diff --git a/cores/esp8266/Arduino.h b/cores/esp8266/Arduino.h
index dbde309fb..f82b3aea9 100644
--- a/cores/esp8266/Arduino.h
+++ b/cores/esp8266/Arduino.h
@@ -205,18 +205,7 @@ void loop(void);
 
 void yield(void);
 
-#ifndef F_CPU
-// single function needed from SDK user_interface.h
-extern "C" uint8 system_get_cpu_freq(void);
-#endif
-
-void __optimistic_yield(uint32_t intvl_cycles);
-
-#if defined(F_CPU)
-#define optimistic_yield(interval_us) (__optimistic_yield(interval_us * clockCyclesPerMicrosecond()))
-#else
-#define optimistic_yield(interval_us) (__optimistic_yield(interval_us * getCpuFreqMHz()))
-#endif
+void optimistic_yield(uint32_t interval_us);
 
 #define _PORT_GPIO16    1
 #define digitalPinToPort(pin)       (((pin)==16)?(_PORT_GPIO16):(0))
diff --git a/cores/esp8266/core_esp8266_main.cpp b/cores/esp8266/core_esp8266_main.cpp
index c4be729fa..bbcfd29ef 100644
--- a/cores/esp8266/core_esp8266_main.cpp
+++ b/cores/esp8266/core_esp8266_main.cpp
@@ -124,7 +124,13 @@ extern "C" void __yield() {
 
 extern "C" void yield(void) __attribute__ ((weak, alias("__yield")));
 
-extern "C" void __optimistic_yield(uint32_t intvl_cycles) {
+extern "C" void optimistic_yield(uint32_t interval_us) {
+    const uint32_t intvl_cycles = interval_us *
+#if defined(F_CPU)
+        clockCyclesPerMicrosecond();
+#else
+        getCpuFreqMHz();
+#endif
     if ((ESP.getCycleCount() - s_cycles_since_yield_start) > intvl_cycles &&
         can_yield())
     {
@@ -132,17 +138,6 @@ extern "C" void __optimistic_yield(uint32_t intvl_cycles) {
     }
 }
 
-#undef optimistic_yield
-extern "C" void optimistic_yield(uint32_t interval_us) {
-    __optimistic_yield(interval_us *
-#if defined(F_CPU)
-        clockCyclesPerMicrosecond()
-#else
-        getCpuFreqMHz()
-#endif
-    );
-}
-
 // Replace ets_intr_(un)lock with nestable versions
 extern "C" void IRAM_ATTR ets_intr_lock() {
   if (ets_intr_lock_stack_ptr < ETS_INTR_LOCK_NEST_MAX)