Modified Barometric Pressure Sensor to make it easier for beginners to understand.

libraries/SPI/examples/BarometricPressureSensor/BarometricPressureSensor/BarometricPressureSensor.pde

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+/*
+  SCP1000 Barometric Pressure Sensor Display
+ 
+ Shows the output of a Barometric Pressure Sensor on a
+ Uses the SPI library. For details on the sensor, see:
+ http://www.sparkfun.com/commerce/product_info.php?products_id=8161
+ http://www.vti.fi/en/support/obsolete_products/pressure_sensors/
+ 
+ This sketch adapted from Nathan Seidle's SCP1000 example for PIC:
+ http://www.sparkfun.com/datasheets/Sensors/SCP1000-Testing.zip
+ 
+ Circuit:
+ SCP1000 sensor attached to pins 6, 7, 10 - 13:
+ DRDY: pin 6
+ CSB: pin 7
+ MOSI: pin 11
+ MISO: pin 12
+ SCK: pin 13
+ 
+ created 31 July 2010
+ modified 14 August 2010
+ by Tom Igoe
+ */
+
+// the sensor communicates using SPI, so include the library:
+#include <SPI.h>
+
+//Sensor's memory register addresses:
+const int PRESSURE = 0x1F;      //3 most significant bits of pressure
+const int PRESSURE_LSB = 0x20;  //16 least significant bits of pressure
+const int TEMPERATURE = 0x21;   //16 bit temperature reading
+cont byte READ = 0b00000000;     // SCP1000's read command
+const byte WRITE = 0b00000010;   // SCP1000's write command
+// pins used for the connection with the sensor
+// the other you need are controlled by the SPI library):
+const int dataReadyPin = 6; 
+const int chipSelectPin = 7;
+
+void setup() {
+  Serial.begin(9600);
+
+  // start the SPI library:
+  SPI.begin();
+
+  // initalize the  data ready and chip select pins:
+  pinMode(dataReadyPin, INPUT);
+  pinMode(chipSelectPin, OUTPUT);
+
+  //Configure SCP1000 for low noise configuration:
+  writeRegister(0x02, 0x2D);
+  writeRegister(0x01, 0x03);
+  writeRegister(0x03, 0x02);
+  // give the sensor time to set up:
+  delay(100);
+}
+
+void loop() {
+  //Select High Resolution Mode
+  writeRegister(0x03, 0x0A);
+
+  // don't do anything until the data ready pin is high:
+  if (digitalRead(dataReadyPin) == HIGH) {
+    //Read the temperature data
+    int tempData = readRegister(0x21, 2);
+
+    // convert the temperature to celsius and display it:
+    float realTemp = (float)tempData / 20.0;
+    Serial.print("Temp[C]=");
+    Serial.print(realTemp);
+
+
+    //Read the pressure data highest 3 bits:
+    byte  pressure_data_high = readRegister(0x1F, 1);   
+    pressure_data_high &= 0b00000111; //you only needs bits 2 to 0
+
+    //Read the pressure data lower 16 bits:
+    unsigned int pressure_data_low = readRegister(0x20, 2);    
+    //combine the two parts into one 19-bit number:
+    long pressure = ((pressure_data_high << 16) | pressure_data_low)/4;
+
+    // display the temperature:
+    Serial.println("\tPressure [Pa]=" + String(pressure));
+  }
+}
+
+//Read from or write to register from the SCP1000:
+unsigned int readRegister(byte thisRegister, int bytesToRead ) {
+  byte inByte = 0;           // incoming byte from the SPI
+  unsigned int result = 0;   // result to return 
+
+  // SCP1000 expects the register name in the upper 6 bits
+  // of the byte. So shift the bits left by two bits:
+  thisRegister = thisRegister << 2;
+  // now combine the address and the command into one byte
+  dataToSend = thisRegister & READ;
+
+  // take the chip select low to select the device:
+  digitalWrite(chipSelectPin, LOW); 
+  // send the device the register you want to read:
+  SPI.transfer(dataToSend); 
+  // send a value of 0 to read the first byte returned:
+  result = SPI.transfer(0x00); 
+  // decrement the number of bytes left to read:
+  bytesToRead--;
+  // if you still have another byte to read:
+  if (bytesToRead > 0) {
+    // shift the first byte left, then get the second byte: 
+    result = result << 8;
+    inByte = SPI.transfer(0x00); 
+    // combine the byte you just got with the previous one:
+    result = result | inByte;
+    // decrement the number of bytes left to read:
+    bytesToRead--;
+  }
+  // take the chip select high to de-select:
+  digitalWrite(chipSelectPin, HIGH); 
+  // return the result:
+  return(result);
+}
+
+
+//Sends a write command to SCP1000
+
+void writeRegister(byte thisRegister, byte thisValue) {
+
+  // SCP1000 expects the register address in the upper 6 bits
+  // of the byte. So shift the bits left by two bits:
+  thisRegister = thisRegister << 2;
+  // now combine the register address and the command into one byte:
+  dataToSend = thisRegister | WRITE;
+
+  // take the chip select low to select the device:
+  digitalWrite(chipSelectPin, LOW); 
+
+  SPI.transfer(dataToSend); //Send register location
+  SPI.transfer(thisValue);  //Send value to record into register
+
+  // take the chip select high to de-select:
+  digitalWrite(chipSelectPin, HIGH); 
+}
+
+
+