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Kai Klaas
02/22/06 08:00
Read: 520 times
Germany


 
#110494 - Pull-downs create problems
Responding to: Suresh R's previous message
Suresh said:
Can i know how, the source impedence could be maintained low, so that the port pin could fall to logic low state.

Assume you want to connect a simple switch to a port. Now you have at least two options:

1. Using the switch in combination with a pull-up resistor, while the switch connects the port line to ground when being activated.
In this case the source impedance seen by port when inputting logic low state is almost zero. No problems occur.

2. Using the switch in combination with a pull-down resistor, while the switch connects the port line to supply voltage of micro when being activated.
In this case the source impedance seen by port when inputting logic low state is identical to the pull-down resistance. And if this is bigger than about 3kOhm, from our example, then problems will occur!!

So, all interfacing techniques using rather high pull-downs should be avoided with the quasi-bidirectional topology.

Kai

List of 19 messages in thread
TopicAuthorDate
AT89s52 Port pin clarifications:      Suresh R      02/16/06 02:14      
   nice copy, where is the question      Erik Malund      02/16/06 06:30      
   output characteristics      Jan Waclawek      02/16/06 06:53      
      most confusing post      Jez Smith      02/16/06 07:34      
   Answers      Kai Klaas      02/16/06 10:35      
      Logic 1 to 0 transition      Suresh R      02/17/06 05:01      
         You haven't read the 'bible' chapter...      Kai Klaas      02/17/06 08:38      
            no details about transition from logic 1      Suresh R      02/18/06 03:27      
               other way 'round      Jan Waclawek      02/18/06 04:21      
               There ARE!!      Kai Klaas      02/18/06 06:54      
                  pFET2      Suresh R      02/19/06 07:29      
                     Weak and very weak pull-ups      Kai Klaas      02/20/06 07:19      
      maintaining low source impedence?      Suresh R      02/20/06 03:39      
         by making thje driver a sink............      Abhishek Singh      02/21/06 10:02      
         Pull-downs create problems      Kai Klaas      02/22/06 08:00      
            Maximum IOL per port pin: 10 mA ?      Suresh R      02/23/06 04:43      
               IOL      Kai Klaas      02/23/06 06:22      
                  IOL = 10mA or 1.6mA? in port 1      Suresh R      02/23/06 22:26      
                     Depends on load      Kai Klaas      02/25/06 07:40      

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