main.cpp

00001 #include "mbed.h"
00002 
00003 // DS18B20 converted to run on mbed
00004 
00005 #define TEMP_PIN  p20
00006 
00007 Serial pc(USBTX, USBRX); // tx, rx
00008 
00009 DigitalInOut temperature_pin(TEMP_PIN);
00010 
00011 void OneWireReset() { // reset.  Should improve to act as a presence pulse
00012     temperature_pin.output();
00013     temperature_pin = 0;     // bring low for 500 us
00014     wait_us(500);
00015     temperature_pin.input();
00016     wait_us(500);
00017 }
00018 
00019 void OneWireOutByte(unsigned char d) { // output byte d (least sig bit first).
00020     for(int n=8; n!=0; n--) {
00021         if ((d & 0x01) == 1) { // test least sig bit
00022             temperature_pin.output();
00023             temperature_pin = 0;
00024             wait_us(5);
00025             temperature_pin.input();
00026             wait_us(60);
00027         } else {
00028             temperature_pin.output();
00029             temperature_pin = 0;
00030             wait_us(60);
00031             temperature_pin.input();
00032         }
00033 
00034         d=d>>1; // now the next bit is in the least sig bit position.
00035     }
00036 
00037 }
00038 
00039 unsigned char OneWireInByte() { // read byte, least sig byte first
00040     unsigned char d = 0, b;
00041     for (int n=0; n<8; n++) {
00042         temperature_pin.output();
00043         temperature_pin = 0;
00044         wait_us(5);
00045         temperature_pin.input();
00046         wait_us(5);
00047         b = temperature_pin;
00048         wait_us(50);
00049         d = (d >> 1) | (b << 7); // shift d to right and insert b in most sig bit position
00050     }
00051     return d;
00052 }
00053 
00054 int main() {
00055     int HighByte, LowByte, TReading, SignBit, Tc_100, Whole, Fract;
00056     pc.printf("mbed 1 wire interface simple test!\r\n");
00057     temperature_pin.output();
00058     temperature_pin = 0;
00059     temperature_pin.input();  // sets the digital pin as input (logic 1) make sure external pullup resistor 4K7
00060     wait(1);
00061     pc.printf("temperature measurements:\r\n");
00062 
00063     while (1) {
00064         OneWireReset();
00065         OneWireOutByte( 0xcc);  //Skip ROM command
00066         OneWireOutByte( 0x44); // perform temperature conversion, strong pullup for one sec
00067 
00068         OneWireReset();
00069         OneWireOutByte( 0xcc);
00070         OneWireOutByte( 0xbe);   //Read Scratchpad
00071 
00072         LowByte = OneWireInByte();
00073         HighByte = OneWireInByte();
00074         TReading = (HighByte << 8) + LowByte;
00075         SignBit = TReading & 0x8000;  // test most sig bit
00076         if (SignBit) { // negative
00077             TReading = (TReading ^ 0xffff) + 1; // 2's comp
00078         }
00079         Tc_100 = (6 * TReading) + TReading / 4;    // multiply by (100 * 0.0625) or 6.25
00080 
00081         Whole = Tc_100 / 100;  // separate off the whole and fractional portions
00082         Fract = Tc_100 % 100;
00083 
00084         if (SignBit) { // If its negative
00085             pc.printf("-");
00086         }
00087         pc.printf("%d", Whole);
00088         pc.printf(".");
00089         if (Fract < 10) {
00090             pc.printf("0");
00091         }
00092         pc.printf("%d", Fract);
00093         pc.printf("\r\n");
00094        
00095         wait(5);         // 5 second delay.  Adjust as necessary
00096     }
00097 }