unprecisse timing on SAMD21 (Arduino Zero) #18
Description
On an Arduino Zero (SAMD21) using this simple sketch shows 12 us or 13 us instead of 10 us:
#include "uTimerLib.h"
/* serial interface */
#define SERIAL_BAUD_RATE 115200 // baud = bits per second
#define SERIAL_TIMEOUT 1000 // timeout in 1e-6 seconds
#define LED_PIN 0
unsigned long microstime_old = 0;
unsigned long microstime_delta = 0;
void handler (void) {
digitalWrite(LED_PIN, !digitalRead(LED_PIN));
const unsigned long mymicros = micros();
microstime_delta = mymicros - microstime_old;
microstime_old = mymicros;
}
void setup() {
// set serial communication:
Serial.begin(SERIAL_BAUD_RATE);
Serial.setTimeout(SERIAL_TIMEOUT);
delay(1000);
pinMode(LED_PIN, OUTPUT);
TimerLib.setInterval_us(handler, 10);
}
void loop() {
const unsigned long mtime = millis();
Serial.print("mtime: ");
Serial.print(mtime);
Serial.print(" SystemCoreClock: ");
Serial.print(SystemCoreClock);
Serial.print(" microstime_delta: ");
Serial.println(microstime_delta);
delay(1000);
}
A more precise measurement with an oscilloscope shows there is an additional time of about 2.5 us (= 2.5e-6 s). So the handler function is called every 12.5 us instead of every 10 us.
The same is true for longer periods. Also for TimerLib.setInterval_us(handler, 100); I get about 102.5 us instead of 100 us.
Unfortunately I have to say that this is not a hardware problem. With my own trigger implementation I get the setting as measurement.
And further it is not clear for my why uTimerLib shows this behavior. Maybe you have an idea?