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Richard Erlacher
03/15/12 17:10
Read: 422 times
Denver, Co

#186712 - think about this for a minute
Responding to: Erik Malund's previous message
Erik Malund said:
I don't think 100 nF is "right" for IC bypass unless in parallel with about 200pF.
fine if ceramic.

Additionally, I believe one should avoid too much power-gnd capacitance, as it will cause far too slow a decay of Vcc.
here we go again, with a proper supervisor chip that is NOT a problem sure, in some cases the suprevisors reset output need be connected to more than just the uC, (that's why there are supers woth both RST and !RST outputs) but that is SOP.

I'm reminded of a couple of friends here in this area who, back in the days of Apple-][ popularity, produced and retailed a real-time-clock board for the Apple-][. This board had a power entry swamping cap of 4.7 uF, and, on one occasion, one of their boards was sold and lost in the mail. They tried out and preset each board before shipping it, and in the case of the lost board, when it came back in the mail, unopened, after 20 days or so, the RTC was still pretty much on the money, meaning that the oscillator had kept on running and the clock/calendar was still counting.

I doubt adding the tiny cap's will hurt
today, in many cases, real estate is at a premium

The O/P didn't say anything about limitations on real estate, cost, or power.

that 4.7 uF at the power entry will keep things smoothed with today's low-power circuits.
not good enough, if you pull, say 20mA each from 3 ports you need a repository at the uC or you will be playing bounce ball.

I'm not persuaded that he's likely to do that, nor is it necessary to have a huge reservoir of capacitor-stored charge. What helps most in managing noise, is high-frequency bypass members. They don't have to be large if the supply powering the on-board regulator is hefty and clean. Hefty means big enough, not necessarily HUGE.

YES, the 'rules' in the FAQ are not necessarily #1 in all cases, but in no case will they hurt, unless, of course, you do not believe in supervisors etc.

Since I've observed peculiar "unspecified" behavior on the part of 805x's during active RESET, while Vcc was decaying, (a) I'm not convinced that supervisors can solve the power-down side of the problem and (b) I'm not sure what a supervisor is supposed to do with components that don't have a RESET pin.

I have a few updates to the FAQ, but last I tried it was not updateable (is that a word?)

it is one now ...


Frankly, I've put those 3-pin parts in a few app's just for good measure, but until someone tells me how to cope with slow decay on Vcc, when various components have different requirements regarding valid levels of Vcc, I'll reserve judgment for now.

That 10 uF cap on the old Intel reset app-note has to be just about as dumb as the stupidest thing anyone ever published. A fast rise time on Vcc fixes most of the problems and I've convinced myself, empirically, that a fast fall-time does that too. I once had a circuit with a supervisor that caused my BBRAM used as a program store to "lose its mind" quite frequently under repetitive power cycles, but when the rise and fall of Vcc was limited to < 100 microseconds, the problem went away for over a million iterations over about 1000 hours. One of these days, I'll get that old system out of storage and hook it up to those two 805x circuits and take some LA pictures to show you. That won't prove much beyond that it happens, but it'll be amusing.


