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???
08/11/07 06:09
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 		 #143086 - Vcc rise time
Responding to: ???'s previous message
Kai Klaas said:
Richard said:
I completely disagree with your interpretation of this statement as saying, simply, that the time constant of a 10 uF cap pulled down by 8K2 is enough. I believe that it says more than that, namely that, first and foremost, you must provide a Vcc rise time of < 1 ms, at the end of which time you must have reached and settled at a "legal" level for Vcc.

I'm afraid you're wrong. The original "bible" by INTEL, which you will find here

http://download.intel.com/design/MC...238302.pdf

tells a more detailed story:






Note, what they state: "On power up, Vcc should rise within approximately 10msec."

This very lax "demand" sounds more like a recommendation than a rigid rule. It simply reflects the fact, that most tiny (and by this weak) mains transformers have no chance to charge up the storage cap in front of 7805 much faster anyway.

Kai



Richard,

I completely agree with Kai. The Vcc rise time spec is ONLY and ONLY to make the crappy RC reset to work, there is no, NO other reason for it. Once you accept that RC reset is unsatisfactory in the majority of application and is in the datasheet just to hide the fact that the manufacturer is unable to integrate a true reset/low power detection circuitry and that you need to spend the extra $1 or whatever to ensure its proper operation (as the majority of manufacturers admit later in form of appnotes), you will stop thinking in this way.

Yes, Richard, they ARE lying. Are you really surprised?

Of course, you can work around this problem using a sophisticated power cycling controller (including essentially the same reference&comparator&timer combo as is in the reset IC, plus the required power components, just to prevent power out shorter than required to discharge the C of RC), and while I am not quite sure it will be satisfying during powerdown, I am absolutely sure it will by several orders of magnitude exceed the cost of $1 reset IC which you were protesting against initially...

---
As for your experiment, can you please post a simplified schematics? It is not clear for me how did you combine the RC reset with the reset IC. Are you aware of the requirement that the teset IC has to "see" the SAME VCC as the mcu? Also, it is suspicious that the Dallas BRAM got corrupt as IIRC it has a builtin gate of CS during low VCC (I don't have access to the datasheets now to tell it for sure).

Jan

List of 29 messages in thread
TopicAuthorDate
reset supervisors            01/01/70 00:00      
   two purposes of the IC (at least)            01/01/70 00:00      
      Unfortunately, they seem to fail at one of them            01/01/70 00:00      
         you are talking about the RC resets, I assume            01/01/70 00:00      
            No, I'm referring to the DS and Max 1232            01/01/70 00:00      
               and what            01/01/70 00:00      
                  It's just not that simple ...            01/01/70 00:00      
                     I am getting tired            01/01/70 00:00      
                        Reset chips do work properly and prevent damage!            01/01/70 00:00      
                           I never doubted that, Kai            01/01/70 00:00      
                              can you please be more precise...            01/01/70 00:00      
                                 Yes            01/01/70 00:00      
                              It is a black box and you will never find out...            01/01/70 00:00      
                                 It\'s the power supply, and probably not the reset.            01/01/70 00:00      
                                    That's not what the bible states...            01/01/70 00:00      
                                       Isn't that the same thing with different syntax?            01/01/70 00:00      
                                          No, is not. Look at INTEL's original C51 manual!            01/01/70 00:00      
                                             Vcc rise time            01/01/70 00:00      
                                                It can, I believe, be done simply.            01/01/70 00:00      
                                             Brown-out Bug Chasing            01/01/70 00:00      
                                                Seems reasonable            01/01/70 00:00      
                                             There's been nothing specific enough to test.            01/01/70 00:00      
                                                Some hints            01/01/70 00:00      
                                                   it's a thorny problem            01/01/70 00:00      
                                                      Add a monoflop            01/01/70 00:00      
                                                         I\'ve got to be careful ...            01/01/70 00:00      
                        Jan, you\'re missing my point!            01/01/70 00:00      
                           that's too easy to answer            01/01/70 00:00      
                              You have stories, but no specifics            01/01/70 00:00      

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