Unexplainable behavior of delayNanoseconds

[nodemand]

I'm trying to generate a precise low frequency short pulsed signal of 500Hz.
This works perfectly with delayMicroseconds, but as soon as I swap it out with delayNanoseconds and adjust the delay accordingly, to me unexplainable things start to happen. I would think the following code should generate a 500Hz signal, but my scope says different. 8.06kHz.
What is going on?
Thanx for any help!

const uint8_t pin = 5;
const uint32_t d = 1995000;

void setup() {
  pinMode(pin, OUTPUT);
}

void loop() {
  digitalWriteFast(pin, HIGH);
  delayNanoseconds(5000);
  digitalWriteFast(pin, LOW);

  delayNanoseconds(d);
}

 

[Angelo]

Why not use the harware timers, in PWM mode for the short pulse ?
It will be precise, independant of software, interrupts, ....

 

[jmarsh]

Your delay is too long, delayNanoseconds is only intended for small values. Since you don't actually need nanosecond resolution try delayMicroseconds() instead.

(You really need to use a timer module for exact precision, not relay on CPU cycle timing.)

 

[nodemand]

Thanx you guys so much!

 

[MarkT]

Looking at the definition of delayNanoseconds in core_pins.h it looks like it ought to work for an input of upto about 470000:

static inline void delayNanoseconds(uint32_t nsec)
{
    uint32_t begin = ARM_DWT_CYCCNT;
    uint32_t cycles =   ((F_CPU_ACTUAL>>16) * nsec) / (1000000000UL>>16);
    while (ARM_DWT_CYCCNT - begin < cycles) ; // wait
}