SPI Write Limitation

[Peter Johnson]

Originally Posted by gerthworm

Seems like it. The bit of research I've done on this seems to indicate SPI is implemented on the FPGA, which I assume means any buffers are of fixed length. What those lengths are, however, I'm not really sure....

Yes, it's a hardware limitation. Per the Zynq documentation [1], the SPI read and write FIFOs are 128 bytes deep. This matches the behavior you're seeing. Conceptually, functionality could be added to wpilib to do the outer loop for longer writes, but I think this case is rare enough (and there's multiple ways to do it, e.g. use interrupts to determine when to continue filling the FIFO, or periodically check), that it's better left to the user code to implement.

[1] http://www.xilinx.com/support/docume...q-7000-TRM.pdf, section 17.2.4.

[s5511]

We are trying to control the AdaFruit DotStars through the SPI bus, too, except we are using LabVIEW. Which ports on the roboRIO did you guys connect the dotstar to?

[gerthworm]

Actually, we ended up using both at one point. We started by using the dedicated 10-pin port on the top, then we switched to the port that's on the custom expansion port in the middle.

I'm not familiar with sending bytes in labview. However, I know in java it was just a single initialization parameter to say which SPI port to use. Everything else was the same.

Also as indicated in this thread, we did have to chunk the send data into 128-length bytes to get it to work nicely.