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[SpiROSE] FPGA, the end is near

Hello everyone, I hadn’t posted since Christmas, so here I go ! Since the expedition (no pun intended) of the PCBs to the manufacturer, I kept mainly focused on FPGA development including source code and tests for various modules, which allowed us to correct many bugs we hadn’t seen so far. I will detail some features later on.

SPI slave

The SPI slave implementation is complete. The protocol for sending commands from the SBC to the FPGA is simple, we have a header command byte, followed by as many bytes of data as needed. Let’s have a quick summary of the different possible commands.

  • Enable/Disable the RGB module: we need to tell the RGB module, the one that writes into RAM the data the FPGA receives from the parallel RGB, to start or stop writing in RAM, since we only want to display relevant data. As the RGB module is the first module in the chain, this command starts everything.
  • Configuration command: change the configuration of the drivers. The command may be issued at every moment, the driver controller module can handle the reconfiguration even when it was streaming data to the drivers.
  • Request rotation information: The FPGA should be able to send its the rotation position (the slice we are at), as well as the speed of the motor.

But how can we get the position of the rotating part ? Let’s look at the Hall effect sensors !

Hall effect sensors

Since we finally use Hall effect sensors instead of the rotary encoder that was originally wanted, I did the module that tells the driver controller and the framebuffer when we enter a new slice, to begin displaying for the current slice.

The issue is that we have only two Hall sensors that are opposite one to another and 256 slices per turn. So we need to infer the slice we are at given only 2 positions, over the 256 for a turn, that we are absolutely sure of. Ideally, the synchronization signal is generated without knowing in advance the motor speed, so it must adapt in “real time” to it. An idea, since we have 128 slices per turn, is to estimate the slice positions of the current half-turn given the number of cycles that it took to make the previous half-turn. In a word, we constantly correct the duration of a half-turn to fit the speed variations of the motor.

The Hall effect sensors we chose are hysteresis sensors which, in their case, means that the output of the sensors is set low when the magnetic flux is beyond a certain threshold and is kept low until the flux reaches a second threshold that is lower than the first one. This provides an integrated anti-bounce mechanism. We thus only need to detect the negative edge of the sensor output to have a “top” to synchronize onto. Between the 2 tops of the opposite sensors, we count the number of cycles and we compute (right shift)  the duration for each slice for the following half-turn, sending the synchronization signal accordingly. To give a few figures, if we have 15 rotations per seconds, around 15600 clock cycles are elapsed between 2 slices.

 

For the upcoming week, while we are waiting for the components to be shipped and since all FPGA modules are complete, we will build end-to-end tests with all of them, hoping that all the tests we have implemented for each modules were thorough enough to spot all possible bugs, for it to be ready for a quick deployment on the actual board.

See you soon !

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