Uno -> HID Many questions about Scancodes and Byte* Variables

Projects Programming
1

Hey Guys,
i flashed the Atmega16U2 from the Uno with the arduino-keyboard-0.3.hex. I downloaded an Example File and everything workfs fine! But i don´t understand the code and thats make me going crazy! The code looks like this:

uint8_t buf[8] = { 0 };	/* Keyboard report buffer */

#define KEY_LEFT_SHIFT	0x02

void setup() 
{
    Serial.begin(9600);
    delay(10000); //some Time to wait
    pinMode(8,INPUT);
    digitalWrite(8,HIGH); //external button
    pinMode(13,OUTPUT);
    digitalWrite(13,LOW); //just to see that the 10 Seconds are over
}

char *str = "arduino";

void loop() 
{
    char *chp = str;
    int button = digitalRead(8);   //only do that if the Button is pushed
    if (button == LOW)
    {
    while (*chp) {
	    
	if ((*chp >= 'a') && (*chp <= 'z')) {
	    buf[2] = *chp - 'a' + 4;
	} else if ((*chp >= 'A') && (*chp <= 'Z')) {
	    buf[0] = KEY_LEFT_SHIFT;	/* Caps */
	    buf[2] = *chp - 'A' + 4;
	} else {
	    switch (*chp) {
	    case ' ':
	    	buf[2] = 0x2c;	// Space
		break;
	    default:
	        /* Character not handled. To do: add rest of chars from HUT1_11.pdf */
		buf[2] = 0x37;	// Period
		break;
	    }
	}

	Serial.write(buf, 8);	// Send keypress
        delay(25); //some Time for the Keyboard Controller
	buf[0] = 0;
	buf[2] = 0;
	Serial.write(buf, 8);	// Release key
	chp++;
    }
        buf[2]= 0x28;
        Serial.write(buf, 8);	// Send keypress
	buf[0] = 0;
	buf[2] = 0;
        delay(25); //dem Controller Zeit geben!
	Serial.write(buf, 8);	// Release key
    delay(100);
    }
}

My questions are:

  1. uint8_t buf[8] = { 0 }; uint8_t buf[8] is the same like byte buf[8] isn´t it? But Why make an array of 8 if we need in the code only 2 byte. Hope you understand me!? In the code we only use buf[0] or buf[2]
    Is there any reason for it? I don´t understand that....

  2. whats about the * before the str where the string is declared? what means *str ?

  3. I don´t understand why "while (*chp)" work. I would make an for loop with textlength or something, but the while loop is much shorter and looks more professional but i have never seen this before, so i just want to ask if that have any handicaps in comparsion with a "textlengt for loop"

Maybe someone knows the answers...
Thanks a lot, Ruediger

2

But Why make an array of 8 if we need in the code only 2 byte. Hope you understand me!? In the code we only use buf[0] or buf[2]

But that's a span of three bytes.

whats about the * before the str where the string is declared?

It makes the variable a pointer

I don´t understand why "while (*chp)" work.

Fetch the value that chip points to, test to see if it is zero

3

Hi Awol,
thanks for that very fast answer!
I understand answer 2 + 3 and will know aks google some things :slight_smile:
But i don´t understand answer 1 !

If I replaye buf[8] with
byte byte1 = 0;
byte byte2 = 0;
and then replaye every buf[0] with byte1 and every buf[2] with byte2 then the code don´t work! But Why??

Thanks a lot for your patience and sorry for that worse english!

Ruediger

4

I don't understand why you're using buf[0] and buf[2] instead of buf[0] and buf[1] if you only need 2 bytes.

5

:slight_smile:
that´s excactly what i also don´t understand! The Code from my first post is the code from the Example i found in the Internet!
And know im wondering why in the code the variable uint8_t buf[8] ist declare with an 8 and not with an 2
Is there something in the code i don´t see??

Greetz, Ruediger

6

And know im wondering why in the code the variable uint8_t buf[8] ist declare with an 8 and not with an 2

because they're writing eight bytes every time to the serial port.
This is presumably because whatever is on the other end of the serial connection expects eight bytes, not two.

7

Doh...
I think your absolutly right! Now i´m al little bit ashamed, because that was very logical
However, thank you very much for the Answers!!
Greetings, Ruediger