Issue with GPIO Clock XTAL calibration method


I would like to use the GPIO Clock method for XTAL trim calibration.
The production test document says: “To configure GPIO0 to output the clock signal, write 0x3 to GPIO_MODE register Reg_0x05:00”. And i should observe a 31.25 MHz signal.

By using the following code (modifying the gpio example) on DWM300EVB:

if (dwt_initialise(DWT_DW_IDLE) == DWT_ERROR)
        test_run_info((unsigned char *)"INIT FAILED     ");
        while (1) { };
dwt_setgpiomode(GPIO0_BIT_MASK, 0x3);

I measure a very unstable clock on GPIO0 which vary from device to device.
For example on 1 device i measure 29.38MHz, on another 28.6MHz etc…
All the results are way off the 31.25MHz.

Is there any known issue with this calibration method or do I do something wrong?

Thank you