In order to test this code and have something to play with, I wrote a simple driver stub. It contains the main() function and accepts typed in characters and can modify the display brightness. It is all in the top.c file. If the driver code were used in a real application, this entire file would be replaced.

/*---------------------------------------------------------------------------
* Filename:  top.c
* $Revision: 1.2 $
* Author: Bob Harbour
* $Date: 2020-11-26 20:59:58 $
*
*
* Revisions:
*------------------------------------------------------------------------*/


//===================================================================
// INCLUDES
//===================================================================
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

#include "std_constants.h"
#include "chargen_5x8.h"
#include "MatrixDriver_MAX7221.h"

//===================================================================
// Defines
//===================================================================

//===================================================================
// Globals
//===================================================================

//===================================================================
// Externals
//===================================================================

//===================================================================
// Function Prototypes
//===================================================================
int strip_newline_from_string (char *instr, int maxlen);

//===================================================================
// Functions
//===================================================================
int main (int argc, char *argv[])
{
  int retval, len, trunc_boundary, done, count;
  char inbuffer [SHORT_BUFFER_LEN], *inptr;
  uint8_t brightness;

  retval = OK;
  done = FALSE;
  brightness = MX_DEFAULT_BRIGHTNESS;

  if ((retval = init_matrix_display_system ()) == OK)
  {
    count = 0;

    fprintf (stdout,"<ESC>Q to quit, <ESC>+ or <ESC>- adjust brightness\n");

    while (!done)
    {
      // get user input
      if (fgets (inbuffer, SHORT_BUFFER_LEN, stdin) != NULL)
      {
        len = strip_newline_from_string (inbuffer, SHORT_BUFFER_LEN);
        if (inbuffer[0] == ESC)
        {
          if ((inbuffer[1] == 'Q') || (inbuffer[1] == 'q'))
            done = TRUE;
          else
            if (inbuffer[1] == '+')
            {
              if (brightness < MX_MAX_BRIGHTNESS)
              {
                brightness += 1;
                set_display_brightness (brightness);
                fprintf (stdout,"brightness: %1d\n", brightness);
              }
              else
                fprintf (stdout,"Already at MAX brightness\n");
            }
            else
            if (inbuffer[1] == '-')
            {
              if (brightness > MX_MIN_BRIGHTNESS)
              {
                brightness -= 1;
                set_display_brightness (brightness);
                fprintf (stdout,"brightness: %1d\n", brightness);
              }
              else
                fprintf (stdout,"Already at MIN brightness\n");
            }
            else
            {
              fprintf (stdout,"Unrecognized Escape Sequence: ESC %c\n",
                       inbuffer[1]);
            }
        }  // pairs with: if (inbuffer[0] == ESC)
        else
        {
          //retval = insert_display_chars_from_left (inbuffer, len, &trunc_boundary);
          retval = insert_display_chars_from_right (inbuffer, len, &trunc_boundary);
          printf ("truncation boundary: %1d\n", trunc_boundary);
          count += 1;
        }  // pairs with: if (inbuffer[0] == ESC) else
      }   // pairs with: if (fgets () )
      else
      {
        if (feof (stdin))
        {
          done = TRUE;
        }
      }      // pairs with: if (fgets () != NULL)  else
    }  // pairs with : while (!done)
    shutdown_matrix_display_system (FALSE);
  }
  else
  {
    fprintf (stderr,"ERROR, init_matrix_display_system() barfed\n");
  }

  exit (retval);
}

//===================================================================
//  strip_newline_from_string()
//  Removes the newline character from a string.
//  Returns the number of characters in the resulting string
//
//  8/2007
//===================================================================
int strip_newline_from_string (char *instr, int maxlen)
{
  char *ptr;
  int len;

  ptr = instr;
  len = 0;
  // leave room for terminator
  maxlen -= 1;

  // count to end of line
  while ((*ptr != CR) && (*ptr != LF) && (*ptr != EOS) && (len < maxlen))
  {
    ptr++;
    len += 1;
  }

  // replace possible CR or LF with end of string
  if ((*ptr == CR) || (*ptr == LF))
  {
    *ptr = EOS;
  }
  else
  {
    if (len == maxlen)
    {
      *ptr = EOS;
    }
  }

  return (len);
}

To build this program on a Raspberry Pi, place the tarball in a directory by itself and unpack the tarball:

> tar -xzvf MAX_Single_Driver_RasPi_11_27_2020.tgz

Build the program with make

> make

Connect the displays to the SPI interface on the Raspberry Pi similarly to the block diagram at the top of this article.

Run the program:

> ./top

Type a few characters, they should show up on your display modules. If you have 4 display modules hooked up, the first 4 characters that you types should  display.

In order to quit, hit the escape key followed by Q and then hit the enter key.

<esc>Q<enter>

To increase the display brightness one step:

<esc>+<enter>

To decrease the display brightnes one step:

<esc>-<enter>