List of 66 messages in thread
Sine wave detection on microcontroller      Yagnesh Mehta      03/07/12 04:59      
   Detect or measure?      Per Westermark      03/07/12 05:25      
      not detection, measurement of sine wave signal      Yagnesh Mehta      03/07/12 06:14      
         What are you measuring?      Andy Peters      03/07/12 10:05      
            RMS and Average - 1st Priority      Yagnesh Mehta      03/07/12 20:59      
               415 v      Mahmood Elnasser      03/08/12 22:07      
                  I have to do using MCU      Yagnesh Mehta      03/08/12 22:48      
                     Start simple      Mahmood Elnasser      03/08/12 23:17      
                        if time is major isssue than.....      Yagnesh Mehta      03/08/12 23:31      
                           People responding to specific questions      Per Westermark      03/09/12 02:14      
                              Same Technique....      Yagnesh Mehta      03/09/12 04:32      
                                 RMS      Maarten Brock      03/09/12 05:49      
                                    Sorry for my laziness        Yagnesh Mehta      03/09/12 20:50      
                                 So what have you done?      Per Westermark      03/09/12 06:19      
                                    Require more work...      Yagnesh Mehta      03/10/12 00:50      
                                    Working on that      Yagnesh Mehta      03/10/12 06:12      
                                       options, options      Per Westermark      03/10/12 06:49      
   Help me      Yagnesh Mehta      03/08/12 21:31      
      Okay      Joseph Hebert      03/09/12 06:13      
         Are many threads available      Per Westermark      03/09/12 06:29      
      when you save a char ...        Erik Malund      03/09/12 08:09      
      Why don't you specify your problem COMPLETELY?      Richard Erlacher      03/09/12 11:36      
         Already specify it      Yagnesh Mehta      03/10/12 02:07      
            there are some details?      Richard Erlacher      03/10/12 07:51      
               comparisions      Yagnesh Mehta      03/12/12 01:16      
                  How are you calculating your needed sample rate?      Per Westermark      03/12/12 01:54      
                     start with simple one....      Yagnesh Mehta      03/12/12 08:02      
                        You do know sin(90+angle)? Or 2-log computations?      Per Westermark      03/12/12 08:48      
                           ADC resolution............      Yagnesh Mehta      03/12/12 12:27      
                              impossible or wrong      Erik Malund      03/12/12 13:30      
                              double post, ignore      Erik Malund      03/12/12 13:30      
                  Precision Rectify:      Erik Malund      03/12/12 06:44      
                     SCR Chopped Wave      Yagnesh Mehta      03/12/12 07:48      
                        What accuracy do you need?      Kalpak Dabir      03/12/12 08:43      
                           CT accuracy      Yagnesh Mehta      03/12/12 12:58      
                              the fundamental      Erik Malund      03/12/12 13:41      
                              50Hz      Per Westermark      03/12/12 14:27      
                              Precision rectifier is suitable for any waveform      Kalpak Dabir      03/13/12 01:47      
                                 are u sure??      Yagnesh Mehta      03/13/12 08:03      
                                    Depends      Per Westermark      03/13/12 09:08      
                                    must be severely bandwidth limited      Erik Malund      03/13/12 09:11      
                                       Maybe was circuit with top-voltage capacitor      Per Westermark      03/13/12 09:17      
                                       Sorry for confusion      Yagnesh Mehta      03/14/12 00:18      
                                          What accuracy do you need?      Kalpak Dabir      03/14/12 01:23      
                                             360/1024      Yagnesh Mehta      03/14/12 01:41      
                                                Not what he (or me) have asked      Per Westermark      03/14/12 04:39      
                                                   you are right....      Yagnesh Mehta      03/14/12 11:51      
                                                Do you believe you can protect your circuit from noise?      Richard Erlacher      03/14/12 08:19      
                                                   Very Noisy environment      Yagnesh Mehta      03/14/12 11:57      
                                                      re Very Noisy environment      Erik Malund      03/14/12 12:23      
                                                         Some of those rules are showing their age      Richard Erlacher      03/14/12 20:00      
                                                            here we go again      Erik Malund      03/15/12 08:15      
                                                               think about this for a minute      Richard Erlacher      03/15/12 17:10      
                                                                  when discussing a FAQ      Erik Malund      03/16/12 06:49      
                                                                     Well, when were those cap sizes determined?      Richard Erlacher      03/16/12 09:53      
                                                                        Technology      Per Westermark      03/16/12 10:13      
                                                                           Steady state currents do not need decaps      Kalpak Dabir      03/16/12 22:48      
                                                                              Often low capacitance so limited charge. But remember load.      Per Westermark      03/17/12 06:41      
                                                                        fixations      Erik Malund      03/16/12 10:40      
                                                                           Nothing. has been proven yet.      Richard Erlacher      03/18/12 13:15      
                                                                              once more for the peanut gallery      Erik Malund      03/19/12 07:23      
                                                                                 Where you sit determines what you see ...      Richard Erlacher      03/20/12 01:56      
                                                                                    Off-topic        Per Westermark      03/20/12 02:01      
                                                                                       difficult when ...      Erik Malund      03/20/12 06:59      
                                                                                       Yes ... I don't know how we got here either      Richard Erlacher      03/20/12 09:32      
                                                                                          i do      Erik Malund      03/20/12 09:45      

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