| ??? 02/26/08 08:34 Modified: 02/26/08 08:37 Read: times Msg Score: -1 -1 Message Not Useful |
#151472 - Page 4 datasheet says Responding to: ???'s previous message |
Brian Reid said:
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) This link already indicated a lot. http://www.8052.com/forum/read.phtml?id=151286 If you see datasheet page 4 , It is stated : "Logic 1-0 Transition Current (Port 2, 3) " you may calculate according to this , use Ohms law. If you use a 5V system it requires 400uA Typ for the port to see logic 1-0 transition . Where as your 100K resistance is only providing 50uA . Now for 400uA to see you require 12500E resistance. AP |
