Forth

Forth on RPi

sudo apt-get install wget
wget http://www.fourmilab.ch/atlast/download/1.2/atlast-1.2.tar.gz
tar -xzvf atlast-1.2.tar.gz
cd atlast-1.2/
make

Start the Forth interpreter with ./atlast and exit with Ctrl-D

atlast.html and atlast.pdf is included and an is an extensive atlast-forth manual. Read it online [1] and read about Forth itself here [2] and here [3] (Beware that the Forth dialect in the book Starting Forth is a bit outdated compared to Atlast Forth). Leo Brodie wrote another book, Thinking Forth, read it here [4]

Adding new words

Most of the power of Atlast Forth derives from the ease with which C coded primitives can be added to the language. In my case I will add some words for controlling my Gertboard. There is a detailed description on how to do this in the atlast-forth manual. And you can copy much of the word implementations from the gertboard_sw directory if you have downloaded the gertboard demo files. It is easy to add your own words, just add a "define GERTBOARD" to atlast.c around line 56.

#define EVALUATE                      /* The EVALUATE primitive */
#define FILEIO                        /* File I/O primitives */
#define GERTBOARD                     /* Gertboard functions */

Include the gb_common.h right after include <math.h>

#ifdef MATH
#include <math.h>
#endif

#ifdef GERTBOARD
#include "gb_common.h"
#endif

Then add your own word definitions at the end of the section with word definitions, around line 2704, right after #endif /* COMPILERW */ in atlast.c

#ifdef GERTBOARD

prim P_gert_io() // state ---
{ // Setup and restore io
  Sl(1);
  if(S0 == 1)
    setup_io(); // Map the I/O sections
  else
    restore_io(); // Unmap and free memory
  Pop;
}

prim P_gert_setport() // channel state ---
{ // Set a digital io port to a specified state
  int rev;
  Sl(2);
  if (S1 == 21)
    { // find out which rev of RPi we have
      rev = pi_revision();
      if (rev != 1)
        S1 = 27; // GP21 on Gertboard is controlled by GPIO27
  }
  INP_GPIO(S1);
  OUT_GPIO(S1);
  if (S0 == 1)
    GPIO_SET0 = (1<<S1);
  else
    GPIO_CLR0 = (1<<S1);
  Pop2;
}

prim P_gert_getport() // channel ---
{ // Get a digital io port
  int rev;
  Sl(1);
  if (S0 == 21)
    { // find out which rev of RPi we have
      rev = pi_revision();
      if (rev != 1)
        S0 = 27; // GP21 on Gertboard is controlled by GPIO27
  }
  INP_GPIO(S0);
  S0 = !!(GPIO_IN0 & (1 << S0));
}

#endif /* GERTBOARD */

And finally, add the actual words to the Table of primitive words, right after #endif /* EVALUATE */ at line 2960 or so.

#ifdef EVALUATE
    {"0EVALUATE", P_evaluate},
#endif /* EVALUATE */

#ifdef GERTBOARD
	 {"0GERTBOARD", P_gert_io},	
	 {"0SETIO", P_gert_setport},		
	 {"0GETIO", P_gert_getport},		
#endif /* GERTBOARD */

As we are using code from the Gertboard demos, copy the files gb_common.o and gb_common.h from the gertboard_sw directory to atlast-1.2 directory (it's there if you have run make in this directory as well).

Add gb_common.o to the file Makefile in atlast-1.2.

ATLOBJ = atlast.o gb_common.o atlmain.o

Now, save and run "make" again to recompile atlast.c.

Test the new words

Wire up the Gertboard according to the information you get when you run the command sudo ./leds in the Gertboard demo directory.

Run sudo ./atlast in the atlast-1.2 directory.

Type 1 gertboard

Type 22 1 setio and press enter, the corresponding LED will go on.

Type 22 0 setio and the LED will go off.

Type 0 gertboard

Play with it

Define your own LED demo, start the interpreter with sudo ./atlast. Define these words:

: use 1 ;
: free 0 ;
: leds 25 24 23 22 21 18 17 11 10 9 8 7 ;
: on 12 0 do 1 setio loop ;
: off 12 0 do 0 setio loop ;

Now type use gertboard leds on and press enter and all leds should go on. Type leds off and they should go off again. And finally type free gertboard

A real Use Case

I have a kWh meter that I would like to read with the RPi. On the meter there is a small light that blinks once per second at 1 kWh. So I need a way to detect time between pulses. I tape up a simple LDR over the blinking light and connects it to Gertboard Buf1 and ground. Make B1 an input with a jumper on the board. Connect GP25 to B1.

Now that you know how to add a primitive word to atlast I just list the code for the word:

prim P_gert_getkwh() // channel --- clocks  clocks_per_sec
{ // Get a digital io port negative edge
  unsigned int i;
  clock_t start, end;

  start = clock();
  Sl(1);
  if (S0 == 21)
    { // find out which rev of RPi we have
      rev = pi_revision();
      if (rev != 1)
        S0 = 27; // GP21 on Gertboard is controlled by GPIO27
  }
  INP_GPIO(S0);
  i = 0;
  while(GPIO_IN0 & (1 << S0))
  {
    i++;
    if(i > 100000000) break;
  }
  long_wait(1);
  end = clock();
  S0 = (double) (end - start);
  So(1);
  Push = (stackitem) CLOCKS_PER_SEC;
}
 

Add #include <time.h> at line 20 in atlast.c

Test the command

1 gertboard
25 getkwh . .

: time_kwh 25 getkwh 2drop 25 getkwh float 2 roll float 2swap f/ ;

time_kwh f.

: hz_kwh 25 getkwh 2drop 25 getkwh float 2 roll float 2swap f/ 1.0 2swap f/ ;

hz_kwh f.

0 gertboard