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???
07/29/06 04:46
Modified:
  07/29/06 04:50
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 		 #121300 - Totally unsuited with push/pulls, unless...
Responding to: ???'s previous message
Richard said:
Whether N-channel or P-channel, these mosfets behave like resistors when "ON."

Which can be astonishingly low impedant with modern micros containing push/pull stages!

Richard said:
As they warm up, they become more resistive, not that this characteristic is, in any relevant way, important. If one turns the connection between row and column on only long enough to sample the state of the column signal, as shown in the subject schematic, and then turns the row off again, it won't be conducting significant current long enough to damage anything.

Even if there's no damage, then you still get enormous ground noise and ground bounce due to the temporary short-circuit, which is not at all good in a microcontroller circuit.

If the rows can only be driven by push/pulls, then the use of these ghosting preventing diodes is a must! Not to prevent damage, but to prevent huge ground noise and ground bounce, eroding the noise margins.

Kai

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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