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???
06/06/06 09:22
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#117840 - logic gain and other
Responding to: ???'s previous message
Abishek Bk said:
The Port 1 output buffers can sink/source four TTL inputs.
Does this mean that that though there are 8 pins in a port only 4 pins can be used as inputs/outputs at a time? I am planning to use all 8 pins of Port2 as output. Will that be a problem?

No. This is called logic gain, it means that when you use the port1 pin as output, you can connect 1-4 TTL-compatible inputs to it.

Abishek said:
Two:
When 1's are written to Port 1 pins they are pulled high by internal pullups and can be used as inputs.

In that case if I send a 1 to an output pin, it becomes an input pin. Can I still write a zero to that pin and keep using it as output?


This is a misleading interpretation.
Please have a look at the port structure (transistors) in the "bible" (Erik gave you the links elsewhere).
It simply means, that the controller can switch an internal transistor on or off, which pulls the line low. The "transistor(s)" pulling it high have a function of resistors (slightly more complicated but you don't need that for the basic description) of severak kOhm. The input is always active, you can read the status of the pin, just if the internal transistor pulls it low, you cannot pull it back to high from external circuit (actually you can but you will damage the internal transistor :-( ). If you switch the internal transistor off (writing "1" to the port by the software), you can put it externally to any level you wish, and if you leave it floating, the internal pullup ("weak transistor") will pull it to 1.

Jan Waclawek


List of 18 messages in thread
TopicAuthorDate
Help Understanding AT89C51 Datasheet            01/01/70 00:00      
   logic gain and other            01/01/70 00:00      
      Should I study more?            01/01/70 00:00      
         do you have any hardware knowledge            01/01/70 00:00      
            No hardware knowledge            01/01/70 00:00      
               what purpose?            01/01/70 00:00      
                  Purpose            01/01/70 00:00      
                     many get confused by this            01/01/70 00:00      
                        Got it !            01/01/70 00:00      
   per-pin, per-port, and whole-chip limits            01/01/70 00:00      
      strictly speaking...            01/01/70 00:00      
         Slight oversimplification            01/01/70 00:00      
            it used to be DC, now it is AC            01/01/70 00:00      
      "per pin"            01/01/70 00:00      
   Good post!            01/01/70 00:00      
      Excellent Community            01/01/70 00:00      
   have you read "the bible"            01/01/70 00:00      
      Yes, I have started            01/01/70 00:00      

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