You’re ‘aving a laugh

I separated the data ready interrupt frequency (666Hz) from the IMU ADC sampling rate (500Hz) with the intention that this might prove better timing accuracy and therefore better angles and velocities.

In flight, timing was better, angle were probably better, but unexpectedly, vertical velocity was completely shot – both Phoebe and Chloe rose to about 4m, and their height increased during hover, and descended slowing during descent before dropping out of the sky from 3m on landing.  Phoebe partially snapped off her USB port, and after an hour trying to desolder and replace it, I gave up and ordered a new A+ arriving today.

In addition, it’s seems now like the 666Hz was just someone having a laugh* at my expense – the one flight I got yesterday was capturing data ready interrupts at 677Hz. This could be

  • 500Hz plus noise or
  • 1kHz with missed samples
  • a code bug

I checked the data ready interrupt frequency with my ‘scope when the new A+ arrived with the ADC sampling frequency set to 500Hz:

500Hz 50us data ready interrupt

500Hz 50us data ready interrupt

Given the ‘scope is saying the hardware interrupt is running at 500Hz, why does the code think it’s running at > 660Hz – clearly something in my customized GPIO library is causing double counting – probably not flushing the epoll fd after a read?


*Unrelated to the Quadcopter, but related to someone having a laugh at my expense, I ordered 2 new monitors yesterday to replace my work and home computers – more screen space for the large number of open apps at work and better full screen viewing of files, and full sRGB colour space rendering for editing photos at home.  I ordered them as a huge motivation to stop smoking.  I can’t afford them unless I give up smoking which costs me £300 a month.  Shortly after ordering, I was sorting out my work PC in preparation for the new monitor, and while the existing monitor was off its vesa wall-mount it smashed to the floor and would only show a white screen.  So stopping smoking is now mandatory, not optional!

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