7-segment display code

Question

What is the meaning of BitSet = bitRead(numeral[number], segment);?

const byte numeral[10] = {
 //ABCDEFG /dp
 B11111100, // 0
 B01100000, // 1
 B11011010, // 2
 B11110010, // 3
 B01100110, // 4
 B10110110, // 5
 B00111110, // 6
 B11100000, // 7
 B11111110, // 8
 B11100110, // 9
};
// pins for decimal point and each segment
// dp,G,F,E,D,C,B,A
const int segmentPins[8] = { 5,9,8,7,6,4,3,2};
void setup()
{
 for(int i=0; i < 8; i++)
 {
 pinMode(segmentPins[i], OUTPUT); // set segment and DP pins to output
 }
}
void loop()
{
 for(int i=0; i <= 10; i++)
 {
 showDigit(i);
 delay(1000);
 }
 // the last value if i is 10 and this will turn the display off
 delay(2000); // pause two seconds with the display off
}
// Displays a number from 0 through 9 on a 7-segment display
// any value not within the range of 0-9 turns the display off
void showDigit( int number)
{
 boolean isBitSet;
 for(int segment = 1; segment < 8; segment++)
 {
 if( number < 0 || number > 9){
 isBitSet = 0; // turn off all segments
 }
 else{
 // isBitSet will be true if given bit is 1
 isBitSet = bitRead(numeral[number], segment);
 }
 isBitSet = ! isBitSet; // remove this line if common cathode display
 digitalWrite( segmentPins[segment], isBitSet);
 }
}

• Code Gorilla: Do you now about chips like the Max7219? It will help you handle a number of 7 segment displays (up to 8).

Answer 1

bitRead(x, y) takes a value x, and looks at bit number y.

So, if:

• y is the number 2;
• x is 53 (binary number 0 0 1 1 0 1 0 1)

   ^

it looks at bit #2. Bits are counted from the right starting at 0 - I have indicated the bit in question above.

So, bitRead(53, 2) would return 1, since bit #2 in 53 is a 1.

In the above program, the clever programmer has coded whether to light or not light the LED for each segment of the display in a single byte for each possible number to display. The digit 8 has all seven of its LEDs lit up, so you'd expect the encoding for 8 to have lots of binary 1s in it - and sure enough, it does (B11111110)! And the digit 7 has only three segments lit up, so you'd expect only 3 bits to be set. Sure enough, B11100000.

The comment at the top describes what each of the bits represent - the last bit is the decimal point, which is never set...