Add busy_wait_at_least_cycles method (raspberrypi#830)

Author: kilograham

src/rp2_common/pico_platform/include/pico/platform.h

@@ -400,6 +400,29 @@ uint __get_current_exception(void);
 }
 #endif
 
+/*! \brief Helper method to busy-wait for at least the given number of cycles
+ *  \ingroup pico_platform
+ *
+ * This method is useful for introducing very short delays.
+ *
+ * This method busy-waits in a tight loop for the give number of system clock cycles. The total wait time is only accurate to within 2 cycles,
+ * and this method uses a loop counter rather than a hardware timer, so the method will always take longer than expected if an
+ * interrupt is handled on the calling core when during the busy-wait; you can of course disable interrupts to prevent this.
+ *
+ * You can use \ref clock_get_hz(clk_sys) to determine the number of clock cycles per second if you want to convert to cycles
+ * from an actual time duration.
+ *
+ * \param minimum_cycles the minimum number of system clock cycles to delay for
+ */
+static inline void busy_wait_at_least_cycles(uint32_t minimum_cycles) {
+    __asm volatile (
+        ".syntax unified\n"
+        "1: subs %0, #3\n"
+        "bcs 1b\n"
+        : "+r" (minimum_cycles) : : "memory"
+    );
+}
+
 #else // __ASSEMBLER__
 
 #define WRAPPER_FUNC_NAME(x) __wrap_##x