HI. I'm not sure you can count on the on-board clocks to be within 1% of the nominal clock without an external reference. I'd check that with an oscilloscope before I did anything else...
John > Hi Casper, > Is there any way to find out the FPGA clock rate precisely? We are > suspecting that our ROACH2s are not running at the frequency (202MHz) as > we specified in the design. We tried the python function in katcp, but it > does not look reliable enough. We hacked that function and increased the > time between counter reads to 30 seconds (instead of 2 seconds), and the > reading is still not that reliable. They all read roughly around 200.2MHz, > and if that's the case, then we must be doing something wrong regarding > the clocks and our 202MHz did not go into the design. We are using > internal FPGA clocks with no ADC or anything. > > Background: > We are testing 4 ROACH1 F-engine to 4 ROACH2 X-engine 10GBE set up, 16 > connections in between. Each F-engine is sending to all 4 X-engines, and > each X-engine is receiving from all 4 F-engines. We have a very simple > sending and receiving FIFO buffer logic. The thing we see is that one > particular X-engine always has all 4 buffers filled up in a fixed amount > of time (32K buffer fills up in ~270 seconds). We swapped a lot of thing > around to test which part is wrong in the system, and we nailed it down to > something physical in that X-engine, and the fact that it fills up > indicate that X-engine clock is running slower than we set it to, because > we set X-engine clock to be 202MHz vs 200MHz on F-engine. Therefore we > would like an definitive measurement of the actual X-engine clock that is > running. > > Plot explanation: > It's plotting one of the four FIFO buffer filling up over 270 seconds. The > vertical axis is the literal number of numbers currently stored in the > FIFO. This FIFO should on average get 1 number every 8 clocks from the > F-engine (200MHz) receiver, and should get 1 number pumped out every 8 > clocks on X-engine (202MHz), so this long term build up definitely suggest > that the clocks are not what we think. > > > Thanks a lot and sorry for the long email! > > Jeff >
