General • System ticks not lasting the expected time (off by several orders of magnitude)

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#include <stdio.h>#include "pico/stdlib.h"// #include "pico/binary_info.h"// #include "hardware/spi.h"// #include "hardware/dma.h"#include "hardware/structs/systick.h"#define GET_SYSTICK systick_hw->cvr#define GET_SYSTICKMAX systick_hw->rvr// Maximum value of system tick.uint32_t max;// A function borrowed from another coder, specifically from:// https://github.com/WestfW/RPi-Pico-Hacks/blob/main/systick_delay/systick_delay.c//// I use it to check if I'm the only person whose code works wrong.// But this also seems to work wrong (runs too fast).void sysTickDelay (uint32_t count) __attribute__((noinline));void sysTickDelay (uint32_t count) {  int32_t start, end, now;  start = GET_SYSTICK;  end = start - count;  if (end > 0) {    while (1) {      now = GET_SYSTICK;      if ((now <= end) || (now > start))        return;    }  } else {    end += max;  // wrap the end time    while (1) {        now = GET_SYSTICK;        if ((now <= end) && (now > start))        return;    }  }}int main() {      // Enable USB output for logging.   stdio_usb_init();      // Ready the LED.   const uint LED_PIN = PICO_DEFAULT_LED_PIN;   gpio_init(LED_PIN);   gpio_set_dir(LED_PIN, GPIO_OUT);         // Configure to get unaltered system ticks.   systick_hw->csr = 0x5;   systick_hw->rvr = 0x00FFFFFF;      // Current system tick.   // Last time we got a system tick.   int32_t cur_tick = 0;   int32_t last_tick = 0;      // Difference in ticks between current and last.   // Cumulative tick counter (goes beyond 24 bit limit).   volatile int32_t diff = 0;   volatile int64_t cumul = 0;   // Current LED state.   volatile int8_t led_state = 0;      // How many ticks do we count before flipping LED state.      volatile int64_t ticks_to_wait = (int64_t) (125e9);   // volatile int64_t ticks_to_wait = (int64_t) (125e6);         // I try to achieve 1 state transion per second (1 blink per 2 seconds).   // At 125 M ticks per second the waiting period should be 125 M ticks.   // However when I use that, the LED blinks faster than my vision limit, or doesn't blink at all.   // I can see it blinking when I set the waiting period to 125 B ticks.   // That seems to contradict sense and available documentation.   // What's the trick?   // Report maximum permissible value of system tick.   int32_t max = GET_SYSTICKMAX;   printf("Max: %lu.\n",max);      // Show that you're alive and allow the coder to read the console for a second. :)   gpio_put(LED_PIN, 1);   sleep_ms(1000);   gpio_put(LED_PIN, 0);   sleep_ms(1000);   // Initialize, start looping.   last_tick = GET_SYSTICK;         while (true) {      cur_tick = GET_SYSTICK;            // NOTE: SYSTICK COUNTS DOWN!      // Invert it here to keep code more understandable.      cur_tick = max - cur_tick;      // If time seems to have run forward, compute time as usual.      // If time has jumped back, compute time until max + the usual.            if (cur_tick >= last_tick) {         diff = cur_tick - last_tick;      } else {         diff = (max - last_tick) + cur_tick;      }            // Accumulate differences      cumul += diff;            // If enough difference has accumulated      if (cumul > ticks_to_wait) {                  // Alter LED state         if (led_state == 0) {            led_state = 1;         } else {            led_state = 0;         }                  // Drive LED to new state         gpio_put(LED_PIN, led_state);                  // Optionally print report.         // Does not influence visually perceivable behaviour,         // but could influence finely measured stuff.         printf("%d %lld\n", led_state, cumul);         // Restart counting         cumul = 0;      }   }      // Uncomment this and comment the loop above   // to test using another programmer's function.      /*   while (true) {      gpio_put(LED_PIN, 1);      sysTickDelay(125e6);      gpio_put(LED_PIN, 0);      sysTickDelay(125e6);   }   */   }

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