Add busy_wait_at_leasy_cycles to host (#1925)

Author: kilograham

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

@@ -148,7 +148,11 @@ uint get_core_num();
 
 static inline uint __get_current_exception(void) {
     return 0;
+
 }
+
+void busy_wait_at_least_cycles(uint32_t minimum_cycles);
+
 #ifdef __cplusplus
 }
 #endif

src/host/pico_platform/platform_base.c

@@ -7,9 +7,9 @@
 #include <stdio.h>
 
 #include "pico.h"
+#include "hardware/timer.h"
 
 PICO_WEAK_FUNCTION_DEF(tight_loop_contents)
-
 void PICO_WEAK_FUNCTION_IMPL_NAME(tight_loop_contents)() {
 
 }
@@ -45,4 +45,10 @@ void __breakpoint() {
     #else
         __builtin_trap();
     #endif
-}
+}
+
+PICO_WEAK_FUNCTION_DEF(busy_wait_at_least_cycles)
+void PICO_WEAK_FUNCTION_IMPL_NAME(busy_wait_at_least_cycles)(uint32_t cycles) {
+    // fairly arbitrary; we'll use 125Mhz as a reference
+    busy_wait_us((cycles + 124)/125);
+}