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???
03/02/07 20:08
Modified:
  03/02/07 21:10
Read: times


 		 #134146 - reset (and diode)
Responding to: ???'s previous message
hi,

Richard Erlacher said:
What does, "no, even diode cannot help due positive reset - sorry." mean?


Well, this is simplest schematic of RC-reset. (This example shows negative reset to understand easy).
Save This Code
-----*------> Vcc
     |
    ---
    | |
    | |
    ---
     |
 (A) *------> /RST
     |
    ---
    ---
     |
     |
    ===

When power is off for long time then capacitor is empty, indeed.
Now, system is turned on. Vcc goes to nominal value (assume it is 5V) and /reset grows up slowly (capacitor takes current itself) so it does provide negative reset to reset pin. It is good - more or less.
Now system is turned off. Vcc falls down to zero but voltage on node A falls not so fast due high resistance of reset pin and current limit resistor from Vcc as well. All this does not allow capacitor to lost its energy fast. In some cases when Vcc is about, for example, 2V nevertheless /RST is still about 4 or more volts so no reset signal is provided in the whole. Internal parts of processor come unstable but reset is not generated yet. It is very bad.

Now imagine that system is turned on once again immediately during this period -- but capacitor is not empty yet! In this case even power-on situation does not generate reset by this RC circuit. How processor starts in this case I cannot say because it will not take reset state! Such situation may occur, for example, when power losts for short time.

I can agree that it may be not so dangerous in situation when power goes off (but it is the question of real device and application, sure). But as you can see such schematic may cause "no reset" state even at power-on process, what is not allowed in most cases, really!

So what is about diode. If we place diode (low-drop one, catode tied to Vcc) over resistor then it helps us to keep voltage on node A not more than +0.4..0.7V of Vcc (average, depend on diode type) during power-off process. At least it helps capacitor to lost its energy more quickly. But is it good practice? My answer - no, it is not.

Regards,
Oleg

P.S. Remember this note in instructions: "switch power off, wait for 10 seconds and then power on". Wait till capacitor... (=

