Re: [SI-LIST] : Why all reset signals are active low?

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From: Ashok Prabhu Masilamani (ashok.prabhu@wipro.com)
Date: Mon Mar 05 2001 - 01:24:23 PST


Hello,

      Mostly all critical signals like reset, CS etc are active low because of the input noise tolerance of the logic levels of the logic families. take the case of TTL where the V(ih) = 2V and V(il) = 0.8V. It means a voltage upto 0.8V can be detected as logic low by the device and a voltage as low as 2V can be detected as logic one. so, for a logic low input a noise voltage of 2V can make the device to detect it as logic one which is not the case for logic one. so logic one has more noise tolerance. This is the case for most of the logic families.

      this is the reason reset signals are active low. In deasserted state they will be in logic level one which means it won't switch to the logic level zero even if a noise input of greater magnitude affects it. otherwise if it had been a active high system, the system would get reset even for a noise input of small magnitude.

      Even though i have not put forward my explanation in a proper flow, i think i have answered your question.

ashok
  ----- Original Message -----
  From: selvaraj subramanian
  To: si-list@silab.eng.sun.com
  Sent: Monday, March 05, 2001 9:37 AM
  Subject: [SI-LIST] : Why all reset signals are active low?

  Hello,

  Why all reset signals are active low?

  Can anyone suggest me on this regard.

  Which one will be preferrable (active low reset or active high reset) and why?

  thanks in advance.

  regards,
  Selva
   

   

   

   

   

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