The voltage divider between the weak pullup and the pulldown resistor is clear. In my previous posts I clarified that the resistance value of the pulldown has to be low enough to ensure the input is below V_IL.

To provide specific information for finding a suitable resistance for a pulldown I provide the following. Consider for example the following data sheet excerpt for a Philips miccrocontroller.


For this specific microcontroller the V_IL voltage is in range of 0.8V to 1.0V over the normal operating V_CC range. If one determines that 0.4V is a sufficient noise margin on inputs to the processor that are at the low level then the inputs should be held at V_IL-0.4V = 0.8V - 0.4V = 0.4V. The data sheet also specifies that the weak pullups on P1, P2 and P3 source a current that is a maximum 75μA at a V_IN of 0.4V. This will require a R_pull-down=0.4V/75μA = 5.33K. This means that a nominal 4.7K ohm resistor should be a maximum resistance pulldown value. In a realistic design with this specific processor I would recommend a value on the order of 2.2K.

With a 2.2K resistor as a pulldown then an external circuit would need to source current of about 2.3mA to drive the input to 5 volts.

Hope this helps those unfamiliar with proper pulldown resistor selection. It is important to use the data sheet for your specific processor.

Michael Karas

PS...Here's for you Erik....