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Pam Klabbers | USLHC | USA

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Delicate Timing

I have been remiss. I haven’t written anything since the beginning of March. Can I blame my husband and my apartment? He’s been working really hard so I have been picking up some of the slack at home…and we’ve been painting our little apartment. I am certain that in the 10 years its been in existence, it hasn’t been painted even once, so it is very necessary. Our walls are heavily textured and suck up the expensive and thick acrylic paint like a sponge. It was hard work, but it looks way better!

CERN recently had Good Friday and Easter Monday as lab holidays, so the whole lab quieted down for a four day break. Just before it started, I decided to do a study of the relationship of two clocks used by my system.

In order to capture the data coming on our links from the calorimeters we have one clock at a specific frequency. It has one tick every 8.33 ns (0.00000000833 s), or 1/3 of the LHC machine’s tick of once per 25 ns (every LHC tick is a possible collision). We do this to speed up the rate at which our data is processed on our boards.

Our second clock has one tick every 6.25 ns or every 1/4 of a LHC machine tick. We have a specially designed high-speed chip, called the Phase ASIC (Application Specific Integrated Circuit) which takes the data coming from the links every 8.33 ns and rearranges it to run at the 6.25 ns clock. This way we can process lots of data quickly, in parallel. However, the clocks’ phase has to be good, that is the relative ticks have to be aligned in a certain way. This is pretty delicate and requires careful study.

I spend two evenings, until 10pm, clicking buttons on display to make a study of this relationship. I can’t say it was exciting. It was necessary to take frequent breaks to avoid clicking too soon or missing one…about like watching my paint dry. But at the end I had a nifty plot, which gave me a great idea of where I should set those clocks.

This has to be repeated, but the next time I will automate! I didn’t have the tools then, but I do now, thank goodness. It can even run without me.

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