GPIO interrupt on rising edge

Hi,

I wanted to count the incoming frequency on a GPIO pin (the input fre is around 50-150kHz)
I plan to use GPIO interrupt on rising edge and count the systick timer to get the frequency value.
However I am not able to get correct value from systick timer, and I am not sure if I am setting up GPIO interrupts correctly

Can someone help me debug this issue

somefun()
{

//start systick timer
systick_usec_units(false);
systick_stop();
systick_start(90000, false);
//enable GPIO
GPIO_ResetIRQ(GPIO1_IRQn);
GPIO_EnableIRQ(GPIO_PORT_1,GPIO_PIN_1,GPIO1_IRQn,false,true,0); //want to trigger on rising edge signal
GPIO_RegisterCallback(GPIO1_IRQn,app_btn_callback();
...
}

void app_btn_callback(void)
{
if(firstEdge == 0)
{
firstEdge=1;
freqCount=SysTick->VAL;
}
else
{
freqCount=freqCount-SysTick->VAL;
systick_stop();
NVIC_DisableIRQ(GPIO1_IRQn);
}

NVIC_ClearPendingIRQ(GPIO1_IRQn);
}