Raspberry Pi - SPI

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Using the SPI interface on the Raspberry Pi with Free Pascal

First of all: It is a good idea to use the pascalio Lib (https://github.com/SAmeis/pascalio) for the hardware components such as SPI, I2C and the GPIOs of the Raspberry Pi.

However, the pascalio is also very extensive and those who want to understand what is happening may be well served with this guide.

SPI on Linux

The Linux principle "Everything is a file" also applies to the SPI interface. However, the advantage that you can access the interface with simple file access is bought at a slower speed - usually uncritically - and that other programs can also access the same interface.

On the Raspberry Pi (we're talking about version 3 with Raspbian Stretch, as of Oct 2017) there is an SPI interface that can address 2 devices: /dev/spidev0.0 and / dev / spidev0.1 (Strictly speaking, there are several, but only one is led outside.)

For the basics of SPI, master-slave, wiring see Wikipedia: https://wikipedia.org/wiki/Serial_Peripheral_Interface

Header

unit test_spi;

{$ mode objfpc} {$ H +}

interface

uses
  Classes, SysUtils, BaseUnix;

type
  Tspi = class

  procedure Init ();
  procedure Close ();

  function DataIn (var din; cnt: byte): integer;
  function DataOut (const dout; cnt: byte): integer;

  procedure TransferSync (const dout; var din; len: byte);
  procedure TransmitSync (const dout; len: byte);
  function FastShift (dout: byte): byte;

  private
    {private declarations}
  public
    {public declarations}
  end;

const
  // SPI mode flags

  cSPI_CPHA = $ 01;
  cSPI_CPOL = $ 02;

  cSPI_MODE_0 = (0 or 0);
  cSPI_MODE_1 = (0 or SPI_CPHA);
  cSPI_MODE_2 = (SPI_CPOL or 0);
  cSPI_MODE_3 = (SPI_CPOL or SPI_CPHA);

  cSPI_CS_HIGH = $ 04;
  cSPI_LSB_FIRST = $ 08;
  cSPI_3WIRE = $ 10;
  cSPI_LOOP = $ 20;
  cSPI_NO_CS = $ 40;
  cSPI_READY = $ 80;

  cSPI_CTRL = $ 6B; // is the "magic byte"

  // control register
  // Read / Write + Size + MagicByte + Register

  cSPI_RD_MODE: uint32 = $ 80016B01;
  cSPI_WR_MODE: uint32 = $ 40016B01;
  cSPI_RD_LSB_FIRST: uint32 = $ 80016B02;
  cSPI_WR_LSB_FIRST: uint32 = $ 40016B02;
  cSPI_RD_BITS_PER_WORD: uint32 = $ 80016B03;
  cSPI_WR_BITS_PER_WORD: uint32 = $ 40016B03;
  cSPI_RD_MAX_SPEED_HZ: uint32 = $ 80046B04;
  cSPI_WR_MAX_SPEED_HZ: uint32 = $ 40046B04;

  cSPI_DEVICE = '/dev/spidev0.0'; // Device 0, ChipSelect 0
// cSPI_DEVICE = '/dev/spidev0.1'; // Device 0, ChipSelect 1
  cSPI_SPEED = 1000000; // data rate in Hz
  cSPI_BITS = 8; // data bits
  cSPI_LSBF = 0; // LSB first = -1
  cSPI_MODE = cSPI_MODE_0;

type
  spi_ioc_transfer = record // Control register required for FileIO
    tx_buf: uint64; //pointer;
    rx_buf: uint64; //pointer; // always 64-bit
    len: uint32; // Number of characters
    speed: uint32; // data rate in Hz
    delay: uint16; // delay CS in usec
    bpw: uint8; // bits per word
    csc: uint8; // CS change
    txn: uint8;
    rxn: uint8;
    pad: uint16;
  end; // a total of 32 bytes

var
  spi: Tspi;
  spihnd: longint; // the filehandle for the interface

implementation

Initialization

// initialize SPI

procedure Tspi.Init ();
var
  val8: byte;
  val32: longword;
begin
  try
    spihnd: = FpOpen (cSPI_DEVICE, O_RdWr); // Open the interface for read / write

    if spihnd <> -1 then begin
      val8: = cSPI_MODE;
      FpIOCtl (spihnd, cSPI_WR_MODE, @ val8); // set fashion
      val8: = cSPI_BITS;
      FpIOCtl (spihnd, cSPI_WR_BITS_PER_WORD, @ val8); // set data bits per byte
      val8: = cSPI_LSBF; //-1
      FpIOCtl (spihnd, cSPI_WR_LSB_FIRST, @ val8); // Set MSB or LSB first
      val32: = cSPI_SPEED;
      FpIOCtl (spihnd, cSPI_WR_MAX_SPEED_HZ, @ val32); // set speed
    end;
  finally

  end;
end;

The interface is opened when the program starts and the parameters are set. The parameters can also be changed later. For example, the clock rate can be set slow and then increased, which is necessary for some SD cards.

'Caution!' The clock rate is derived from the current system clock. If the clock changes, the clock rate changes. A clock rate of 1Mhz with 600MHz system clock with higher processor load and 1200MHz system clock goes up to 2MHz.

