lf clock config prove of concept

Author: nvlsianpu

hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF51822/system_nrf51.c

@@ -104,8 +104,32 @@ void SystemInit(void)
         *(uint32_t volatile *)0x4006EC14 = 0xC0;
     }
 
+    
+    #define NRF_LF_SRC_XTAL  2 
+    #define NRF_LF_SRC_SYNTH 3 
+    #define NRF_LF_SRC_RC    4
+    
+        //MBED_CONF_APP_NORDIC_NRF_LF_CLOCK_SRC
+    #if MBED_CONF_APP_NORDIC_NRF_LF_CLOCK_SRC == NRF_LF_SRC_SYNTH
+        #define CLOCK_LFCLKSRC_SRC_TO_USE (CLOCK_LFCLKSRC_SRC_Synth)
+        #warning synhetic clock
+    #elif MBED_CONF_APP_NORDIC_NRF_LF_CLOCK_SRC == NRF_LF_SRC_XTAL
+        #define CLOCK_LFCLKSRC_SRC_TO_USE (CLOCK_LFCLKSRC_SRC_Xtal)
+        #warning crystal clock
+    #elif MBED_CONF_APP_NORDIC_NRF_LF_CLOCK_SRC == NRF_LF_SRC_RC // and undefined
+        #define CLOCK_LFCLKSRC_SRC_TO_USE (CLOCK_LFCLKSRC_SRC_RC)
+        #warning RC clock
+    #else
+        #error Bad LFCLK configuration. Declare proper source through mbed configuration.
+    #endif
+    
+    #undef NRF_LF_SRC_XTAL
+    #undef NRF_LF_SRC_SYNTH
+    #undef NRF_LF_SRC_RC
+    
     // Start the external 32khz crystal oscillator.
-    NRF_CLOCK->LFCLKSRC             = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
+    //NRF_CLOCK->LFCLKSRC             = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
+    NRF_CLOCK->LFCLKSRC             = (CLOCK_LFCLKSRC_SRC_TO_USE << CLOCK_LFCLKSRC_SRC_Pos);
     NRF_CLOCK->EVENTS_LFCLKSTARTED  = 0;
     NRF_CLOCK->TASKS_LFCLKSTART     = 1;