Software
Downloadable Code Files.txt
This device uses TIM2 and TIM14 to generate pulses and measure the time of flight (TOF).
TIM2 is set up the the PWM mode and used to generate 8 pulses. In code we assume the minimum time we can measure is 700us because, like a bell, the transducer rings and dies out after the pulses stop (Figure 1).
Transducer 1 and 2
Tx
Rx
Ringing time
Object Seen
Figure 1
TIM14 is set up in input capture mode which allows the timer to run until the receiver send back pulses. When it does, using TIM14_IRQHandler(); the first stop pulse after 700us is captured and the time is recorded.
distance = (speed_of_sound_cm*(TIM14_us + 700us)/1000000)/2;
distance = (34300.0584*(EchoTime)/1000000)/2;
There is also some error handling where if the distance is larger than 200cm or less than 10cm the distance will be equal to -2 or -1. If there is no return signal for 50ms (1500cm) the code will rerun.
Object Seen, Record time of first pulse.
Ignore all pulses for 700us
Start Time