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Per Westermark
01/02/12 08:57
Read: 637 times

#185261 - RC resets are evul
Responding to: ???'s previous message
Adam Collins said:
Lol.Funny but u got a point. But I am new to all this. But thanks for the tips! and what do you mean"asking for it" as in something will fail without the vdd and with rc??

Selecting a "cheap" design, that cuts lots of corners, means that you end up with a design that "may" work. It may "look" like it works, except that in real life, the probability of big problems exists.

RC designs often fails when you have a power glitch or the power button bounces or the PSU have slow ramp-up or ramp-down of the voltage. A design that contains flash memory is extremely sensitive to a lost or incorrect reset, or to getting slow voltage ramps while the reset signal isn't held - it's very easy to get corrupt memory content.

A 3-pin reset chip is a cheap investment to get a well-working product that you can trust reasonably well.

Unless the processors happens to have special hardware on the inside of the reset pin (and you don't use other chips requiring a reset in your design), you really should not use a RC-based reset. And the chip manufacturers should be shot for publishing RC-based reset circuits. You get a solution where customer A says it always works. And customer B says the unit fails to boot 8 times out of 10. Just because there are some differences to their power supply behaviour that makes one customer almost always getting a "good enough" reset, while customer B almost never gets a "good enough" reset.

It might even work 100 of 100 when tested in a laboratory, just because you had a good PSU, a good power switch and tested in room temperature.

List of 32 messages in thread
AT89S52 and ISP problem      Adam Collins      01/01/12 13:38      
   Quick Schematic Obversation      Michael Karas      01/01/12 18:28      
      hmm      Adam Collins      01/01/12 22:20      
         Your Reset Input      Michael Karas      01/02/12 03:56      
            Thanks!!      Adam Collins      01/02/12 07:28      
   Your schematic is wrong      David Prentice      01/02/12 03:35      
   The AT89S52 needs a reset chip!      Kai Klaas      01/02/12 05:44      
      The AVRISP-2 is a USB device      David Prentice      01/02/12 06:02      
   what an AWFUL design      Erik Malund      01/02/12 07:35      
      Isn't mine!      Adam Collins      01/02/12 08:06      
         Compare several schematics      David Prentice      01/02/12 08:16      
            Thanks!!      Adam Collins      01/02/12 08:41      
               Sink or source? Does matter a lot!      Per Westermark      01/02/12 08:50      
                  Not granted      Adam Collins      01/02/12 09:07      
                     Processor can be used      Per Westermark      01/02/12 09:20      
                        Yeah..      Adam Collins      01/02/12 09:33      
                     cure for ignorance      Erik Malund      01/02/12 11:11      
         you forgot the quotes      Erik Malund      01/02/12 08:23      
         well      Erik Malund      01/02/12 08:30      
            Funny      Adam Collins      01/02/12 08:47      
               RC resets are evul      Per Westermark      01/02/12 08:57      
                  So?      Adam Collins      01/02/12 09:02      
                     Not necessarily,,,      Kai Klaas      01/02/12 09:18      
                     I guess you are a student / hobbyist      David Prentice      01/02/12 09:19      
                        Oh..      Adam Collins      01/02/12 09:30      
                        Even or better just then...      Kai Klaas      01/02/12 09:45      
                           Where to apply 100nF?      Adam Collins      01/02/12 10:01      
                              Shortest possible distance      Per Westermark      01/02/12 10:34      
                        especially not for a hobbyist      Erik Malund      01/02/12 11:23      
                  RC RESET worked fine for years ,,, but ...      Richard Erlacher      01/02/12 20:01      
               no, not "will" but "is likely to"      Erik Malund      01/02/12 11:17      
                  an added IMPORTANT comment      Erik Malund      01/02/12 11:26      

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