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Reduce the number of bytes over SPI for MCP2515Class::parsePacket() by up to 2.8x #46

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Reduce the number of bytes over SPI for MCP2515Class::parsePacket() by up to 2.8x #46

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Commits on Aug 4, 2020

  1. Optimize MCP2515Class::parsePacket() 

    1) Use the RX STATUS command instead of READ(CANINTF) to check availability of
   data to read. This reduces SPI usage for a simple "is data available" check
   from 3 bytes to 2.

2) Use the READ RX BUFFER command to read the RXFn* registers as well as the
   received data. This requires doing only a single CS pull, and only N+6 bytes
   transferred over SPI. READ RX BUFFER also takes care of resetting the RXnIF
   flag.

If my math is right, the old code needed N+6 readRegister() calls for a standard
frame, and N+9 readRegister() calls for an extended frame, plus one
modifyRegister() call to reset the RXnIF flag. Each readRegister() call requires
a CS pull and transferring 3 bytes over SPI.

For N = 8,
- we now send 16 bytes over SPI vs 45 for a standard frame (>2.8x reduction!)
- we now do just 2 CS pulls vs 16 for a standard frame (8x reduction!)
    @timurrrr
    timurrrr committed Aug 4, 2020
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