List of 147 messages in thread
TopicAuthorDate
Pierce oscillator runs down to Vcc=1.3V!            01/01/70 00:00      
   you should get a probing set...            01/01/70 00:00      
      I keep the micro always reseted, ...            01/01/70 00:00      
      we already know            01/01/70 00:00      
   Low voltage operation            01/01/70 00:00      
      What puzzles me ...            01/01/70 00:00      
         True Reset            01/01/70 00:00      
            Thanks, Lynn, for giving us this deep insight!            01/01/70 00:00      
               Scary warnings            01/01/70 00:00      
               do you not know ...            01/01/70 00:00      
         What I know about this...            01/01/70 00:00      
            I didn't hear complaints about reset back then            01/01/70 00:00      
               MAY???            01/01/70 00:00      
                  Those were different times            01/01/70 00:00      
                     which troubles?            01/01/70 00:00      
                        I don\'t understand your question, Oleg.            01/01/70 00:00      
                           If I could do that, I would be a millionaire from            01/01/70 00:00      
                           There is an overwhelming evidence!            01/01/70 00:00      
                              Conjecture, not Evidence            01/01/70 00:00      
                                 already answered            01/01/70 00:00      
                                    HORSEFEATHERS! ... Read the question            01/01/70 00:00      
                                       Do you put in question, that...            01/01/70 00:00      
                                          Comments on Vdd and Flash            01/01/70 00:00      
                                             not quite true            01/01/70 00:00      
                                                I agree...            01/01/70 00:00      
                                                   by the way            01/01/70 00:00      
                                                      state machine decoding...            01/01/70 00:00      
                                          well Richard did not read it            01/01/70 00:00      
                                             No, I did read it ... in fact, I read it long ago            01/01/70 00:00      
                                                of course you are not            01/01/70 00:00      
                                          reset etc.            01/01/70 00:00      
                                          stories-part II            01/01/70 00:00      
                                             aka experience            01/01/70 00:00      
                                                Another Low VDD Story            01/01/70 00:00      
                                                   That's how many reset chips work            01/01/70 00:00      
                                          about that I have no doubt ...            01/01/70 00:00      
                                       You, even less like to read the answers            01/01/70 00:00      
                                          As I said before ... HORSEFEATHERS!            01/01/70 00:00      
                                             a horse of a different color            01/01/70 00:00      
                                                Flogging a dead (reset?) horse            01/01/70 00:00      
                                                   would that not be the case of            01/01/70 00:00      
                                                      Not defective, just powered down            01/01/70 00:00      
                                                         then where is the difference            01/01/70 00:00      
                                                            Good chips and state machine failures            01/01/70 00:00      
                                                               Reset seems to be different with static 8052            01/01/70 00:00      
                                                                  What do you think about my lines, Lynn?            01/01/70 00:00      
                                                                     Static and Dynamic Reset, and Lines            01/01/70 00:00      
                                                                        Thanks, the hope dies last...            01/01/70 00:00      
                                                                        why?            01/01/70 00:00      
                                                                           Isn't it shocking...            01/01/70 00:00      
                                                                              I know a lot ...            01/01/70 00:00      
                                                                              ...partially...            01/01/70 00:00      
                                                                                 The names and functions            01/01/70 00:00      
                                                                                    the analogy            01/01/70 00:00      
                                                                     Flash and Lines            01/01/70 00:00      
                                                                        So, what the hell is the remedy, then??            01/01/70 00:00      
                                       So long post...            01/01/70 00:00      
                                          Kai's remarks are germane to the issue            01/01/70 00:00      
                                             Stoping the oscillator            01/01/70 00:00      
                                                Lynn, pray explain            01/01/70 00:00      
                                                   Stopped clocks and reset            01/01/70 00:00      
                                                      waitaminute....            01/01/70 00:00      
                                                         This leads to the conclusion that ...            01/01/70 00:00      
                                                            Some further reading            01/01/70 00:00      
                                                   Let's be general            01/01/70 00:00      
                                                      General Reset            01/01/70 00:00      
                                                         So ... how do we fix it?            01/01/70 00:00      
                                                            one thing you could do would be to ..            01/01/70 00:00      
                                                               Tying reset ot crystal enable            01/01/70 00:00      
                                                it's already there... well in some form...            01/01/70 00:00      
                                                   Clock stopping on power-up            01/01/70 00:00      
                                                      we have alternatives...            01/01/70 00:00      
                                             comments            01/01/70 00:00      
                                             reset (and diode)            01/01/70 00:00      
   trying to conclude            01/01/70 00:00      
      Total agree!            01/01/70 00:00      
         That's not quite been my experience            01/01/70 00:00      
      Erik ... That proves nothing!            01/01/70 00:00      
         what else do you want 'proven'            01/01/70 00:00      
            suggestion vs. proof            01/01/70 00:00      
               I need a link            01/01/70 00:00      
                  well, you do not need tp prove it            01/01/70 00:00      
                     I had not considered these ATMEL (yechh) parts            01/01/70 00:00      
                        is there no end?            01/01/70 00:00      
                           coincidence is not proof            01/01/70 00:00      
                              Why on earth would I do that?            01/01/70 00:00      
                                 I would make me do something            01/01/70 00:00      
                                    Sadly, few who are in a position to do something            01/01/70 00:00      
                                 I am not demanding proof because is isn't there.            01/01/70 00:00      
                                    Maybe you can get support from Silabs?            01/01/70 00:00      
                        All manufacturers have problems            01/01/70 00:00      
                           function, not the device            01/01/70 00:00      
                              I like the way you go into the details!            01/01/70 00:00      
                                 It puzzles me that no chip maker has done this.            01/01/70 00:00      
                                    Vdd slew rate as a factor.            01/01/70 00:00      
                                       I have looked, but since it is not one of 'my' der            01/01/70 00:00      
                                          1 ms Vdd slew rates            01/01/70 00:00      
                                             I'd be afraid...            01/01/70 00:00      
                                                nope            01/01/70 00:00      
                                                   if I specify the slew rate for powerdown...            01/01/70 00:00      
                                                No ... I see this as limiting board capcitance            01/01/70 00:00      
                                                   I know...            01/01/70 00:00      
                                                      they get what they deserve!            01/01/70 00:00      
                                                         be realistic            01/01/70 00:00      
                                                            it takes more than a supervisor            01/01/70 00:00      
                                                               shooting sparrows with RPGs            01/01/70 00:00      
                                                                  it's not a sparrow, Erik ... it's a vulture            01/01/70 00:00      
                                                                     as it does not, then why worry            01/01/70 00:00      
                                                                        Erik, I don't know where your head's wedged ...            01/01/70 00:00      
                                                                           An almost off-topic comment on Xilinx JTAG            01/01/70 00:00      
                                                                           there is no problem, so why do you state there is            01/01/70 00:00      
                                                                              ?            01/01/70 00:00      
                                                                                 calrification            01/01/70 00:00      
                                                                              If you would get your head out of your ...            01/01/70 00:00      
                                                                                 well, if you do not want to 'test' the derivatives            01/01/70 00:00      
                                                                                    If you want yours tested, YOU test 'em            01/01/70 00:00      
                                                                                       I did not            01/01/70 00:00      
                                                                                          It's like breaking off the Vcc pin            01/01/70 00:00      
                                                                                             the datasheet states..            01/01/70 00:00      
                                                                                                It's probably ATMEL (yechhh!) that said that            01/01/70 00:00      
                                                                                                rephrased to match this subject            01/01/70 00:00      
                                                               the bulletproof '51            01/01/70 00:00      
                                                                  Custom 8051!            01/01/70 00:00      
                                                                  That is not the goal.            01/01/70 00:00      
                                             very true ...            01/01/70 00:00      
                                          I don't understand, Erik ...            01/01/70 00:00      
                                             READ!!!!!!!!!!!!!!!!!!!!!            01/01/70 00:00      
                                                when SiLabs builds a DIP-40 or PLCC-44 805x            01/01/70 00:00      
                                                   what does the package have to do with slew rate            01/01/70 00:00      
                                                      Erik ... it has to do with what I'll test.            01/01/70 00:00      
                                                         well, if you restrict your test to ancients, then            01/01/70 00:00      
                                                            current parts, but in DIP40 or PLCC44            01/01/70 00:00      
                                                               well you may find that someone sinned but did            01/01/70 00:00      
                                       No manufacturer warns about slew rates!!            01/01/70 00:00      
                                          actually I recall one            01/01/70 00:00      
                           I reread the whole link and            01/01/70 00:00      
                              To Richard            01/01/70 00:00      
                              it's just "lip-service" and means nothing            01/01/70 00:00      
               Please, Richard, only facts            01/01/70 00:00      
                  Kai, you can\'t compare the two environments            01/01/70 00:00      
   re 'lip service'            01/01/70 00:00      
      there you go again ...            01/01/70 00:00      
         unless the chairman of the board confess his sin            01/01/70 00:00      
            What you\'re spewing is nonsense            01/01/70 00:00      
               show what nonsesne you are communicating            01/01/70 00:00      
   what is this still about?            01/01/70 00:00      
      Times change, and so do IC's...            01/01/70 00:00      

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