Hi all,

First off, let me say "Thank You" to all of you for your replies. Just reading through them all and working through your logic has really helped me get a better understanding of how to size resistors properly, how the 8051 ports operate, etc. It's a huge help and it's finally starting to make sense so thank you all!

To reply to a few concerns you had, the first thing I did was disconnect the sensor from the circuit and ensure it was still working properly. I confirmed that it was indeed giving me a step from 5V to ~0.1V so that's how I came to the conclusion that it was the series resistor causing the problem.

Being new to embedded systems, I had missed the -660uA maximum 1-0 transition current in the spec sheet for my MCU and after replacing the 100k ohm resistor with a 2.2k ohm resistor to ensure I can get the max of -660uA at the worst case of 2.0V, everything worked exactly as it should. I wrote a quick test program in assembly to blink an LED in time with the steps from the speed sensor and it's working great. I checked it with a scope also and I get a nice square wave at the port pin up to the maximum required frequency.

Like many of you said, I do believe it was the 1-0 transition current requirement that was causing the problem as there was just no way to get that kind of current out of the port with such a large series resistor. However, I decided not to completely eliminate the resistor "just in case." If something shorts out in the sensor, if I get a cut in one of the wires and a short to ground, etc. I certainly would like to have something between the MCU pin and ground to at least try to keep the chip safe. Maybe it's just me being paranoid(??) but I rather err on the side of caution.

Re. what this sensor is actually doing, it's actually sensing wheel speed for an anti-lock braking system I'm designing and building so there's no inductive noise from a large AC motor to worry about. Mechanically and electrically, I'm ready to test the entire system as a whole and now that all my I/O is working as it should, I'm very close to testing my control algorithm. I'm excited!

Thank you all again for your help and patience :-)
Brian