nrf52840 3 wire spi using sdk

[Shinicy99]

Is it possible to use SDK of nrf52840 to configure a 3wire SPI?

I saw in some posts that i can not use directly the MISO pin and not assing it in the sdk's nrf function, but is the best way to proceed?

[hmolesworth]

By SDK hopefully you mean nRF52 SDK, but in any case here's a guide using bare-metal programming which you can use to alter other examples. Tested with LIS2DH12 which starts as 4-wire SPI but can be set to 3-wire SPI. (This code assumes the SPI slave is always 3-wire so the 4-wire to 3-wire slave command is not shown). Sequence is send a 1-byte command, no rx, reverse the pin then send a dummy command and receive whatever number of rx bytes are required, in this example a single byte "I am .." response but can be longer

#define CSPIN    11  // spi chip select
#define SCKPIN   12  // spi clock
#define MOSIPIN  13  // spi mosi
//#define MISOPIN  14  // spi miso - not used in 3-wire mode

#define LIS2DH12_WHO_AM_I  0x0F // Read as 0x33

uint8_t spi3wTXBuf[] = {0x80|LIS2DH12_WHO_AM_I, 0xFF};
uint8_t spi3wRXBuf[sizeof(spi3wTXBuf)] = {0,0};
static const uint8_t m3wRxTxBufLength = sizeof(spi3wTXBuf);

volatile uint32_t mSpiInterruptCounter = 0UL;

void notSPIM0_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQHandler(void)
{
  if (NRF_SPIM0->EVENTS_END) NRF_SPIM0->EVENTS_END = 0;
  mSpiInterruptCounter++;
}

static void Test3WireSPI(void)
{
  NRF_GPIO->PIN_CNF[CSPIN]   = 0x301;  // output, high drive high and low H0H1
  NRF_GPIO->PIN_CNF[SCKPIN]  = 0x301;  // output, high drive high and low H0H1
  NRF_GPIO->PIN_CNF[MOSIPIN] = 1;      // output, standard drive S0S1
//NRF_GPIO->PIN_CNF[MISOPIN] = 0;      // input pin, input buffer connected, no pull, S0S1, sense disabled
  NRF_GPIO->OUTSET = 1 << CSPIN;       // deactivate by setting chip select high
  NRF_SPIM0->PSEL.SCK = SCKPIN;
  NRF_SPIM0->PSEL.MOSI = MOSIPIN;
  NRF_SPIM0->PSEL.MISO = 0xFFFFFFFFUL; // MISOPIN;
  NRF_SPIM0->CONFIG = 0;               // CPOL 0 -- clock polarity active high, CPHA 1 -- sample on trailing clock edge, send Msb first
  NRF_SPIM0->FREQUENCY = 0x80000000UL; // 8 Mbps
  NRF_SPIM0->ORC =0;                   // Unused Tx bytes, set all low

  // Configure registers for 3-wire mode
  NRF_SPIM0->PSEL.MOSI = MOSIPIN;
  NRF_SPIM0->PSEL.MISO = 0xFFFFFFFFUL; // MISOPIN;
  NRF_GPIO->PIN_CNF[MOSIPIN] = 1;       // output
  NRF_SPIM0->EVENTS_ENDTX = 0;
  //NRF_SPIM0->EVENTS_ENDRX = 0;
  // Disable all interrupts
  NRF_SPIM0->INTENCLR = 0xFFFFFFFFUL;
  // Enable selected interrupts
  NRF_SPIM0->INTENSET = 0x40;   // END
  // Set interrupt priority and enable interrupt
  NVIC_SetPriority(SPIM0_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQn, 6);
  NVIC_ClearPendingIRQ(SPIM0_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQn);
  NVIC_EnableIRQ(SPIM0_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQn);
  NRF_SPIM0->ENABLE = 7;               // enable SPI
  // Transfer registers to set 3-wire SPI mode on target
  // - not required if slave only has or is already in 3-wire mode

  // 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
  spi3wRXBuf[0] = 99; spi3wRXBuf[1] = 98;
  NRF_SPIM0->TXD.PTR = NULL;
  NRF_SPIM0->TXD.MAXCNT = 0;
  NRF_SPIM0->RXD.PTR = (uint32_t)spi3wRXBuf;
  NRF_SPIM0->RXD.MAXCNT = 1;
  NRF_SPIM0->EVENTS_ENDRX = 0;
  NRF_SPIM0->TASKS_START = 1;
  while(!NRF_SPIM0->EVENTS_ENDRX);    // last byte received

  NRF_SPIM0->TASKS_STOP = 1;
  while(!NRF_SPIM0->EVENTS_STOPPED);
  NRF_GPIO->OUTSET = 1 << CSPIN;
  while(1);
}

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