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Bare Metal Programming Tool Kit
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Bare Metal Programming Tool Kit
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interface to an SR04 ultrasonic distance sensor More...
#include <sr04.h>
Static Public Member Functions | |
| static void | init () |
| constructor, supply the trigger and echo pins | |
| static bmptk::time | pulse_width_get (bmptk::time error_value=-1 *bmptk::us) |
| measure and return the echo pulse witdh | |
| static int | distance_mm_get (int error_value=-1, int speed_of_sound=3430) |
| measure and return the distance to a detected object | |
interface to an SR04 ultrasonic distance sensor
This sensor can be triggered to output a short ultrasonic sound burst, and detect the reflection of this burst from a nearby solid object. The datasheet claims that it can detect objects at distances of 2 .. 400 cm. My estimate is that 4 .. 100 cm is more realistic.
The sensor needs a 5V supply (it won't work a 3.3 V), but its trigger input will trigger on a 3.3V pulse.
The sensor's echo output will produce a pulse that starts ~ 500 us after the trigger pulse, and ends when the echo is received. Hence the width of the pulse is proportional to the distance of the object.
The get_pulse_length() method (and hence the get_distance() method) uses busy waiting (in a tight loop, not by calling bmptk wait function) to generate the trigger pulse and to measure the echo pulse. When the pulse does not start within 1 ms after the trigger (expected time is 500 us), or when the echo pulse is longer than 30 ms (corresponding to > 5 m) an error is returned.
To avoid problems with late reflexions of a pulse influencing a next measurement it is advised to observe a time interval of at least 60 ms between calls to the get_pulse_length() or get_distance() methods.
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inlinestatic |
measure and return the distance to a detected object
This method needs the speed of sound to calculate the distance from the echo pulse length it gets from get_pulse_width(). This speed must be supplied as units of distance per microsecond. The returned result is expressed in the same unit of distance. The default speed_of_sound parameter value is 3430 mm/us, hence the default unit of the returned value is mm.
This method uses busy waiting. It can take up to 32 ms. When no echo pulse is seen or it is too long the error_value is returned.
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inlinestatic |
measure and return the echo pulse witdh
The unit is the bmptk timing unit, so to convert for instance to microseconds divide the result by bmptk::us.
This method uses busy waiting. It can take up to 32 ms. When no echo pulse is seen or it is too long the error_value is returned.
1.8.2
