Since mbed revision 82, an "automatic" clock configuration has been introduced in the Nucleo SDKs.

It works this way:

1) At startup, try to configure the system clock using an external clock (PLL + HSE_BYPASS used). The external clock can be provided by the MCO coming from the U2 device present on the upper part of the Nucleo board. To enable this mode you need to close SB16 and SB50, open SB54 and SB55 and remove R35 and R37. You can also provide your own the clock but it must be at 8 MHz otherwise the SDK code must be changed to adapt the different clock dividers and PLL setting.

2) If the first configuration does not work, then try to configure the system clock using an external xtal (PLL + HSE_ON used). The xtal must be equal to 8 MHz. If the xtal value is different the SDK code must be changed to adapt the different clock dividers and PLL setting.

3) If the second configuration does not work, then the system clock is configured using the internal clock (HSI).

For more informations please read the Nucleo board user manual : http://www.st.com/st-web-ui/static/active/en/resource/technical/document/user_manual/DM00105823.pdf