Hi all,

I just tested the inputs to my 8052 derivative chip(ADuC831) and I'm having some trouble I hope you can help with. I have a sinking output sensor connected to P3.5 that *should* generate a TTL compliant step from 5V to ~0.1V. However, I'm seeing a step from 5V to 4.82V at P3.5, resulting in the port never going low.

Looking at my diagram, I'm guessing it's because the series resistor I'm using is too large but I wanted to run it by the experts before I mess something up because I am new to all this after all. I chose the 100k ohm value for the series resistor to ensure I don't exceed the max sourcing current of 80 microamps just in case I write a zero to P3.5 by accident but perhaps that is being too cautious?

Looking at the sensor diagram, I had thought the 470 ohm pullup resistor would pull the port low when in the active state but I was obviously wrong.

Your help would be greatly appreciated. Please see below.

Brian

My P3.4 Input Diagram

MCU Datasheet (Digital input specs on p.4)