SPI constantly sending Clock and OxFF over MOSI after enabling SPI in half duplex.

[milindmishra87]

Hi,

I have been trying to communicate with an SPI peripheral (NOR Flash). I want to use SPI in half duplex mode. The problem is that as soon as enable the SPI peripheral I see SPI constantly sending OxFF over MOSI even though I have not written anything on the SPI DR register.

Pasting the screenshot of the Salea LA

Below is the simple SPI TX code

void SPI_SendData(uint8_t* Txdata, uint32_t len) {
    if((spi_dev->SR & TXE_FLAG)) {
      spi_dev->DR = *(uint8_t*)Txdata;
      Txdata++;
      len--;
    }

  if(!(spi_dev->CR1 & SPI_EN)) {
    spi_dev->CR1 |= SPI_EN;
  }
  while(len > 0) {
    // Wait for TXE FLAG
    if((spi_dev->SR & TXE_FLAG)) {
      spi_dev->DR = *(uint8_t*)Txdata;
      Txdata++;
      len--;
    }
  }
}

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Edited to apply source code formatting - please see How to insert source code for future reference.

[TDK]

In half duplex mode, you need to toggle between sending and receiving. When receiving, it is clocked constantly as shown in your logic analyzer traces.

[FBL]

Hello @milindmishra87 

The 0xFFs and clock activity after enabling SPI is normal behavior of the SPI master.. /CS does not stop the SPI peripheral from toggling SCK/MOSI; it only tells the flash when to pay attention.

To be sure there is no issue with your NOR flash, I recommend verifying the CPOL/CPHA, and half‑duplex direction bits and sharing your SPI register dump just before starting the transfer.