Joined: 07 Mar 2006
Location: So. Cal. USA
|Posted: Tue Nov 04, 2008 8:10 am Post subject: Multi-board synchronization
|We need some more help for the proposal we are doing. We anticipate an architecture similar to that shown in the attached drawing.
The Clock generator block is ClockGen PMC. We are planning to use 8 X3-Sd XMC (only 4 are on the drawing)
We have some questions:
1) How can we implement GPS absolute time stamping in the architecture (every acquired sample should have its own time)?
2) How can we trigger all eight X3 modules simultaneously?
In order to support acquisition from eight X3-SD modules, you will require eight synchronous clock outputs. Since the ClockGen supports only four outputs, you would need to use two ClockGen modules to drive the X3-SDs. Each of the ClockGen SYNC inputs would be driven by the 10 MHz output of the Trimble GPS. In this arrangement, all eight ClockGen outputs will be synhronous to the GPS 10 MHz reference and its world time.
Additionally, you could route the epoch (1 PPS) output from the GPS into one of the digital inputs on the X3-SD module designated as the MASTER in the system. Eight individual digital outputs from the MASTER would be routed to the TRIGGER inputs on all eight X3-SD modules.
The application software would poll the epoch output from the GPS until the 1 second prior to the desired acquisition time. Then, the eight trigger digital output bits on the MASTER program would be asserted simultaneously. Subsequently, all channels on all modules will begin sampling simultaneously. Sampling will remain synchronous throughout due to the GPS sync input to each ClockGen.
Tagging of each sample is therefore unnecessary. Since acqusition started at a known GPS time, and remains synchronous to GPS time thereafter, simple dead-reckoning into the acquired data set is sufficient. That is, knowing the exact GPS start time, the time for any specific sample in the set is equal to:
|sample time = start time + (sample # / sample rate) |