main.cpp

00001 #include "mbed.h"
00002 
00003 Serial pc(USBTX,USBRX);
00004 
00005 DigitalInOut pingPin(p18);
00006 
00007 Timer tmr;
00008 
00009 long microsecondsToInches(long microseconds);
00010 long microsecondsToCentimeters(long microseconds);
00011 
00012 int main() {
00013     while (1) {
00014         // establish variables for duration of the ping,
00015         // and the distance result in inches and centimeters:
00016         long duration, inches, cm;
00017 
00018         // The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
00019         // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
00020         pingPin.output();
00021         pingPin = 0;
00022         wait_us(2);
00023         pingPin = 1;
00024         wait_us(5);
00025         pingPin = 0;
00026 
00027         // The same pin is used to read the signal from the PING))): a HIGH
00028         // pulse whose duration is the time (in microseconds) from the sending
00029         // of the ping to the reception of its echo off of an object.
00030         pingPin.input();
00031 
00032         // pulseIn
00033         while (!pingPin); // wait for high
00034         tmr.start();
00035         while (pingPin); // wait for low
00036         duration = tmr.read_us();
00037 
00038         // convert the time into a distance
00039         inches = microsecondsToInches(duration);
00040         cm = microsecondsToCentimeters(duration);
00041 
00042         pc.printf("in=%4d, cm=%4d\n", inches, cm);
00043         wait_ms(100);
00044     }
00045 }
00046 
00047 long microsecondsToInches(long microseconds) {
00048     // According to Parallax's datasheet for the PING))), there are
00049     // 73.746 microseconds per inch (i.e. sound travels at 1130 feet per
00050     // second).  This gives the distance travelled by the ping, outbound
00051     // and return, so we divide by 2 to get the distance of the obstacle.
00052     // See: http://www.parallax.com/dl/docs/prod/acc/28015-PING-v1.3.pdf
00053     return microseconds / 74 / 2;
00054 }
00055 
00056 long microsecondsToCentimeters(long microseconds) {
00057     // The speed of sound is 340 m/s or 29 microseconds per centimeter.
00058     // The ping travels out and back, so to find the distance of the
00059     // object we take half of the distance travelled.
00060     return microseconds / 29 / 2;
00061 }