You can try TPS3824-5 (for 5V Power supply). This is processor supervisory circuit with watch dog timer in SOT-23 package. Watch Dog is better than in P89. I have used them in my last design. I have P89C51RB2HBBD powered with SPS (on the other side of insulated barrier)in my current design and works well. Reset circuit is standard 8k2 resistor to the +5V and 10uF capacitor to the ground.
Reset mistakes was in the first couple series of 89C51RD2. Please contact philips.
G.