duality of port 2 of 8051

[electroboy]

hi...

it is said that port 2 of 8051 has internal pull up in both functions (i.e when used as i/o or higher order address). but according to the figure given in manual, it is not possible for port 2 to have internal pull ups in both functions.




the control should be zero either during i/o or address operation. please help ... some websites show port 2 without that AND gate. which is correct?

also my question is, Should we use external pull up when using port 2 for higher order address ?

 

[ud23]

**broken link removed**

this is very good link to newbies

 

[electroboy]

thank you ud23... i saw the same website. we can see the difference in the following picture..



the AND gate which is in the manual is missing here... this creates a great difference in the operation of port. please clear my doubt.

 

[ud23]

no need of AND gate for port 2 its only needed in port 0 u can compare below two images you can see that for port 0 you must have use external pull ups
in port 2 case no need that so they don't have AND gate

 

[electroboy]

yes thank you... but what makes me to be still in confusion is that it is the lNTEL's manual that shows the presence of AND gate in port 2. Can u please clarify?

 

[ud23]

intel manuals have and gate because when you are using port 2 for addressing external ram you have to use it with port 0 so for that you addressing 16bit address for say 64k of ram then higher 8bit address is provided by port 2 so you need only addressing mode then you cant use port 2 as normal i/o port so AND gate is for mode selection.