[TriEmbed] C Code Question Ralated to Rotary Encoder

Pete Soper pete at soper.us
Tue Feb 23 14:43:18 CST 2016


All those variables are "binary variables". The computer is only capable 
of holding binary values. The code is expressing them as decimal values 
(via formatting library routines like 'printf'), while the comments are 
expressing the values as a confusing hodge podge of decimal and binary.

The C language (and let's pretend this is C code and not go crazy by 
mentioning C++) has these data types, among others:

    (signed) char   - if you leave off "signed" you get that for free
    unsigned char
    (signed int)
    unsigned int
    etc.

Typedefs (think synonyms) allow additional names to stand for these type 
names. In Arduino Land "byte" is a synonym for "unsigned char". In 
Arduino land char is an eight bit type and int is 16 bits. This means 
they can hold this many binary digits of information. If they're signed, 
the most significant bit is interpreted as a sign bit (one means 
"negative", zero means "positive").

This means signed char variables can hold values from -128 to +127 
decimal, or 11111111 to 01111111 binary. Unsigned char variables (or 
something named "byte") can hold values from 0 to 255 decimal or 
00000000 to 11111111 binary.

Int variables are the same but twice as big (e.g. an unsigned int can 
express the number 65535 decimal).

An array element is the same whether it's indexed by a literal integer 
value or an integer variable and the size of the variable only 
determines the range of index values.

When you say "When the code sees the binary index as a byte it knows 
that 10 means read the third element value and when it sees 11 it knows 
to read the fourth element value" you're imagining there's some kind of 
radix conversion going on (radix is the "base" of the value, such as two 
for binary). Again, writing "11" to express the number three decimal is 
just confusing.

Imagine every occurrence of "11" being written as "0b11", every "10" 
being "0b10", etc. This is the proper "C way" to express a binary 
number. If the author had used these prefixes consistently he'd be 
plainly saying what he intended to say.

