main.cpp

00001 /*
00002  * Honeywell Pressure Sensor
00003  *
00004  */
00005  
00006 #include <math.h>
00007 
00008 #include "mbed.h"
00009 
00010 DigitalOut myled(LED1);
00011 
00012 // http://sensing.honeywell.com/index.cfm/ci_id/157750/la_id/1/document/1/re_id/0
00013 // http://sensing.honeywell.com/index.cfm/ci_id/156988/la_id/1/document/1/re_id/0
00014 
00015 #define HSC_2A 0x28
00016 
00017 int main() {
00018 
00019 // BMP085 bmp085(p28, p27); //bmp085(p9, p10); // sda, scl
00020 
00021     printf("\rhsc\n\r");
00022 
00023     I2C i2c(p28, p27);
00024     
00025     int id = HSC_2A;
00026 
00027     i2c.frequency(100000); // 100k - 400k
00028 
00029     while(1) {
00030 
00031 #if 0    
00032         i2c.start();
00033         i2c.write((id<<1) | 1);
00034         int b1 = i2c.read(1);
00035         int b2 = i2c.read(1);
00036         int t1 = i2c.read(1);
00037         int t2 = i2c.read(0);
00038         i2c.stop();
00039         printf("%02X %02X  %02X %02X\n\r", b1, b2, t1, t2);
00040 #else
00041         unsigned char data[4];
00042         i2c.read((id<<1)|1, (char*)data, sizeof(data));
00043         //printf("%02X %02X  %02X %02X\n\r", data[0], data[1], data[2], data[3]);
00044         
00045         int st = data[0]>>6; // status
00046         int bd = ((data[0] & 0x3f) << 8) | data[1]; // bridge data
00047         int td = ((data[2]<<8) | data[3]) >> 5; // temp data
00048         
00049         // P: 14 bits
00050         // Cmin=2^14-1*.1 Cmax=2^14-1*.9
00051         // Pmin=0 Pmax=15
00052         float p = (((float)bd-1638)*(15-0)/(14745-1638))+0; // convert raw data to psi
00053         p = p * 68.9475729; // convert psi to hPa
00054         
00055         // T: 11 bits (Cmax=2047=2^11-1)
00056         float t = (((float)td*200)/2047)-50; // convert raw data to deg C
00057         printf("%d: %.2f hPa  %.2f C\n\r", st, p, t);
00058         
00059         wait(1);
00060 #endif    
00061 
00062         myled = 1;
00063         wait(0.2);
00064         myled = 0;
00065         wait(0.2);
00066         
00067     } // while
00068     
00069 } // main
00070 
00071 
00072 // http://en.wikipedia.org/wiki/Barometric_formula
00073 const float layers[][4] = {
00074 //  Height[m], Pressure[Pa], Temp[K], Lapse[K/m], Subscript
00075         0,     101325.0,     288.15,  -0.0065,    // 0
00076     11000,      22632.1,     216.65,   0.0,       // 1
00077     20000,       5474.89,    216.65,   0.001,     // 2
00078     32000,        868.019,   228.65,   0.0028,    // 3
00079     47000,        110.906,   270.65,   0.0,       // 4
00080     51000,         66.9389,  270.65,  -0.0028,    // 5
00081     71000,          3.95642, 214.65,  -0.002      // 6
00082 };
00083 
00084 enum { L_HEIGHT=0, L_PRESS, L_TEMP, L_LAPSE };
00085 
00086 const float g = 9.80665;
00087 const float M = 0.0289644;
00088 const float R = 8.31432;
00089 const float Ta = -273.15; // Absolute zero [deg C]
00090 
00091 // Search for subscript which matches pressure range
00092 // Choose appropriate formula and plug in values
00093 // Lookup derived h in table and check subscript is still same, otherwise recalculate
00094 // Add height correction
00095 
00096 // Pass in pressure and temperature from sensor
00097 // Return height[m]
00098 float pressureToHeight(float p, float t) {
00099     float h = -999; // error
00100     int b;  
00101     int tries;
00102 
00103     // Search backwards thru layers for our pressure
00104     for(b = 6; b >= 0; --b) {
00105         if(p <= layers[b][1]) break;
00106     }
00107     if(b < 0) b = 0; // adjust so can have -ve heights
00108     int b1 = b;
00109     
00110     // Only retry limited times
00111     for(tries = 10; tries; --tries) {
00112           
00113         float Hb = layers[b][0];
00114         float Pb = layers[b][1];
00115         float Tb = layers[b][2]; //??? Tb = t - Ta;
00116         float Lb = layers[b][3];
00117         
00118         if(Lb != 0.0) 
00119             //h = (log(g*M/R/Lb)/log(p/Pb)*Tb - Tb)/Lb + Hb; // Equation 1 (non-zero lapse rate)
00120             h = (Tb/exp(R*Lb/g/M*log(p/Pb))-Tb)/Lb + Hb;
00121         else 
00122             h = -log(p/Pb)*R*Tb/g/M + Hb; // Equation 2 (zero lapse rate)
00123         
00124         // Search again for derived height
00125         for(b = 6; b >= 0; --b) {
00126             if(h <= layers[b][0]) break;
00127         }
00128     
00129         if(b == b1) break; // gave same subscript, so we can stop
00130         b1 = b;
00131         
00132     } // for - tries    
00133     
00134     if(tries <= 0) h = -999; // error
00135 
00136         
00137     return h;    
00138 
00139 } // pressureToHeight
00140 
00141 /*end*/
00142