maybe this defaults to 16 MHz. there is no XTAL on the board
--- a/NUCLEO-F411RE/blinky.c Thu Feb 18 23:47:15 2016 +0000
+++ b/NUCLEO-F411RE/blinky.c Fri Feb 19 00:17:37 2016 +0000
@@ -23,6 +23,7 @@
* Blink NUCLEO-F411RE LED on PA5
*
* TOGGLE RATE IS 1138 KHz WITHOUT CLOCK SET UP
+ * (Internal 16 MHz factory-trimmed RC?)
*/
int