esp32s3_delay.c

Busy-wait timing, counted with the CPU cycle counter.

There is no timer interrupt and no scheduler here - these spin. All of them scale with the CPU frequency reported by cpu_mhz(), so they stay correct across a clock switch.

API

Declared in esp32s3_delay.h

Functions

static inline uint32_t cycle_count(void);

The cycle counter: increments once per CPU clock, wraps after 2^32.

returns: the current cycle count

void delay_cycles(uint32_t cycles);

Waits the given number of CPU cycles.

cycles: how many cycles to wait
returns: nothing

void delay_us(uint32_t us);

Waits the given number of microseconds.

us: how many microseconds to wait
returns: nothing

void delay_ms(uint32_t ms);

Waits the given number of milliseconds.

ms: how many milliseconds to wait
returns: nothing

Source

board/esp32s3_delay.c

26 lines

/*
 * Busy-wait timing, counted with the CPU cycle counter.
 */

#include "esp32s3_delay.h"
#include "esp32s3_clock.h"

void delay_cycles(uint32_t cycles)
{
    uint32_t start = cycle_count();
    while (cycle_count() - start < cycles) { }
}

void delay_us(uint32_t us)
{
    delay_cycles(us * cpu_mhz());
}

// Stepping 1 ms at a time keeps the cycle count well clear of overflow:
// at 240 MHz a single delay_cycles() call can only span ~17 s.
void delay_ms(uint32_t ms)
{
    while (ms--) {
        delay_us(1000);
    }
}

include/esp32s3_delay.h— the header, in full

include/esp32s3_delay.h

38 lines

/*
 * Busy-wait timing, counted with the CPU cycle counter.
 *
 * There is no timer interrupt and no scheduler here - these spin. All of them
 * scale with the CPU frequency reported by cpu_mhz(), so they stay correct
 * across a clock switch.
 */

#ifndef ESP32S3_DELAY_H
#define ESP32S3_DELAY_H

#include <stdint.h>

// The cycle counter: increments once per CPU clock, wraps after 2^32.
// returns: the current cycle count
static inline uint32_t cycle_count(void)
{
    uint32_t count;
    __asm__ __volatile__("rsr.ccount %0" : "=a"(count));
    return count;
}

// Waits the given number of CPU cycles.
// - cycles: how many cycles to wait
// returns: nothing
void delay_cycles(uint32_t cycles);

// Waits the given number of microseconds.
// - us: how many microseconds to wait
// returns: nothing
void delay_us(uint32_t us);

// Waits the given number of milliseconds.
// - ms: how many milliseconds to wait
// returns: nothing
void delay_ms(uint32_t ms);

#endif // ESP32S3_DELAY_H