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???
12/08/08 17:37
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 		 #160743 - reset vs supervisor
Responding to: ???'s previous message
I have to chime in with support for Erik's position.

Richard initially said said:
I've recently encountered one 805x circuit with a 1000 uF cap on its Vcc rail, that had unidentified problems which were initially called "reset" issues. By removing the 1000 uF electrolytic cap, I made the problem "disappear" though I'm not yet sure why.


Now we don't know any other details here, including possible impedances in series with the micro's VCC rail and whether the design used the really bad 8x51 data sheet RC reset. But if you have 1000 uF or more hanging on your VCC rail, you slow down the VCC rise time and it's possible that this slow ramp is baffling the micro's internal logic -- parts of it come out of reset earlier than others, other parts may not be completely reset, etc. Also RTFDS to see if it has any specific VCC rise-time recommendations.

Anyways, Erik's recommendation of using a reset supervisor for one of its intended purposes -- to ensure that the processor is held in reset until AFTER the power supply has ramped up fully and is stable -- is completely reasonable and should not have been dismissed out of hand!

I don't think the 1000 uF itself is bad, although it's likely overkill (it's not a kilowatt audio amplifier). I do think the micro did have "reset" issues which would be completely addressed by a proper supervisor.

-a



List of 26 messages in thread
TopicAuthorDate
Decoupling capacitance can kill the regulator?            01/01/70 00:00      
   Yes, LDOs are sensitive...            01/01/70 00:00      
   and that's not all ...            01/01/70 00:00      
      was there a supervisor?            01/01/70 00:00      
         No ...            01/01/70 00:00      
            you have yourself posted re ....            01/01/70 00:00      
               as often happens ...            01/01/70 00:00      
                  who 'injected'?            01/01/70 00:00      
                     It's on you, Erik!            01/01/70 00:00      
                        what was 'injected'            01/01/70 00:00      
                           What I injected was potential failure modes ...            01/01/70 00:00      
                              not if Vdd drop below            01/01/70 00:00      
                                 This is not the place to argue that matter!            01/01/70 00:00      
                              reset vs supervisor            01/01/70 00:00      
                                 I think it is equally important that ...            01/01/70 00:00      
                                    Lethal ramp-down            01/01/70 00:00      
                                       a funny thing happened ...            01/01/70 00:00      
                                          Brownout interrupts can be nice            01/01/70 00:00      
                                 RESET is not germane to this thread            01/01/70 00:00      
      diode in reverse bias (from output to input)            01/01/70 00:00      
         The best way to pass the ce testing...            01/01/70 00:00      
      High frequency ripple            01/01/70 00:00      
         Use a LRC-lowpass-filter!            01/01/70 00:00      
            What kind of L in the filter?            01/01/70 00:00      
      Big caps at output were used in the past            01/01/70 00:00      
         They used large L in the input in the past, too            01/01/70 00:00      

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