Cornell University ECE4760
Language Interpreters
for Atmel Mega644 microcontrollers

Introduction

It is useful to be able to write small programs in a scripting language. This example of an interpreter is based on a C program written by Adam Dunkels, originally to implement, in his words: A really tiny and stupid BASIC interpreter, written in a few hours for the fun of it. (see http://www.sics.se/~adam/ubasic/).

I hacked it to be more of a tiny and stupid, block-oriented, bracket-delimited, language and ported it to the Mega644 in WINAVR/GCC. I also made a very simple interactive download utility so that source code (see example below) can be written on a PC editor, then sent to the microcontroller as a text file, terminated with the @ sign.

# test program ;  
print 'starting demo'; 
peek 3+32, c ; # look at pinb ;
while c=255 { 
   print 'press button'; 
   delay(200); 
   peek 3+32, c ; 
} ; 
poke 7+32, 255 ;
for i= 0 to 10 {
   peek 3+32,c ;
   poke 8+32,i ;
   print 'i=',i ;
   for j=10 to 13 {print 'i-j=', i-j;};
   if c=255 {print 'press button';};
   if c<255 {
      print 'button pushed';
      delay(500);
      if c=254 {print 'button0';};
   }
   else {
      print 'no button' ;
   };
   delay(500);
};
print 'done' ;
end;
@

Files

The first pass of modification just uses a string embedded in the C interpreter as source. You need to download the following files. The source string and main are in use-ubasic.c.

These files are modified versions of the programs on Adam Dunkel's site.

Program structure

The following table shows the syntax of the existing statements. You can easily add your own statements.

The more formal BNF notation for the language is:

All statments are terminated with a semicolon. Spaces, tabs, newlines are all ignored. All program constructs must be lower-case.

statement example
for var = expr to expr { statement; statment; ... } ; for i=1 to 10 { j=j+1; print j; } ;
while relation { statement; statment; ... } ; while i<10 { i=i+1; print 'i=',i; } ;

if relation { statement; statment; ... }
else { statement; statment; ... } ;

 if c<255 {print c; delay(500);}
else {print 'no button';};
print expr, 'string', ... ; print 'hi there', i+1;
peek address, var ; peek 3+32, x; # peek address, variable;
poke address, expr ; poke 40, i+2; # poke address, value
delay expr ; delay 500; # milliseconds ;
# this is a comment statement ; # comment ;
var = expr ; a = (i+1)/j; # assignment ;

 

Performance

A for-loop executes in about 100 microSec, and a 2-level nested for-loop in 200 microSec. A 16-bit add, subtract, multiply, or divide takes 200 microSecs.

Interactive version.

A simple interactive interface was built on top of the intrepeter so that a program can be downloaded from a PC using hyperterm's sendfile feature, then executed or saved in eeprom. The source code sent from the PC must end with an at sign @. The revised interperter files are here.
There are six commands:

Command meaning
help
 Prints the other 5 commands.
down
 Sets download of program from PC using hyperterm transfer>send text file the text file is terminated by an @ symbol. Prints download complete when it finishes.
run
 Executes the program.
load
 Gets a program from eeprom. Prints load complete when it finishes.
save
 Puts a program into eeprom. Prints save complete when it finishes.
list
 Prints the program text.

 

 

 

 

Copyright Cornell University September 1, 2011