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AR# 64614

Spartan-6 - asynchronous release of reset on the IOB FF


Asynchronous resets can result in a logic High when asynchronously released on clock edges.


In Spartan 6, asynchronous release of reset on the IOB FF can potentially cause a pulse on the output even when the input is tied low.

In Spartan-6 the asynchronous reset selects between the D-input and an initialization value.

For asynchronous controls, the initialization value will be a logic High when released.

While the asynchronous reset is applied, the IOB FF is held in reset, and the IOB FF operates as expected.

After releasing the asynchronous reset, the D-input will switch from the initialization value back to the D-input.

During this switching period if the IOB FF receives a clock, the IOB FF can potentially load the initialization value.


  1. Use a synchronous reset with the IOB FF
  2. Use a CLB FF with either an asynchronous reset or synchronous clear

AR# 64614
Date Created 05/19/2015
Last Updated 05/27/2015
Status Active
Type General Article
  • Spartan-6