Commits on Aug 5, 2020

1. Optimize MCP2515Class::endPacket() …

   1) Use LOAD TX BUFFER instruction to set TXBn* registers sequentially, as well
      as to write into the TX data registers.
      This is done with a single CS pull, and requires sending N+6 bytes over SPI.
      The old code was doing N+5 writeRegister() calls, each doing 1 CS pull and
      sending 3 bytes.
   
   2) Use the 1-byte RTS SPI instruction instead of WRITEing to TXBnCTRL.
      This reduces the number of bytes sent over SPI by 2.
   
   For N = 8,
   - we now send 15 bytes over SPI before the while(...TXBnCTRL...) loop vs 42
     (2.8x reduction)
   - we do just 2 CS pulls before the while(...TXBnCTRL...) loop vs 13
     (6.5x reduction)
   
   We still do at least 3 CS pulls and send at least 10 bytes over SPI for the
   condition of the while(...TXBnCTRL...) loop, to clear TXnIF and to check for
   errors.
   
   That means for N = 8, in the case when we do zero iterations of the while loop,
   we end up sending 25 bytes over SPI instead of 52, which is >2x reduction.

   

   timurrrr committed Aug 5, 2020
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