ClockP_usleep(usec) for FreeRTOS calculates the delay passed to vTaskDelay(), using portTICK_PERIOD_MS. This macro is defined in FreeRTOS/Source/portable///portmacro.h as: #define portTICK_PERIOD_MS ( ( TickType_t ) 1000 / configTICK_RATE_HZ ) If configTICK_RATE_HZ were ever set to a value > 1000, this would be 0. It is probably also better to use defines that are in FreeRTOSConfig.h. Instead, we could calculate the delay for ClockP_usleep(usec) as: tickPeriouUs = 1000000 / configTICK_RATE_HZ; /* Tick period in microseconds */ delay = (usec + tickPeriodUs - 1) / tickPeriodUs; /* Take the ceiling */