3 WIRE SPI bi-directional communication

Question

Hello! 
 Does the SDK support a 3-wire SPI interface with bi-directional communication? 
 Like this:

Thanks!

ketiljo · Accepted Answer

No initially, but you can connect the MISO and MOSI pins externally with a resistor.

hmolesworth · Answer

Yes you can :-) This is a bare-metal example, but the same idea can be used with the Nordic drivers.

Start in 3-wire command transmit; Send the command, then configure for 3-wire receive:

  // Send 3-wire whoami command byte (1 byte only)
  // Connect output drive, connect 1-wire rx/tx output to MOSI
  NRF_SPIM0->PSEL.MISO = 0xFFFFFFFF; // MISOPIN;
  NRF_SPIM0->PSEL.MOSI = MOSIPIN;
  NRF_GPIO->PIN_CNF[MOSIPIN] = 1;    // output
  NRF_SPIM0->TXD.PTR = (uint32_t)spi3wTXBuf;
  NRF_SPIM0->TXD.MAXCNT = 1;
  NRF_SPIM0->RXD.PTR = NULL;
  NRF_SPIM0->RXD.MAXCNT = 0;
  NRF_SPIM0->EVENTS_ENDTX = 0;
  NRF_GPIO->OUTCLR = 1 << CSPIN;       // drive cs low to initiate spi comm
  NRF_SPIM0->TASKS_START = 1;
  while(!NRF_SPIM0->EVENTS_ENDTX);     // last byte transmitted

  // Manual says disable SPI before changing pins, but actually doesn't matter
  NRF_SPIM0->ENABLE = 0;               // disable SPI
  // Disconnect output drive, connect 1-wire rx/tx input to MISO
  NRF_SPIM0->PSEL.MOSI = 0xFFFFFFFFUL; // MOSIPIN;
  NRF_GPIO->PIN_CNF[MOSIPIN] = 0;      // input pin, input buffer connected, no pull, S0S1, sense disabled
  NRF_SPIM0->PSEL.MISO = MOSIPIN;      // MISOPIN;
  NRF_SPIM0->ENABLE = 7;               // enable SPI if it was disabled

This works fine; on some SPI devices remember the internal 3-wire register mode has to be set first before reading.

3 WIRE SPI bi-directional communication

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