Ages ago I modified my Nucleo F103RB board not using the internal oscillator but the external XTAL 72MHZ (there is a hack in the manual how to co use the crystal from the ST-Link section, see my manuals about the nucleo). This was done by myself to be compatible with all other boards in the past.
The interesting stuff:
It works with the new core out of the box. Even Serial give the correct baud, it looks like there is no mismatch with the timing.
Maybe this is exactly this, what Slammer wrote:
Slammer wrote:The Nucleo F103RB board uses 8MHz HSE EXT (External Input Clock) generated from the other MCU on board. It is possible to change a bit the clock initialization function to use both HSE sources, eg. to try first with HSE_EXT and if this fails to try again with HSE_XTAL, with this trick the same code will work in Nucleo and other F103 boards with external crystals.
The Nucleo L476RG is different story. The HSE_EXT connection is disconnected on the board, the MCU uses the new internal clock MSI (it is like HSI but much more stable and accurate) at 4 MHz.