Using SPI to read a Barometric Pressure Sensor

/*

 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

const byte READ = 0b11111100;     // 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

  Serial.print(thisRegister, BIN);

  Serial.print("\t");

  // 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

  byte dataToSend = thisRegister & READ;

  Serial.println(thisRegister, BIN);

  // 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:

  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);
}