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Chapter 4

The GPS-based Clock

Synchronization System

To overcome the issues related to the synchronization of geographically separated nodes

we decided to use the Global Positioning System (GPS) to provide a time reference. The

method that is proposed uses custom made clock boards and GPS cards to do the synchro-

nization. The computers part of the testbed are organized in sites that use a local method

of synchronization — all computers in a site are interconnected with wires that transmit

the timing information. To synchronize two remote sites we use a global synchronization

method that uses the GPS as a reference of time.

4.1 Resources that are used

We are using oﬀ-the-shelf GPS receivers with the satellite signal rec eived via the exterior

aerials and connected to the PCI bus of the computer. The current implementation uses

Meinberg GPS167PCI cards. The card has a 9-pin connector that provides two important

outputs: one is a 10MHz clock signal and the other is PPS signal (Pulse-Per-Second) issued

at every change of the UTC second with accuracy of 250 nanoseconds. The GPS can also

provide the time to the hosting computer – the minimum time between two readings is of

1.5ms.

The clock cards are designed at CERN and contain an FPGA programmed to run a 32 bit

counter (the clock). These counters are used by the traﬃc generators to time-stamp the

packets and measure latency. Normally the counter runs according to an external clock

signal, but it c an be reseted on request. Currently the frequency of the counter is 40MHz

and is derived from the GPS clock signal (the 10MHz frequency from the GPS is multiplied

by 4 in hardware and we obtain a frequency of 40MHz). At each site we have one master

clock card and several slave clock cards — the master card can send commands to the slaves

to trigger a reset of the counters.

The software is composed of two kernel modules (one for the clock card and one for the

GPS) and some manager programs. The manager is installed on the master computer in

each site and the other computers in a site run a program that accepts commands from the

site manager and that c ontrols the clock cards.

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Masterclock GPSPCI Spécifications Page 17