PING.cpp

00001 #include "PING.h"
00002 #include "mbed.h"
00003 
00004 /*
00005  * Constructor
00006  */
00007 PING::PING(PinName trigger) : _trigger(trigger)
00008 {
00009    // Attach interrupts
00010    _ticker.attach(this, &PING::_startRange, 0.1);
00011 }
00012 
00013 void PING::_startRange()
00014 {
00015    // send a trigger pulse, 20uS long
00016    _trigger.output(); // set pin to output trigger pulse
00017    _trigger = 0; // ensure pin starts low
00018    wait_us(2); // wait two microseconds
00019    _trigger = 1; // send trigger pulse by setting pin high
00020    wait_us(5); // keep sending pulse for 5 microseconds
00021    _trigger = 0; // set pin low to stop pulse
00022 
00023    _trigger.input();
00024 
00025    // pulseIn
00026    while (!_trigger); // wait for high
00027    _timer.reset();
00028    _timer.start();
00029    while (_trigger); // wait for low
00030    _timer.stop();
00031    _dist = _timer.read_us(); //provides echo time in microseconds
00032 }
00033 
00034 // returns distance in meters
00035 // The speed of sound is 340 m/s or 58 microseconds per meter.
00036 // The ping travels out and back, so to find the distance of the
00037 // object we take half of the distance travelled.
00038 float PING::read(void)
00039 {
00040    // spin until there is a good value
00041    return (_dist/29/2);
00042 }
00043 
00044 // returns distance in inches
00045 // According to Parallax's datasheet for the PING))), there are
00046 // 73.746 microseconds per inch (i.e. sound travels at 1130 feet per
00047 // second).  This gives the distance travelled by the ping, outbound
00048 // and return, so we divide by 2 to get the distance of the obstacle.
00049 // See: http://www.parallax.com/dl/docs/prod/acc/28015-PING-v1.3.pdf
00050 float PING::read_in(void)
00051 {
00052    // spin until there is a good value
00053    return (_dist/74/2);
00054 }
00055 
00056 PING::operator float()
00057 {
00058    return read();
00059 }