-Pete


On 02/23/2016 03:15 PM, Dwight Morgan via TriEmbed wrote:
>
> Thanks Rodney and Mike for the responses. In Mr. Churchward's code he 
> declares the byte variable "state" and uses the function 
> "knobTurned()" as follows -- code snippets:
>
> byte state = 0;           // will store *_two bits_*for pins A & B on 
> the encoder which we will get from the pins above
>
> void knobTurned(){
>
> /* AH HA! the knob was turned */
>
> state = 0;    // reset this value each time
>
> state = state + digitalRead(pinA);   // add the state of Pin A
>
> state <<= 1;  // shift the bit over one spot
>
> state = state + digitalRead(pinB);   // add the state of Pin B
>
>   /* now we have a two bit binary number that holds the state of both pins
>
> 00 - something is wrong we must have got here with a key bounce
>
> 01 - sames as above - first bit should never be 0
>
> 10 - knob was turned backwards
>
> 11 - knob was turned forwards
>
> */
>
>   /* We can pull a value out of our truth table and add it to the 
> current level */
>
> level = level + bump[state];
>
>   /* Let's see what happened */
>
> Serial.print(bits[state] + "    ");  // show us the two bits
>
> Serial.print(bump[state],DEC);       // show us the direction of the turn
>
> Serial.print("    ");
>
> Serial.println(level);               // show us the new value
>
> }
>
> I've never used bytes much in code and never really  thought about it 
> much and I always saw calls to specific array elements with a decimal 
> value in the square brackets like this: X = myarray[2]; for instance 
> to get the 3rd element value.
>
> It looks like when a byte is used as the index the value is a binary 
> number -- using ones and zeros -- and it can simply because it is a byte!
>
> If you look at the knobTurned() function it is building a two place 
> binary index stored in the byte variable named "state". When the code 
> sees the binary index as a byte it knows that 10 means read the third 
> element value and when it sees 11 it knows to read the fourth element 
> value.
>
> This seems like a very handy thing to know for new learners like me 
> with Arduino and RPi.
>
> That's my now understanding of it. Please shim up any parts as needed!
>
> Thanks again!
>
> Dwight
>
> *From:*Rodney Radford [mailto:ncgadgetry at gmail.com]
> *Sent:* Monday, February 22, 2016 11:22 PM
> *To:* Dwight Morgan
> *Cc:* Triangle Embedded Computing Discussion
> *Subject:* Re: [TriEmbed] C Code Question Ralated to Rotary Encoder
>
> I had to search for the video you watched to see what was being done 
> and then find the code.  For those following along, the video is part 
> of a 3 part video of using a quadrature rotary encoder as an input for 
> a morse code sending device.  The one necessary for this discussion is 
> the part 2 which can be found at 
> https://www.youtube.com/watch?v=FGxLXqzPe3Q
>
> In the video, as the knob is turned to the left or right, a line is 
> printed with three values on it, like this:
>
> 11      1      70
> 11      1      71
> 10      -1     70
> 10      -1     69
>
>
> The first column represents the two bits of the quadrature, the second 
> column indicates whether we are counting up (+1) or down (-1) and the 
> third column represents the current value.
>
> The question is how can the compiler know that the 11 represents a 
> binary number whose value is 3, and how does it know that 10 
> represents a binary number whose value is 2.
>
> The answer is that it doesn't have to... the internal variable is 
> called state and it either has a 0, 1, 2, or 3.
>
>
> When printing, the author decided to show the value of the number in 
> binary, instead of decimal, so he printed the first value like this:
>
>    /* For demo purposes we will create an array of these binary digits */
>    String bits[] = {"00","01","10","11"};
>
>    /* Let's see what happened */
>    Serial.print(bits[state] + "    ");  // show us the two bits
>
> So as you see, the number is the index into a character string array 
> and it prints out either the string "00", "01", "10", or "11".  This 
> is just to make it easier for you to visualize that the number is 
> really a two bit number representing the two outputs of the quadrature 
> knob.
>
> The second value in the line is printed with this line (this is the 
> one you showed).  This prints out only the
>
>    int bump[] = {0,0,-1,1};
>    Serial.print(bump[state],DEC);       // show us the direction of 
> the turn
>
> Here we see the same 0..3 state variable indexing into a numerical 
> array and printing out either 0, 0, -1 or +1 as a DECimal number.
>
> And then the third value is printed with these lines:
>
>     Serial.print("    ");
>     Serial.println(level);               // show us the new value
>
> Here we just print out the integer level variable that has already 
> been incremented or decremented.
>
> On Mon, Feb 22, 2016 at 10:10 PM, Dwight Morgan via TriEmbed 
> <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
>
> I'm trying to understand and use a rotary encoder on an Arduino 
> project and I have a simple demo circuit working with the encoder and 
> output to the serial monitor. I see how the code is working but I 
> can't find documentation that confirms what I'm seeing.
>
> The C code builds a two digit binary number that is used as a pointer 
> to values in an array that is used to output either a 1 or -1 to 
> indicate if the encoder is rotating clockwise or counterclockwise -- 
> all well and fine.
>
> The variable used as a pointer is a type byte initially set to zero 
> (byte state = 0) with a function called by an interrupt to build the 
> two digit pointer. I looked it up and a byte can either hold 8 bits or 
> a decimal value from 0 to 255.
>
> The pointer works fine and prints out either a 1 or -1 on the serial 
> monitor.
>
> My question is, how does the code know that a binary 10 is not the 
> number ten as the pointer or binary 11 is not the number eleven as the 
> pointer instead of knowing it is the number 2 or 3 to be used as the 
> pointer to pick out values in the following array?
>
> int bump[] = {0,0,-1,1};
>
> The output is like this: Serial.print(bump[state]),DEC); //state being 
> the built pointer of either binary 10 or 11 for a value of either 
> decimal 2 or 3, respectively.
>
> Thanks to C code by Budd Churchward on YouTube.
>
> Input appreciated. Thanks!
>
> Dwight
>
>
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