As per ATMEL data sheet following are known :

Input High Voltage for Reset pin : Min = 0.7V and Max = Vcc + 0.5.

And the pull down resistor conneted to Reset pin internally is 10K Ohm minimum and 30K Ohm maximum.

Thus for a worst case design take the 10 K Ohm and this will sort of form a potential divider with YOUR external resistor. Assuming a 5V supply, I would guess a 10 K Ohm resistor would be enough - you would get 2.5V at Rsest and that would be OK.

PS : This is assuming that your ResetIC can source 5/20,000 Amps. Just check out this.


Raghu