The TMC2660 has an internal clock but it can be used with an external clock as well. This slide gives some general guidelines on how to select the clock source and frequency. If the internal clock is used, the CLK pin must be tied to Ground, it must not be left floating. If an external clock is used, the first rising edge on the CLK pin switches to external clock. It must be made sure that the external CLK is always present during operation. If it is missing, the driver should be disabled using ENN pin to not damage the MOSFET driver stage.

