Arduino - create dynamic rules - arduino

Question

I want to create a program for arduino in order to be able to add and remove rules controlling some pins, whenever i want and not hardcode them.
For example a rule like this:
if pin1=HIGH and pin2=LOW then pin3=HIGH
i want to turn into this:
if(pin1 == HIGH && pin2 == LOW){
digitalWrite(pin3, HIGH);
}
Lets say we pass the rule via the command line as a string.
How could i convert this string into a rule? Is something like this possible?

Answer 1

Your second piece of code:
if(pin1 == HIGH && pin2 == LOW){
digitalWrite(pin3, HIGH);
}
is compiled by your IDE into machine code. That machinecode is then uploaded to the Arduino and then executed.
Therefore you cannot just send some string like this
if pin1=HIGH and pin2=LOW then pin3=HIGH
to the Arduino because after your programm transformed that into the desired form the complete IDE and compiler stuff is missing on the Arduino side. And a C++ compiler tool chain is big - several tens of megabytes! There is no place for that on the Arduino.
What you can do: Invent a simple language for your rules and develop an interpreter for that which then runs on the Arduino.
Perhaps you also don't need to reinvent the wheel - Google for simple BASIC interpreters running on the AVR CPUs and adapt one of them.

Answer 2

What you are asking for is possible but with much work on your part.
Approach 1 (think big):
What you are looking for is a software implementation equivelant to how PAL's (programmable array logic) and CPLD's (complex programmable logic device) operate. If you read this article, you will get some ideas on how this is done in hardware:
Wikipedia article on PLD's
A PAL can create arbitray combinational logic rules between a set of inputs and outputs, i.e. anything you can express as a logical equation of AND's, OR's and NOT's can be programmed. It is programmed by "burning" a set of fuses that connect the inputs to logics gates and then to outputs. What is uploaded to these devices is just a set of 0's and 1's.
You could implement such a thing in software with an array of 0's and 1's to represent the fuses. The hard code would run over the array and calculate the output. You would need to develop the method to load the array with the correct fuses.
A common method by which PAL's are programmed is with the language VHDL. The compiler for this language takes an expression like yours and translates it to the set of AND's, OR's and NOT's that the PAL can use. A search will yield endless discussion of this language, for example:
A VHDL tutorial
You would have to create the compiler that takes the text input and determines the fuses. You would be undertaking some significant tasks:
a domain specific language parser (I can recommend ANTLR),
a PAL compiler (I can't recommend anyone do this themselves), and
the Arduino code to emulate a PAL.
By the time you create your own equivalent of VHDL and PAL's, you'll probably be thinking you could have just put a PAL chip on a proto board and been done.
Approach 2 (more practical):
The simpler method to achieve the same result is a truth table. A truth table is the equivalent of some set of logical expressions. To implement a truth table, translate your logic expressions to a table with one row for each input case. Here is an example of a truth table of two inputs that drive an output:
IN1 IN2 OUT
0 0 1
0 1 0
1 0 1
1 1 1
The code for such a truth table implementation looks like this:
const int IN1 = 6;
const int IN2 = 7;
const int OUTA = 13;
byte table[4] = {1, 0, 1, 1};
void loop() {
int in1;
int in2;
byte outbit;
size_t ix;
//
in1 = digitalRead(IN1);
in2 = digitalRead(IN2);
ix = in2 * 2 + in1;
outbit = table[ix];
digitalWrite(OUTA, outbit);
}
The complete expression of the logic rules is the array of 4 bytes. To "program" an new output equation, you just send a new set of 4 values, for your specific equation you send "0 0 1 0". Your program would receive the list of values and store them in the table array. As soon as the new values/rules are stored, the function of the output would change.
This type of programmable logic is feasible as long as you have enough RAM to store the table. For 4 inputs you just need 16 values, for 5 you need only 32.
Your question raises a keen point in electronics design: you need to pick the right device for the right problem. Code is not always better or easier. Avoid the hammer-nail trap (when your only tool is a hammer every problem looks like a nail). A PAL/CPLD and microcontroller are a powerful combination.

Answer 3

To add to the truth table approach mentioned by #jdr5ca, it's a good idea to understand the controller used in the arduino as well as using the ardiuno libraries.
Pins 0 to 7 are port D and pins 8 to 15 port B. Each port is represented as three single byte registers - PORT_ which is output/bias resistor, PIN_ which is input state and DDR_ which is direction. When you call digitalRead(pin) the code sees which range the pin is in, reads PIN_ for that port, then bit-shifts and masks the value to give just that pin's state. Which is handy for reading one pin at a time, but a less convenient than if you're reading several.
So if you create a truth table with 256 entries, you can write the output of pins 8 to 15 from the values of inputs 0 to 7 using a single line, rather than decoding the registers to pins then encoding them again:
byte table[256] = {0};
void loop() {
PORTB = table[PIND];
}
You can then add something to read from the serial and load new values into the table on the fly.