I ordered some Feather nRF express boards. I use the board with an ultrasonic sensor (HC SR-04) for continuos distance measuring (this is only one task in a more complexe project). The sensor works fine with the pulseIn() function. To avoid the timeblocking, caused by pulseIn() I wanted to substitute the pulseIn by reading the sensors result via an ISR. The ISR shall generate a timestamp when the the "echo-Pin" - pulse starts and one when it ends. The difference of both is the required pulse-duration, related to the distance. I kept the ISR very small to avoid timer overflow problems. I am aware that using micros() in ISR can become problematic.
After a longer investigation I came to a curious result (not a solution...): The measured duration jumps in steps of 976 microseconds. When cross-checking by continous scanning the echo-Pin without ISR, suddenly also the ISR worked properly. Even, if I comment out the recording of the micros, it works, as long there is a continous scan of the echo-Pin. As soon I remove the echo-pin scanning, the 976 us steps return. Crazy! There might be a bug in the core (wiring.h / .c or somewhere else). Here my test-sketch for this issue:
Code: Select all
#include <Wire.h>
#define trigPin 5
#define echoPin 6
static volatile unsigned long pulseStart, pulseStop;
static volatile boolean pinHL;
void setup() {
pinMode(trigPin, OUTPUT); // Sets the trigPin as an OUTPUT
pinMode(echoPin, INPUT); // Sets the echoPin as an INPUT
dwt_enable();
Serial.begin(115200);
attachInterrupt(digitalPinToInterrupt(echoPin), ultrasonic_interrupt, CHANGE);
}
void ultrasonic_interrupt(){
pinHL ? pulseStop = micros() : pulseStart = micros(); // detect falling or rising edge ->
// start or stop of the pulse;
// pinHL state is before the interrupt;
pinHL = !pinHL;
}
void loop() {
unsigned long delta;
pinHL = LOW;
digitalWrite(trigPin, HIGH);
delayMicroseconds(200);
digitalWrite(trigPin, LOW); // Pulse to start the measurement of the sensor
while(!digitalRead(echoPin)){;}
// pulsStart = micros();
while(digitalRead(echoPin)){;}
// pulsStop = micros();
delay(20);
delta = pulseStop - pulseStart;
Serial.print(" start: ");
Serial.print(pulseStart);
Serial.print(" stop: ");
Serial.print(pulseStop);
Serial.print(" delta: ");
Serial.println(delta);
}