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How To: SPI Bus

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This chapter goes over how to use TOWER SPI SDK Module if you want to read about the bus itself read the SPI Interface chapter.

References​

SPI Speed​

You can choose from several communication speeds.

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The communication speed is limited by the maximal speed that the slave device can communicate, the length of the wires, noise, current consumption or electromagnetic radiation limits.

TWR_SPI_SPEED_1_MHZ
TWR_SPI_SPEED_2_MHZ
TWR_SPI_SPEED_4_MHZ
TWR_SPI_SPEED_8_MHZ
TWR_SPI_SPEED_16_MHZ

SPI Mode​

Clock polarity and clock phase define when the output data is valid. Whether it's on the rising or falling edge.

This information can be found in the datasheet of the slave device.

TWR_SPI_MODE_0 // SPI mode of operation is 0 (CPOL = 0, CPHA = 0)
TWR_SPI_MODE_1 // SPI mode of operation is 1 (CPOL = 0, CPHA = 1)
TWR_SPI_MODE_2 // SPI mode of operation is 2 (CPOL = 1, CPHA = 0)
TWR_SPI_MODE_3 // SPI mode of operation is 3 (CPOL = 1, CPHA = 1)

Transmitting and Receiving Data Examples​

Synchronous transfer​

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You need to create transmit and receive buffer.

Then you start the blocking transfer and you have to wait for it to finish.

Synchronous SPI Transfer Code Example

uint8_t tx_buffer[2] = { 0x20, 0x00 };
uint8_t rx_buffer[2];

twr_spi_transfer(tx_buffer, rx_buffer, sizeof(rx_buffer));

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If you are just transmitting data, then replace the rx_buffer with NULL and vice-versa for just receiving.

The function returns false if the previous asynchronous transfer has not ended yet.

Asynchronous transfer​

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This is a non-blocking transfer where the callback function is called when the transfer is completed.

Asynchronous SPI Transfer Code Example

// In async transmit the buffers must be global or
// in the function but defined as a static
uint8_t tx_buffer[2] = { 0x20, 0x00 };
uint8_t rx_buffer[2];

void send_data(void)
{
    // Check if previous asynchronous transfer is not running
    if (twr_spi_is_ready())
    {
        // Set event handler and optional parameter (NULL for now)
        twr_spi_async_transfer(tx_buffer, rx_buffer, sizeof(tx_buffer), _twr_spi_event_handler, NULL)
    }
}

void _twr_spi_event_handler(twr_spi_event_t event, void *event_param)
{
    (void) event_param;

    if (event == TWR_SPI_EVENT_DONE)
    {
        // Transfer done, you can for example handle received data or initiate a new transfer
    }
}