Updates code to use low level registers. Untested with real eeprom

arduino_program/arduino_program.ino

@@ -1,5 +1,151 @@
+#define SHIFT_DATA 2
+#define SHIFT_CLK 3
+#define SHIFT_LATCH 4
+#define EEPROM_D0 5
+#define EEPROM_D7 12
+#define EEPROM_WE 13
+
+#define LAST_OP_WRITE 0
+#define LAST_OP_READ 1
+
+#define READ_MODE true
+#define WRITE_MODE false
+
+// Digital pins 0 to 7 correspond to PD0 to PD7
+// Digital pins 8 to 13 correspond to PB0 to PB5
+
+byte lastOp;
+
+void latchOutput() {
+  static byte latchMask = 0b00010000;
+
+  PORTD &= ~latchMask;
+  delayMicroseconds(1);
+  PORTD |= latchMask;
+  delayMicroseconds(1);
+  PORTD  &= ~latchMask;
+}
+
+void commitWrite() {
+  static byte writeEnableMask = 0b00100000;
+  
+  delayMicroseconds(1);
+  PORTB &= ~writeEnableMask;
+  delayMicroseconds(1);
+  PORTB |= writeEnableMask;
+}
+
+void setBusMode(bool busMode) {
+  // When dealing with DDR registers. 1 is output and 0 is input.
+  if (busMode == READ_MODE) {
+    // When reading we want the pins to be inputs.
+    DDRD &= 0b00011111;
+    DDRB &= 0b11100000;
+  } else {
+    // When writing we want the pins to be outputs.
+    DDRD |= 0b11100000;
+    DDRB |= 0b00011111;
+  }
+}
+
+void setAddress(int address, bool outputEnable) {
+  // Position of the output enable bit withing the data to be shifted out. 
+  static byte outputEnableMask = 0x8000;
+  // Position of the data bit within PORTD.
+  static byte dataMask = 0b00000100;
+  // Position of the clock bit within PORTD.
+  static byte clockMask = 0b00001000;
+
+  if (outputEnable) {
+    // Clear output enable bit.
+    address &= ~outputEnableMask;
+  } else {
+    // Set output enable bit.
+    address |= outputEnableMask;
+  }
+  
+  // Make sure the clock pin is low.
+  PORTD &= ~clockMask;  
+
+  // Shift the data out LSB first.
+  for (uint8_t i = 0; i < 16; i++, address >>= 1) {
+    if (address & 1) {
+      PORTD |= dataMask;
+    } else {
+      PORTD &= ~dataMask;
+    }
+
+    // Clock the bit out.
+    PORTD |= clockMask;
+    delayMicroseconds(1);
+    PORTD &= ~clockMask;
+    delayMicroseconds(1);
+  }
+
+  latchOutput();
+}
+
+byte readEEPROM(int address) {
+  // TODO: Think about adding a delay if the last operation was a write and where it is appropriate to put it.
+  // If the last operation wasn't a read we need to set our pins as input.
+  if (lastOp != LAST_OP_READ) {
+    setBusMode(READ_MODE);
+  }
+
+  // In contrast with the write function, it's okay to set output enable after setting the
+  // pins as inputs. This way, the arduino pins are already ready to sink current when the
+  // EEPROM starts driving the bus.
+  byte result = (PINB << 3) | (PIND >> 5) ;
+
+  lastOp = LAST_OP_READ;
+  return result;
+}
+
+
+/**
+ * Writes a single byte of data to the specified address.
+ * If cooldown is true, the function adds a delay to avoid
+ * reading incorrect data after a write.
+ */
+void writeEEPROM(int address, byte data, bool cooldown) {
+  // For the write case, must turn output enable off before setting the pins as outputs.
+  // If we don't do this, for a brief period of time, both the arduino and the EEPROM would
+  // drive the bus and this could case problems.
+  setAddress(address, false);
+  
+  // If the last operation wasn't a write we need to set our pins as output.
+  if (lastOp != LAST_OP_WRITE) {
+    setBusMode(WRITE_MODE);
+  }
+  
+  // EEPROM_D0 to D2 correspond to PD5 to PD7
+  // EEPROM_D3 to D7 correspond to PB0 to PB4
+  PORTD &= (data << 5) & 0b00011111;
+  PORTB &= (data >> 3) & 0b11100000;
+
+  commitWrite();
+  lastOp = LAST_OP_WRITE;
+  
+  if (cooldown) {
+    delay(10);
+  }
+}
+
+
 void setup() {
   // put your setup code here, to run once:
+  pinMode(SHIFT_DATA, OUTPUT);
+  pinMode(SHIFT_CLK, OUTPUT);
+  pinMode(SHIFT_LATCH, OUTPUT);
+
+  digitalWrite(EEPROM_WE, HIGH);
+  pinMode(EEPROM_WE, OUTPUT);
+  
+  // Setting an invalid value.
+  lastOp = -1;
+
+  Serial.begin(9600);
+  
 
 }