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Richard Erlacher
11/30/11 12:43
Read: 402 times
Denver, Co

#184965 - It's not the spec's ... it's the lack of them.
Responding to: Per Westermark's previous message
Per Westermark said:
An RC reset doesn't too often have a problem with the length of the reset pulse.

Electronics have minimum requirements but very seldom maximum reset pulse length.

I'll accept that. Many times, it's been connected to a pushbutton, which can have a pretty long duration. My main concern is its behavior in combination with a weak PSU and lots of on-the-PCB capacitance, which will slow the Vcc rise time. That, in turn, will charge the reset cap rather slowly. Worse yet, however, the Vcc decay when power is turned off will be very slow. As I've mentioned, things get "strange" when Vcc is on its way down, over a period of many seconds, the oscillator is still running, and the RC reset circuit doesn't really do anything desirable. That's where a supervisor/reset-generator comes in. Of course, their behavior on power-down isn't always ideal either. Unfortunately, "ideal" isn't really well specified.

So it is (relatively) easy to get a reset pulse to work in laboratory environments. Most (but obviously not all) power supplies have fast enough ramp-up of the voltages that the ramp-up isn't the big issue.

The really big problem is that since it only uses passive components, it suffers from a "memory effect". It can't instantly arm itself for a new full-length pulse in case there is a tiny glitch on the input power.

It is quite common that the power switch results in bounces on the power, in which case the RC reset "consumes" itself based on the first power on, while the microcontroller needs a proper reset pulse timed from the last of the multiple power-on events.

With a proper reset chip, the power is either good enough to keep the hardware running, or (if there is even a tiny glitch below the rated supply voltage) there will be a full reset pulse counted from after the supply voltage has reached acceptable levels.

With a RC reset, there may be a danger zone from microseconds and up to seconds where the RC solution will not give a proper reset. So people have to learn to disconnect power, count to ten and then turn on power. And hope that there wasn't any power bounces when the power plug was inserted or the power switch pressed.

I'm not sure about the rest of your comments. I'll have to consider them further. I'm not disputing them, though, as they do make some sense.


List of 25 messages in thread
P89V51RD2 And Flash Magic      Arif Deshmukh      11/26/11 18:48      
   please suggest      Andy Neil      11/26/11 23:27      
   reset button      Jan Waclawek      11/27/11 01:52      
      Reset Button      Arif Deshmukh      11/27/11 09:26      
         reset circuit      Jan Waclawek      11/27/11 13:04      
            That's the fundamental flaw with RC resets!      Andy Neil      11/28/11 14:40      
   Use a MAX700      Matthias Arndt      11/29/11 00:15      
      Rest supervisor        Steve M. Taylor      11/29/11 03:56      
         How Many Times      Michael Karas      11/29/11 05:03      
            This will continue until ...      Richard Erlacher      11/29/11 15:08      
               Power dip      Per Westermark      11/30/11 05:01      
                  real, but worse if flash      Erik Malund      11/30/11 07:17      
                  note, that here the main problem is NOT power related...      Jan Waclawek      11/30/11 07:17      
                     Yes      Per Westermark      11/30/11 07:42      
                        the ultimate joke      Erik Malund      11/30/11 08:20      
                           SiLabs watchdog      Andy Peters      11/30/11 09:25      
                              clarification      Erik Malund      11/30/11 09:43      
                                 Beware flash erase time      Maarten Brock      12/01/11 10:03      
                           brownout detection      Maarten Brock      12/01/11 10:07      
                              well,      Erik Malund      12/01/11 10:09      
                  It's not the spec's ... it's the lack of them.      Richard Erlacher      11/30/11 12:43      
   Reset Chip?      Arif Deshmukh      11/29/11 21:04      
      Why Ask???      Michael Karas      11/29/11 21:33      
         Thanks      Arif Deshmukh      11/30/11 00:29      
            a plethora of possible chips      Erik Malund      11/30/11 07:19      

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