You can built it by using just a 555 astable circuit with one inverter at the output, pin 3 of 555. You can generate a trigger pulse after certain time period from internal code running within 8051, one pin can drive low the trigger pin (pin2) of 555, and the output will be contineously high and will be inverted by invertor as low, wheneven the 555 will not receive the trigger pulse, the output will be low after specified time (specified by RC network within 555) and invertor will make its output high which can reset the 8051.

Thanks

Rauf