add hw_systick example

Author: hathach

libraries/Bluefruit52Lib/examples/Hardware/hw_systick/hw_systick.ino

@@ -0,0 +1,47 @@
+/*********************************************************************
+ This is an example for our nRF52 based Bluefruit LE modules
+
+ Pick one up today in the adafruit shop!
+
+ Adafruit invests time and resources providing this open source code,
+ please support Adafruit and open-source hardware by purchasing
+ products from Adafruit!
+
+ MIT license, check LICENSE for more information
+ All text above, and the splash screen below must be included in
+ any redistribution
+*********************************************************************/
+#include <Arduino.h>
+
+// Note: Extern "C" is required since all the IRQ hardware handler is
+// declared as "C function" within the startup (assembly) file.
+// Without it, our SysTick_Handler will be declared as "C++ function"
+// which is not the same as the "C function" in startup even it has
+// the same name.
+extern "C"
+{
+
+/* This is hardware interupt service function exectuing in non-RTOS thread
+ * Function implementation should be quick and short if possible.
+ * 
+ * WARNING: This function MUST NOT call any blocking FreeRTOS API 
+ * such as delay(), xSemaphoreTake() etc ... for more information
+ * http://www.freertos.org/a00016.html
+ */
+void SysTick_Handler(void)
+{
+  ledToggle(LED_BLUE);
+}
+
+} // extern C
+
+void setup()
+{
+  // Set up systick to fire 10 times per second
+  SysTick_Config(F_CPU/10);
+}
+
+void loop()
+{
+  // do nothing here
+}