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Richard Erlacher
01/11/12 23:35
Read: 214 times
Denver, Co
USA


 
#185443 - You can do it, though you may not want to
Responding to: Erik Malund's previous message
Erik Malund said:
If you can find a solid specification for exactly how long the RESET/nRESET pin on your particular MCU must be held high or low, then, with the aid of a calculator and a schmidt-trigger inverter, you can fabricate a circuit that produces a sufficiently long active-high/low pulse of appropriate voltage and stability during both power-up/power-down conditions and occasional brownouts, should they occur.

no, you can't

a) a schmitt trigger requires a minimum Vdd and thus the output is unknown at low Vdd
b) there is no way in hades a RC circuit with or without a schmitt trigger can generate a reset at power down

Erik

Erik Malund said:
If you can find a solid specification for exactly how long the RESET/nRESET pin on your particular MCU must be held high or low, then, with the aid of a calculator and a schmidt-trigger inverter, you can fabricate a circuit that produces a sufficiently long active-high/low pulse of appropriate voltage and stability during both power-up/power-down conditions and occasional brownouts, should they occur.

no, you can't

a) a schmitt trigger requires a minimum Vdd and thus the output is unknown at low Vdd

For MCU's with Vdd defined as +5 volts, which is still a goodly number of them, one can certainly use gates specified for operation down to 3 volts.


b) there is no way in hades a RC circuit with or without a schmitt trigger can generate a reset at power down

there is no way in hades that a RESET during power-down, specifically, during the sometimes very long and slow decay of Vdd, can guarantee anything. Stopping the oscillator to the MCU will do that, however, yet no supervisor or other reset generator seems to have attempted that.


Erik


As they say, "Where there's a will there's mourners." If you really want to do it, you'll find a way.

I've seen at least one CMOS schmidt gate that operates down to 2.5 volts. Where there's one, there's more ...

RE



List of 7 messages in thread
TopicAuthorDate
AT89ISP Reset Pin Issue      Jon Wilder      01/11/12 12:02      
   If you want a positive-going RESET ...      Richard Erlacher      01/11/12 13:10      
      no, you can't        Erik Malund      01/11/12 13:21      
         You can do it, though you may not want to      Richard Erlacher      01/11/12 23:35      
   The AT89ISP will expect active-high RESET      David Prentice      01/11/12 13:11      
   Try to run the initialize target command...      Kai Klaas      01/11/12 17:13      
      By the way...      Kai Klaas      01/12/12 07:57      

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