DRV8711 stall not detected

I've got a BOOST-DRV8711 dev board (http://www.ti.com/tool/boost-drv8711) to have some fun with the DRV8711 IC.

I'm interested in stall detection, to detect when the motor looses steps. Using a general purpose MCU I wired the SPI and the STEP/DIR pins. I'm able to read/write the internal registers and to move the motor.

I read a lot of other threads and the relevant docs. I'm aware I have to find the correct values by experiments, they depend upon motors, voltage, speed, I cannot use full-step, etc....

To try this feature I set these parameters:

• torque = 128, smplth = 200 us
• microstepping = 1/16
• sdthr = 0xFF, sdcnt = 1 step, vdiv = / 32
• motor voltage: 12 V

All other settings are left on their default value. This is the motor I'm using (with no load):

http://www.applied-motion.com/sites/default/files/HT17-268_RevC.pdf

I set the motor to run @ 5000 step/s and I clamped the axis, but nothing happened (I was expecting the stall led turns on). Because the stall condition is met when the BEMF drops below the sdthr value I reduced both torque and speed settings - with no luck.

I'm using the most sensitive settings to detect a stall condition (maximum value for sdthr and lowest value for vdiv).
Anyway I tried to set different values to that register with no luck.

Perhaps I miserunderstood the datasheet?