Port register not working

Question

I am trying to read my ultrasonic sensor. It sends a pulse in the trigger pin and the echo pin receives the pulse. Using pulseIn I can know exactly the time it took to the pulse go and back. But I need to use port register which is much faster and I need it to be fast in this specific case.

My code is very simple:

void setup() {
 Serial.begin(9600); 
 //pin 12 as output
 DDRB = B00010000;
 //pin 12 as low
 PORTB = B00000000;
}
void loop() {
 //pin 12 as high
 PORTB = B00010000;
 delayMicroseconds(10);
 //pin 12 as low
 PORTB = B00000000;
 long start_time = micros();
 //While reading high in pin 13 keep running the loop
 while (PINB & B00100000)) {
 }
 Serial.println(micros() - start_time);
 delay(1000);
} 

The problem is that it does not work, I mean, the while loop is not being processed so the serial is always printing a very small number. It doesnt matter if I move the ultrasonic sensor to distant walls the while loop still does not work. What am I doing wrong?

• Ignacio Vazquez-Abrams: How have you wired it up?
• Samul: VCC to 5V, GND to GND, trigger to 12 and echo to 13.
• Ignacio Vazquez-Abrams: And does echo follow the behavior described in your code (normally high, pulls low on receive)? Where is the datasheet for the sensor?
• Samul: It's the HC-SR04 ultrasonic sensor. With pulseIn it works perfectly, but using PINB it does not.

Answer 1

According to anything I could find, the Echo output is normally low and then goes high for a duration equal to the range. Your initial while condition is inverted, and you need to check for both edges.

unsigned long up, down;
// trigger pulse
while (!(PINB & _BV(PB5)))
 ; // wait for leading edge
up = millis();
while (PINB & _BV(PB5))
 ; // wait for trailing edge
down = millis();
Serial.println(down - up);

• Samul: Thank you so much for helping but there is a problem: in your code you putted PIND, but it should be PINB right? Another thing: when I try to copile your code I get error saying that PB5 was not declared. I changed it to only 5 ok? But it still does not work. Could you please check what am I doing wrong? I edited your question to add my code cause I couldnt add it here in the coments ok? Sorry for that.
• Gerben: Use PINB5 instead.
• Samul: still having problem. Look: when my sensor is doing nothing it registers LOW. When an ultrasonic pulse arrives it registers HIGH right? So why do I need 2 whiles? Couldnt I just start a timer when the trigger sends a pulse and stop the timer when sensor reads HIGH? I tried "while sensor is not high keep looping" but it does not work. What am I doing wrong? The HIGH/LOW logic of mine is wrong?
• Ignacio Vazquez-Abrams: The duration the Echo signal is held high is proportional to the distance detected.
• Samul: So you are saying that this image is wrong? forum.arduino.cc/…