Nuts and Volts - January 2017

 count += piecesTemp[row][4];
  piecesTemp[row][5] = !digitalRead(7);
  count += piecesTemp[row][5];
  piecesTemp[row][6] = !digitalRead(8);
  count += piecesTemp[row][6];
  piecesTemp[row][7] = !digitalRead(12);
  // Oddball because we skipped 9, 10
  count += piecesTemp[row][7];
  // for serial RX, TX, Serial2 on Teensy 3.1
  return count;
  // return number of pieces found in this row
}

unused by circuitry external to the microcontroller. Being able to use Serial.print() statements in your code to be read in real time using the embedded serial monitor in the Arduino GUI is a great aid in debugging code. Pins 9 and

10 were also left unconnected and brought out to pads on the PCB. This pair of pins implements RX2 and TX2 on the Teensy 3.1 and can be used to connect the chessboard to other external circuits or a PC.

// Read all 8 lines of the Hall Effect Sensors
int readHall(byte piecesTemp[][8])
{
  // The function calls readLine() 8 times to read all
  // 8 rows of Hall Effect sensors
  // When finished piecesTemp[8][8] has positions of
  // pieces on board
  // Before each read we set the 3 bit address to the
  // 74138
  // The total number of pieces on the board is
  // returned
  int count = 0;
  digitalWrite(A0, LOW);
  digitalWrite(A1, LOW);
  digitalWrite(A2, LOW); // 000
  delay(1); // let 3144 settle
  count += readLine(0, piecesTemp);
  digitalWrite(A0, HIGH);  // 001
  delay(1);
  count += readLine(1, piecesTemp);
  digitalWrite(A0, LOW); // 010
  digitalWrite(A1, HIGH);
  delay(1);
  count += readLine(2, piecesTemp);
  digitalWrite(A0, HIGH);  // 011
  delay(1);
  count += readLine(3, piecesTemp);
  digitalWrite(A0, LOW); // 100
  digitalWrite(A1, LOW);
  digitalWrite(A2, HIGH);
  delay(1);
  count += readLine(4, piecesTemp);
  digitalWrite(A0, HIGH);  // 101
  delay(1);
  count += readLine(5, piecesTemp);
  digitalWrite(A0, LOW); // 110
  digitalWrite(A1, HIGH);
  delay(1);
  count += readLine(6, piecesTemp);
  digitalWrite(A0, HIGH);  // 111
  delay(1);
  count += readLine(7, piecesTemp);
  delay(100);
  return count;
  // return count of all pieces on the board
}

Instead of a D port read, we use eight digitalRead() statements that are negated to read a single row of sensors. The Hall effect sensors are open collector, and when activated they read as a 0. So, for clarity, this is negated to a 1, representing a chess piece in that location.

The readHall() function calls readLine() eight times, after setting the three-bit address to the 74HCT138 to select each row. Here again, a port C output could have simplified things a little, but that could have interfered with the A4 (SDA) and A5 (SCL) I2C signals which are reserved for communicating with an LCD display or RTC. After the call to readHall(), the pieces Temp array will show the locations of all the pieces on the chessboard and return the total number of pieces found on the board.

At this point, all of the functions that interact with the hardware need to be described.

We will pause here for this month. Next time, there will be an overview of the main sections of the software that contain the logic for supervising a chess game between two players. The loop() section of the program takes you through the first player's turn and then, on the next pass through, changes the turn to the second person. Inside the logic is the ability to detect which piece is lifted and display all its possible moves. There will also be some error checking and looking for a king in check or checkmate. If you just can't wait, you can obtain the entire Arduino program from the download section. Keep in mind that this program is intended to run on a Teensy 3.1.

NV

Teensy 3.1 microcontroller

74HCT138 3-to 8 decoder

7414 Dual Schmidt trigger Allegro 3144 Hall effect sensors ( 64)

PARTS

2N3906 PNP transistors ( 8)

470 ohm Resistors ( 8)

LIST

2700 μF 6. 3 volt Electrolytic capacitor

0.1 μF Disk capacitors ( 2)

Five volt/two amp Power supply, wall wart type Power jack to match power supply LCD display, four-line/20 character with I2C two-wire

The functions readLine() and readHall() do the work of scanning the 64 board positions and creating an 8 x 8 array telling us where pieces are located, and also return the number of pieces found. The readLine() function scans a single row. Notice here that it would have been simpler to do a read of the D port pins 0-7 on the Teensy 3.1. In one line of code, we would have read all eight Hall effect sensors in one row. However, that would mean that we could not use pins 0 and 1 with our serial monitor.