80902-116

k.maehlert@stemmer-imaging.dewww.stemmer-imaging.de

Your contact: Klaus Mählert