Macro define vs var data type

Hi,
I'm just curios on how this work.
Is this a general C/C++ process or C51?

In main.h, I have a:

#define MAX_TIME_OUT      3000         //This is beyond U8

In Worker.c, I have Timer 2 ISR:

#include "main.h"

U8 _temp_cntr;                          //unsigned char and not initialized

//T2 init here

void T2_ISR() interrupt using 0
{
   //some flag being set here

   _temp_cntr++;                        //Should this be back to 0 when reach 255?
   if(_temp_cntr == MAX_TIME_OUT)       //What will happen here?
   {
       _temp_cntr = 0;
   }
}

Hope somebody can enlighten me on this.
My UART1 on F387 mcu won't able to run when Timer 2 ISR is enabled because of the specific condition under the ISR.

thanks
gp

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