Hello Daniel,
You do not say whether you are using a DEMO board, or a board of your own design. If your own design, do you have adequate bypassing of the supply rail, with the high frequency capacitors and the larger tantalum capacitor, as recommended in the data sheet? Do you also have a 100nF capacitor connected between the reset pin and Vss, and an external pullup resistor to Vdd?
Is the motor driver device currently on the same board as the MCU? If so, the layout of ground tracks may be quite critical, as Peg has already suggested. You do not mention the current draw of the motor - do you have sufficient track width for all the tracks handling high current, and in particular the ground track between the power supply feed and the SENSE/GND pins of the motor driver. For example, assuming 0.5 oz PCB laminate, for a current capacity of 3.5 A, the track width would need to be at least 0.1" (2.5 mm). The high current tracks should be as short as possible, and kept well away from low current components, including the MCU.
Regards,
Mac