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???
08/01/06 17:12
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#121505 - Some encoders "see" pretty high impedances
Responding to: ???'s previous message
The "classic" (74C922/923) keypad encoders (not necessarily a "good buy," BTW) recognize a 50K-ohm impedance as a switch closure. They don't use any external resistors and simply drive the rows and sense the columns. This kind of encoding can be done with a "johnson counter" e.g. CD4017, and a 74xx148 priority encoder. That's essentially what the firmware attempts to do when one's using microcontroller.

There's a penalty for using low-impedance push-pull configuration of the port in question. In the simplest case, it's that one has to provide current-limiting in the form of series resistors. That requires choices, e.g. "do they go on the high side or the low side?" That requires study of the port behavior. When using other families of MCU's, one can configure a port to be either input or output. That sometimes solves the problem. In this family, one has to know whether that particular MCU at that particular port, is capable of open-drain or weak-pullup operation.

That's why one has to READ the specifications before proceeding.

RE


List of 41 messages in thread
TopicAuthorDate
connecting a keypad . HOW?            01/01/70 00:00      
   Because...            01/01/70 00:00      
      How many times will this come up?            01/01/70 00:00      
         To be fair            01/01/70 00:00      
         Help him!            01/01/70 00:00      
            OK ... but just this once!            01/01/70 00:00      
   it's quite simple ... therein lies the beauty.            01/01/70 00:00      
      In minute detail            01/01/70 00:00      
         Credit goes to original author Michael Karas            01/01/70 00:00      
            Yes - Credit to Michael Karas            01/01/70 00:00      
      Diodes prevent \"ghosting\"            01/01/70 00:00      
         indeed, but rarely needed            01/01/70 00:00      
            Think of a piano, where each key counts...            01/01/70 00:00      
               How often do you intend to use multiple keys?            01/01/70 00:00      
                  There is the case of...            01/01/70 00:00      
                     True, but how often?            01/01/70 00:00      
                        Agreed            01/01/70 00:00      
                        general input            01/01/70 00:00      
                  I agree, but...            01/01/70 00:00      
      a small catch            01/01/70 00:00      
         In this case, the pullups were external            01/01/70 00:00      
            HUH????            01/01/70 00:00      
               Sorry, I should have said you must not ...            01/01/70 00:00      
                  just visualize            01/01/70 00:00      
                     It happens all the time ...            01/01/70 00:00      
                        current limits            01/01/70 00:00      
                           transistors open won't conduct            01/01/70 00:00      
                              Totally unsuited with push/pulls, unless...            01/01/70 00:00      
                                 This is true, which is why one should READ first            01/01/70 00:00      
                                    there is no reason, but 'linear thinking' (outputs            01/01/70 00:00      
                                       Some encoders "see" pretty high impedances            01/01/70 00:00      
                                          cruel and unusual            01/01/70 00:00      
                                       No!            01/01/70 00:00      
                                          OK, rephrase            01/01/70 00:00      
                                             Ah, sorry!            01/01/70 00:00      
                                    Benefit of additional pull-ups            01/01/70 00:00      
   Disappearing Original Post!            01/01/70 00:00      
      that is not all that is missing!            01/01/70 00:00      
         Bug            01/01/70 00:00      
            Bugs            01/01/70 00:00      
               OOps            01/01/70 00:00      

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