SPI is a very popular way of connecting devices to the Raspberry Pi and the good news is that it is well supported by Linux drivers - once you know how.

This content comes from my newly published book:

Raspberry Pi IoT In C Using Linux Drivers

By Harry Fairhead

Buy from Amazon.

Contents

1.  Choosing A Pi For IoT

2. C and Visual Studio Code

3.  Drivers: A First Program

4.  The GPIO Character Driver
        Extract: GPIO Character Driver

5. GPIO Using I/O Control

6.  GPIO Events

7.  The Device Tree
       Extract: The DHT22
   

8.  Some Electronics

9.  Pulse Width Modulation
   Extract:  The PWM Driver 

10. SPI Devices
    Extract: The SPI Driver 

11. I2C Basics

12. The I2C Linux Driver ***NEW!

     

13. Advanced I2C

14. Sensor Drivers – Linux IIO & Hwmon
      Extract: Hwmon  

15. 1-Wire Bus
      Extract: 1-Wire And The DS18B20 

16. Going Further With Drivers

17. Appendix I

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In chapter but not in book:

• SPI Devices
• SPI Bus Basics
• Pi SPI Interfaces
• SPI Protocol

SPI Driver

Before you can use the SPI bus you have to load its driver. You can do this by adding:

dtparam=spi=on

to the /boot/config.txt file. This loads a driver for SPI0 using two chip select lines. To find out how to activate other SPI channels see later. For the rest of this section we will be using SPI0.

Alternatively you can activate the driver dynamically:

FILE *doCommand(char *cmd)
{
    FILE *fp = popen(cmd, "r");
    if (fp == NULL)
    {
        printf("Failed to run command %s \n\r", cmd);
        exit(1);
    }
    return fp;
}
void checkSPI0()
{
    FILE *fd = doCommand("sudo  dtparam -l");
    char output[1024];
    int txfound = 0;
    char indicator[] = "spi=on";
    char command[] = "sudo dtparam spi=on";
    while (fgets(output, sizeof(output), fd) != NULL)
    {
        printf("%s\n\r", output);
        fflush(stdout);
        if (strstr(output, indicator) != NULL)
        {
            txfound = 1;
        }
    }
    if (txfound == 0)
    {
        fd = doCommand(command);
        sleep(2);
    }
    pclose(fd);
}

This works by first using the dtparam -l command to list the loaded overlays. If the spi overlay is already loaded nothing is done. If it isn’t then it runs the command:

dtparam spi=on

SPIDev

The interface to the SPI driver is generally referred to as SPIdev and there is an spidev.h header file which provides all of the definitions you need to make use of it. When you load the SPI driver to install SPI Channel n a number of character devices are created in /dev of the general form spidevn.m where n is the channel number and m is the chip select line used to control the device.

For example, the basic SPI driver uses channel 0, i.e. SPI0 with two chip select lines, and thus you will find spidev0.0 and spidev0.1 which control the SPI device connected to SPI0 on chip select 0 and 1 respectively. By default, spidev0.0 uses GPIO8 pin 26 and spidev0.1 uses GPIO7 pin 28 for chip selects.

To work with an SPI device all you have to do is use ioctl to send requests to the relevant file. If you want to know more about ioctl see Chapter 4.

There are a range of configuration requests:

• SPI_IOC_WR_MODE    	
  sets mode

• SPI_IOC_WR_LSB_FIRST  	
  sets LSB first or last

• SPI_IOC_WR_BITS_PER_WORD 	
  sets number of bits per word 

• SPI_IOC_WR_MAX_SPEED_HZ 
  sets SPI clock if possible

Note: SPI_IOC_WR_LSB_FIRST isn’t supported on Pi OS.

There are also requests with WR replaced by RD which read, rather than write, the configuration.

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