Attempting SPIM3 reads with MOSI and MISO Tied Together

Question

Hello, 
 
 We are currently using SDK v17.0.2 with NRF52840, driving a display with SPIM3. We are able to successfully communicate with our display and command it to render various things. Because it's a display, we have MOSI and MISO tied together as the same pin (the display does not utilize a MISO line). 
 I am currently attempting to read back the temperature from the display, but not getting a sensible read. With MOSI and MISO tied together, is there special pin driving logic that needs to be performed? 
 
 Here is my code: 
 
 // Set up logic
 #define EINK_XFER_BUF_LEN ( 2048 ) // bytes
 #define EINK_COMMAND_LEN ( 1 ) // bytes
 #define EINK_RECEIVE_BUF_LEN ( 1 ) // bytes
 #define TEST_DELAY_US ( 150 ) // In microseconds
 static uint8_t m_tx_buf[EINK_XFER_BUF_LEN] __attribute__( (section( ".errata198_workaround" ) ) ) = { 0 };
 static uint8_t m_rx_buf[EINK_RECEIVE_BUF_LEN] __attribute__( (section( ".errata198_workaround" ) ) ) = { 0 };
 nrfx_spim_xfer_desc_t xfer_receive_desc = NRFX_SPIM_XFER_TRX( m_tx_buf, 1, m_rx_buf, 1 );
 
 
 // Send temperature read command
 m_tx_buf[0] = TSC; 
 einkCtx->commandSent = m_tx_buf[0];
 einkCtx->isCommand = true;
 einkCtx->spiXferDone = false;
 // Signal to the EPD that this is a command
 memset(m_rx_buf, 0, EINK_RECEIVE_BUF_LEN);
 einkCtx->retCode = nrfx_spim_xfer_dcx( &spim_eink, &xfer_receive_desc, 0, 1 ); 
 nrf_delay_us( TEST_DELAY_US ); 
 
 
 // Read back logic in callback function
 if ( xsferCtx->commandSent == READ_TEMP )
 {
 // Wait until the Display is no longer busy
 while ( !nrf_gpio_pin_read( E_INK_BUSY_PIN ) );

 if ( m_rx_buf[0] != 0 )
 {
 NRF_LOG_INFO(" TSC Received:");
 xsferCtx->epd_temperature = m_rx_buf[0];
 NRF_LOG_INFO("m_rx_buf[0] == 0x%X ", m_rx_buf[0]);
 
 }
 }

hmolesworth · Answer

The SPI code you are using assumes 4-wire SPI, but to connect MOSI to MISO 3-wire SPI mode is required, which I don't think the driver you are using supports. Here is low-level code showing how it's done:

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_SPIM3->PSEL.SCK = SCKPIN;
  NRF_SPIM3->PSEL.MOSI = MOSIPIN;
  NRF_SPIM3->PSEL.MISO = 0xFFFFFFFFUL; // MISOPIN;
  NRF_SPIM3->CONFIG = 0;               // CPOL 0 -- clock polarity active high, CPHA 1 -- sample on trailing clock edge, send Msb first
  NRF_SPIM3->FREQUENCY = 0x80000000UL; // 8 Mbps
  NRF_SPIM3->ORC =0;                   // Unused Tx bytes, set all low

  // Configure registers for 3-wire mode
  NRF_SPIM3->PSEL.MOSI = MOSIPIN;
  NRF_SPIM3->PSEL.MISO = 0xFFFFFFFFUL; // MISOPIN;
  NRF_GPIO->PIN_CNF[MOSIPIN] = 1;      // output

  NRF_SPIM3->EVENTS_ENDTX = 0;
  //NRF_SPIM3->EVENTS_ENDRX = 0;

  // Disable all interrupts
  NRF_SPIM3->INTENCLR = 0xFFFFFFFFUL;
  // Enable selected interrupts
  NRF_SPIM3->INTENSET = 0x40;   // END

  // Set interrupt priority and enable interrupt
  NVIC_SetPriority(SPIM3_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQn, 6);
  NVIC_ClearPendingIRQ(SPIM3_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQn);
  NVIC_EnableIRQ(SPIM3_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQn);

  NRF_SPIM3->ENABLE = 7;               // enable SPI
  // Transfer registers to set 3-wire SPI mode on target
  NRF_SPIM3->TXD.PTR = (uint32_t)mACC_ControlRegisterRequiredSettings;
  NRF_SPIM3->TXD.MAXCNT = sizeof(mACC_ControlRegisterRequiredSettings);
  NRF_SPIM3->RXD.PTR = NULL;
  NRF_SPIM3->RXD.MAXCNT = 0;

  NRF_GPIO->OUTCLR = 1 << CSPIN;    // drive cs low to initiate spi comm
  NRF_SPIM3->TASKS_START = 1;
  while(!NRF_SPIM3->EVENTS_ENDTX);  // last byte transmitted
  NRF_SPIM3->TASKS_STOP = 1;
  NRF_GPIO->OUTSET = 1 << CSPIN;

  // Send 3-wire whoami command byte (1 byte only)
  // Connect output drive, connect 1-wire rx/tx output to MOSI
  NRF_SPIM3->PSEL.MISO = 0xFFFFFFFF; // MISOPIN;
  NRF_SPIM3->PSEL.MOSI = MOSIPIN;
  NRF_GPIO->PIN_CNF[MOSIPIN] = 1;    // output
  NRF_SPIM3->TXD.PTR = (uint32_t)spi3wTXBuf;
  NRF_SPIM3->TXD.MAXCNT = 1;
  NRF_SPIM3->RXD.PTR = NULL;
  NRF_SPIM3->RXD.MAXCNT = 0;
  NRF_SPIM3->EVENTS_ENDTX = 0;
  NRF_GPIO->OUTCLR = 1 << CSPIN;       // drive cs low to initiate spi comm
  NRF_SPIM3->TASKS_START = 1;
  while(!NRF_SPIM3->EVENTS_ENDTX);     // last byte transmitted

  // Manual says disable SPI before changing pins, but actually doesn't matter
  NRF_SPIM3->ENABLE = 0;               // disable SPI
  // Disconnect output drive, connect 1-wire rx/tx input to MISO
  NRF_SPIM3->PSEL.MOSI = 0xFFFFFFFFUL; // MOSIPIN;
  NRF_GPIO->PIN_CNF[MOSIPIN] = 0;      // input pin, input buffer connected, no pull, S0S1, sense disabled
  NRF_SPIM3->PSEL.MISO = MOSIPIN;      // MISOPIN;
  NRF_SPIM3->ENABLE = 7;               // enable SPI
  spi3wRXBuf[0] = 99; spi3wRXBuf[1] = 98;
  NRF_SPIM3->TXD.PTR = NULL;
  NRF_SPIM3->TXD.MAXCNT = 0;
  NRF_SPIM3->RXD.PTR = (uint32_t)spi3wRXBuf;
  NRF_SPIM3->RXD.MAXCNT = 1;
  NRF_SPIM3->EVENTS_ENDRX = 0;
  NRF_SPIM3->TASKS_START = 1;
  while(!NRF_SPIM3->EVENTS_ENDRX);    // last byte received

  NRF_SPIM3->TASKS_STOP = 1;
  while(!NRF_SPIM3->EVENTS_STOPPED);
  NRF_GPIO->OUTSET = 1 << CSPIN;
}

This link give more details 3-wire-spi-bi-directional-communication