File content as of revision 1:d01abb5e6f52:

/*
 * Honeywell Pressure Sensor
 *
 */
 
#include <math.h>

#include "mbed.h"

DigitalOut myled(LED1);

// http://sensing.honeywell.com/index.cfm/ci_id/157750/la_id/1/document/1/re_id/0
// http://sensing.honeywell.com/index.cfm/ci_id/156988/la_id/1/document/1/re_id/0

#define HSC_2A 0x28

int main() {

// BMP085 bmp085(p28, p27); //bmp085(p9, p10); // sda, scl

    printf("\rhsc\n\r");

    I2C i2c(p28, p27);
    
    int id = HSC_2A;

    i2c.frequency(100000); // 100k - 400k

    while(1) {

#if 0    
        i2c.start();
        i2c.write((id<<1) | 1);
        int b1 = i2c.read(1);
        int b2 = i2c.read(1);
        int t1 = i2c.read(1);
        int t2 = i2c.read(0);
        i2c.stop();
        printf("%02X %02X  %02X %02X\n\r", b1, b2, t1, t2);
#else
        unsigned char data[4];
        i2c.read((id<<1)|1, (char*)data, sizeof(data));
        //printf("%02X %02X  %02X %02X\n\r", data[0], data[1], data[2], data[3]);
        
        int st = data[0]>>6; // status
        int bd = ((data[0] & 0x3f) << 8) | data[1]; // bridge data
        int td = ((data[2]<<8) | data[3]) >> 5; // temp data
        
        // P: 14 bits
        // Cmin=2^14-1*.1 Cmax=2^14-1*.9
        // Pmin=0 Pmax=15
        float p = (((float)bd-1638)*(15-0)/(14745-1638))+0; // convert raw data to psi
        p = p * 68.9475729; // convert psi to hPa
        
        // T: 11 bits (Cmax=2047=2^11-1)
        float t = (((float)td*200)/2047)-50; // convert raw data to deg C
        printf("%d: %.2f hPa  %.2f C\n\r", st, p, t);
        
        wait(1);
#endif    

        myled = 1;
        wait(0.2);
        myled = 0;
        wait(0.2);
        
    } // while
    
} // main


// http://en.wikipedia.org/wiki/Barometric_formula
const float layers[][4] = {
//  Height[m], Pressure[Pa], Temp[K], Lapse[K/m], Subscript
        0,     101325.0,     288.15,  -0.0065,    // 0
    11000,      22632.1,     216.65,   0.0,       // 1
    20000,       5474.89,    216.65,   0.001,     // 2
    32000,        868.019,   228.65,   0.0028,    // 3
    47000,        110.906,   270.65,   0.0,       // 4
    51000,         66.9389,  270.65,  -0.0028,    // 5
    71000,          3.95642, 214.65,  -0.002      // 6
};

enum { L_HEIGHT=0, L_PRESS, L_TEMP, L_LAPSE };

const float g = 9.80665;
const float M = 0.0289644;
const float R = 8.31432;
const float Ta = -273.15; // Absolute zero [deg C]

// Search for subscript which matches pressure range
// Choose appropriate formula and plug in values
// Lookup derived h in table and check subscript is still same, otherwise recalculate
// Add height correction

// Pass in pressure and temperature from sensor
// Return height[m]
float pressureToHeight(float p, float t) {
    float h = -999; // error
    int b;  
    int tries;

    // Search backwards thru layers for our pressure
    for(b = 6; b >= 0; --b) {
        if(p <= layers[b][1]) break;
    }
    if(b < 0) b = 0; // adjust so can have -ve heights
    int b1 = b;
    
    // Only retry limited times
    for(tries = 10; tries; --tries) {
          
        float Hb = layers[b][0];
        float Pb = layers[b][1];
        float Tb = layers[b][2]; //??? Tb = t - Ta;
        float Lb = layers[b][3];
        
        if(Lb != 0.0) 
            //h = (log(g*M/R/Lb)/log(p/Pb)*Tb - Tb)/Lb + Hb; // Equation 1 (non-zero lapse rate)
            h = (Tb/exp(R*Lb/g/M*log(p/Pb))-Tb)/Lb + Hb;
        else 
            h = -log(p/Pb)*R*Tb/g/M + Hb; // Equation 2 (zero lapse rate)
        
        // Search again for derived height
        for(b = 6; b >= 0; --b) {
            if(h <= layers[b][0]) break;
        }
    
        if(b == b1) break; // gave same subscript, so we can stop
        b1 = b;
        
    } // for - tries    
    
    if(tries <= 0) h = -999; // error

        
    return h;    

} // pressureToHeight

/*end*/