The TMC5130 normally will be operated with an external, precisely known clock, as most systems have a microcontroller which has a precise crystal oscillator. When no clock is available, e.g. in de-centralized drives controlled via the single wire UART interface, the internal clock may be sufficient. It can alternatively be calibrated upon power up, by measuring it from a master. If the internal clock is used, the CLK pin must be tied to ground, it must never be left floating, as the first pulse switches off the internal clock generator. 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 clock is always present during operation, if it is missing; the driver should be disabled using ENN pin to prevent the driver stage from remaining in a continuous “on” condition. A procedure in the datasheet describes how to calibrate the ramp generator settings for the internal clock, which uses a known ramp to measure the actual clock rate.