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AR# 13312

SelectIO - What is the maximum Tin (input signal transition time)?

Description

What is the maximum allowable rise time for general inputs and clock inputs?

Some older data sheets (prior to Virtex-II) have a Tin value which indicates the maximum allowable signal transition time, but it is not published for newer devices.  

Solution

The Tin is not specified for as it represents an endorsement of poor design techniques.  

Generally, slow input transitions should be avoided for the following reasons:

  1. Poor design timing -- you do not know precisely when the threshold is crossed.
  2. More power consumption -- the input consumes extra mA while in the threshold. 
  3. More susceptibility to noise -- the input picks up noise from the board and from ground bounce while near the threshold.


In a slow design, combinatorial signals are allowed to be slow. Clocks should always be fast, as any ringing or ground bounce can lead to double-triggering.

Generally speaking maintaining montonicity in a signal transition is the most desireable solution and is easiest to achieve with faster edge rates.

AR# 13312
Date 09/04/2017
Status Active
Type General Article
Devices
  • Virtex-7
  • Virtex-6
  • Virtex-5
  • More
  • Spartan-6
  • Spartan-3
  • Kintex-7
  • Artix-7
  • Kintex UltraScale
  • Zynq-7000
  • Spartan-3A
  • Spartan-3AN
  • Less
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