How to decrease fixed step size in Simulink for STM32F4 ?

Hi, I'm using embedded coder support package for STM32F4. When my Simulink model fixed sample time is above 1e-4, my controller works perfectly fine. When I decrease my fixed sample time, say 1e-6, Simulink successfully generates the .hex file. But when I burn that .hex file to the controller, GPIOs don't give any output. I've also tried to change GPIOs speed but it didn't help. Can anyone please help so that I can generate a pulse of 1MHz using my embedded coder support package for STM32F4. Anxiously waiting for a reply.

Regards