Muhamad:
Can you please tell us if it is the schmitt-trigger-buffer that gets hot OR is it the AT89C2051 processor that gets hot?

It would also help if you could show us the circuit that you used for the buffer chip.

I also recommend that you purge the RC circuit with the schmitt-trigger-buffer and use a reset generator chip. A very good choice for your application would be the MCP101 series chip from Microchip. These have active high outputs and they come in very small 3 pin packages. You would need to select the part from the series that monitors at the proper voltage for your processor. These parts are also a very economical.

Michael Karas