Datasheet

TMC2660 DATASHEET (Rev. 1.01 / 2013-AUG-01) 42

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13 System Clock

The clock is the timing reference for all functions. The internal system clock frequency for all

operations is nominally 15MHz. An external clock of 10MHz to 20MHz can be supplied for more exact

timing, especially when using coolStep and stallGuard2.

USING THE INTERNAL CLOCK FREQUENCY

To use the on-chip oscillator of the TMC2660, tie CLK to GND near the chip. The actual on-chip

oscillator clock frequency can be determined by measuring the delay time between the last step and

assertion of the STST (standstill) status bit, which is 2

clocks. There is some delay in reading the

STST bit through the SPI interface, but it is easily possible to measure the oscillator frequency within

1%. Chopper timing parameters can then be corrected using this measurement, because the oscillator

is relatively stable over a wide range of environmental temperatures.

In case well defined precise motor chopper operation are desired, it is supposed to work with an

external clock source.

USING THE EXTERNAL CLOCK FREQUENCY

An external clock frequency of up to 20MHz can be supplied. It is recommended to use an external

clock frequency between 10MHz and 16MHz for best performance. The external clock is enabled and

the on-chip oscillator is disabled with the first logic high driven on the CLK input.

Attention:

If the external clock is suspended or disabled after the internal oscillator has been disabled, the chip

will not operate. Be careful to switch off the power MOSFETs (by driving the ENN input high or setting

the TOFF parameter to 0) before switching off the clock, because otherwise the chopper would stop

and the motor current level could rise uncontrolled. If the short to GND detection is enabled, it stays

active even without clock.

13.1 Frequency Selection

A higher frequency allows faster step rates, faster SPI operation, and higher chopper frequencies. On

the other hand, it may cause more electromagnetic emission and more power dissipation in the

digital logic. Generally, a system clock frequency of 10MHz to 16MHz should be sufficient for most

applications, unless the motor is to operate at the highest velocities. If the application can tolerate

reduced motor velocity and increased chopper noise, a clock frequency of 4MHz to 10MHz should be

considered.