Hi, im building a simple alarm circuit using an AT89C55wd uC, and im using port 0 to as my data lines for my 16x2 LCD screen, do these connections need pull up resistors as there writing data to the lcd not reading data, if i do need them any suggestions on what size resistors???? Thanks!!!

 

Sam, you wanna use the requirements as shown on the datasheet for the chip. A floating input will cause headaches since they tend to move high over time and may cause erratic operation of you devices or system, Most chip manuf are recomending 10K, but you can experiment with that till you find a resistor that holds the pin high or low according to your device and needs. But you should include them to stabilize your device and insure it works as desired. I am stil a newcomer to uC's, but that is the first lesson I learned the hard way.
Bob

 

Normally if a port is configured as output you don't need pull-up resistors but in this case the data sheet states that port 0 is an open drain port so you will need to provide pull-up resistors for any pins that are used on the port.

 

Thanks for the fast reply lads!! iv scanned the data sheets for an answer but im still confused to what size...iv blown one chip already , what info am i looking for on the datasheet???

 

The size of the pull-up will depend on your application and how much current you want the pin to drive and the voltage drop that you can tolerate. Bear in mind however that the maximum current that any AT89C55wd port pin can sink is 10mA and the maximum total current for all port 0 pins is 26mA.