And: The clock rate is derived from the system clock by a divider 2^n and is never exactly correct. If you specify 1MHz, the resulting rate is somewhere around 800kHz.

Exit

// exit SPI

procedure Tspi.Close ();
begin
  if spihnd <> -1 then begin
    FpClose (spihnd);
  end;
end;

Yes, at the end of the program we close the interface again.

Read out data

// SPI Buffer Read

function Tspi.DataIn (var din; cnt: byte): integer;
begin
  if spihnd <> -1 then begin
    DataIn: = FpRead (spihnd, din, cnt);
  end;
end;

Since "everything is a file" we can apply a simple read to the SPI interface. This outputs the clock for the specified number of bytes and reads the response of the device in an array din .

Send data

// SPI Buffer Write

function Tspi.DataOut (const dout; cnt: byte): integer;
begin
  if spihnd <> -1 then begin
    DataOut: = FpWrite (spihnd, dout, cnt);
  end;
end;

We can also send data with Write. The specified number of bytes is output from the dout array.

SPI Transfer

However, most SPI devices require a slightly more complex interaction than simply sending and receiving. Often data is sent and received at the same time, and that overwhelms the capabilities of read and write. Here we need a transfer function.

// Send, receive SPI data

procedure Tspi.TransferSync (const dout; var din; len: byte);
var
  outbuf: array of byte;
  inbuf: array of byte;
  transfer: spi_ioc_transfer;
begin
  if len> 0 then begin
    SetLength (outbuf, len);
    FillByte (outbuf [0], len, 0);
    Move (dout, outbuf [0], len);

    SetLength (inbuf, len);
    FillByte (inbuf [0], len, 0);

    FillByte (transfer, SizeOf (transfer), 0);
    transfer.tx_buf: = uint64 (@outbuf [0]);
    transfer.rx_buf: = uint64 (@inbuf [0]);
    transfer.len: = len;
    transfer.delay: = 0;
    transfer.speed: = cSPI_SPEED;
    transfer.bpw: = cSPI_BITS;
    transfer.csc: = 0;

    try
      FpIOCtl (spihnd, $ 40206B00, @transfer);
    finally
      Move (inbuf [0], din, len);
    end;
  end;
end;

Now it becomes more difficult. To be able to send and receive at the same time, we need two buffers outbuf and inbuf , in which we mirror the data. We transfer their start addresses - absolutely as 64-bit values, even with a 32-bit OS - into the transfer register. The register also receives the number of bytes to be transferred and a few settings.

Funnily enough, you can change the settings here every time, e.g. adjust the clock rate.

We then transfer the transfer register with the settings to the file handler, which neatly scans the arrays and sends out and reads the data. The mystical $ 40206B00 is again a write ($ 40), followed by the number of bytes in the transfer register ($ 20 = 32 bytes), the magic byte ($ 6B, see above) and the control register ($ 00) the interface. Just leave it like that and don't fiddle with it.

'Disclaimer:' This works here and now on the Raspberry Pi 3 under Raspbian. This can look different on other Linux OS and hardware platforms, especially if the byte order of the addresses is different.

What if I only want to receive data? Then I just pass an empty array. I can also specify the same array for dout and din , then the old array data will be overwritten with the new data.

SPI Transmit

// Send SPI data without receiving

procedure Tspi.TransmitSync (const dout; len: byte);
var
  outbuf: array of byte;
  transfer: spi_ioc_transfer;
begin
  if len> 0 then begin
    SetLength (outbuf, len);
    FillByte (outbuf [0], len, 0);
    Move (dout, outbuf [0], len);

    FillByte (transfer, SizeOf (transfer), 0);
    transfer.tx_buf: = uint64 (@outbuf [0]);
    transfer.rx_buf: = uint64 (@outbuf [0]); // same buffer
    transfer.len: = len;
    transfer.delay: = 0;
    transfer.speed: = cSPI_SPEED;
    transfer.bpw: = cSPI_BITS;
    transfer.csc: = 0;

    try
      FpIOCtl (spihnd, $ 40206B00, @transfer);
    finally

    end;
  end;
end;

If data is only to be written, one can use the above DataOut, or this slimmed-down variant of the transfer routine.

SPI FastShift

// Send, receive SPI single byte

function Tspi.FastShift (dout: byte): byte;
var
  din: byte;
  transfer: spi_ioc_transfer;
begin
  din: = 0;
  FillByte (transfer, SizeOf (transfer), 0);
  transfer.tx_buf: = uint64 (@dout);
  transfer.rx_buf: = uint64 (@din);
  transfer.len: = 1;
  transfer.delay: = 0;
  transfer.speed: = cSPI_SPEED;
  transfer.bpw: = cSPI_BITS;
  transfer.csc: = 0;

  try
    FpIOCtl (spihnd, $ 40206B00, @transfer);
  finally
    FastShift: = din;
  end;
end;

If only a single byte is sent and received, for example for a status query, this version is recommended. Here the result is not in a buffer, but is returned as a function value.

Dismissed!

end.