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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<TITLE>Reset System Design</TITLE>
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<TITLE>Reset System Design</TITLE>
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<META NAME="GENERATOR" CONTENT="LibreOffice 3.5 (Linux)">
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<META NAME="GENERATOR" CONTENT="LibreOffice 3.6 (Linux)">
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<META NAME="CREATED" CONTENT="0;0">
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<META NAME="CREATED" CONTENT="0;0">
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<META NAME="CHANGED" CONTENT="20120817;10052500">
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<META NAME="CHANGEDBY" CONTENT="Ouabache Designworks">
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<META NAME="CHANGED" CONTENT="20130725;19592200">
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<META NAME="KEYWORDS" CONTENT="start">
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<META NAME="KEYWORDS" CONTENT="start">
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<META NAME="Info 3" CONTENT="">
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<META NAME="Info 3" CONTENT="">
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<META NAME="Info 4" CONTENT="">
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<META NAME="Info 4" CONTENT="">
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<META NAME="date" CONTENT="2008-01-08T12:01:41-0500">
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<META NAME="date" CONTENT="2008-01-08T12:01:41-0500">
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<META NAME="robots" CONTENT="index,follow">
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sums up what the thing you are designing will do. This is
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sums up what the thing you are designing will do. This is
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important because everything after this point must be traceable back
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important because everything after this point must be traceable back
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to this statement. The statement will tell you
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to this statement. The statement will tell you
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what steps you must follow. Anything that you cannot trace back
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what steps you must follow. Anything that you cannot trace back
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to something in the mission statement is not part of the design<BR><BR><BR>The
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to something in the mission statement is not part of the design<BR><BR><BR>The
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mission statement for the reset system is:<BR><BR><BR><BR>The reset
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mission statement for the reset system is:<BR><BR><BR><BR>The mission
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system will force all the nodes in a system or subsystem into a known
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of the reset system is to NOT do anything while reset triggers are
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good state while a reset trigger is active.<BR><BR><BR><BR>
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inactive and then to force all the nodes in a system or subsystem
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into a known good state while a reset trigger is active.</P>
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<P><BR><BR>
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</P>
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<P>The main reason we have so much trouble with resets is that most
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designers don't realize that their first duty is to ensure that
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nothing happens when you aren't doing a reset. They do everything
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they can to ensure that any reset is quickly and forcefully delivered
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throughout the entire design and then are surprised when their
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product fails ESD testing.
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</P>
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<P><BR><BR>
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</P>
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</P>
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<H3 CLASS="western">2) Define the reset triggers</H3>
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<H3 CLASS="western">2) Define the reset triggers</H3>
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<P>We must now make a list of all the events that will cause us to
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<P>We must now make a list of all the events that will cause us to
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reset all or part of the system. Our list is:</P>
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reset all or part of the system. Our list is:</P>
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<UL>
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<UL>
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<UL>
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<UL>
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<LI><P>The reset signal has a metastable filter to sync it
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<LI><P>The reset signal has a metastable filter to sync it
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with the clock.</P>
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with the clock.</P>
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</UL>
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</UL>
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<P>The last is important because some designers will forget that the
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<P>The last is important because some designers will forget that the
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filtered output is actually it's own separate reset domain<BR><BR><BR><BR><BR>
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filtered output is actually it's own separate reset domain.</P>
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<P><BR><BR><BR>
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</P>
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<P><BR><BR>
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</P>
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<P><BR><BR>
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</P>
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<P><BR><BR>
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</P>
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<P><BR><BR>
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</P>
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<H3 CLASS="western">3) Design a filter</H3>
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<P>Any Signal from the PCA into a chip might also contain an ESD
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glitch. If this appears on the reset signal then it could disrupt the
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products operation and result in a failure. A low pass filter must be
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used to block these transitiates</P>
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<P><BR><BR>
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</P>
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<H3 CLASS="western"><BR><BR>
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</H3>
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<P><BR><BR>
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</P>
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<P><BR><BR>
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</P>
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</P>
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<H3 CLASS="western">3) Define a known good state</H3>
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<H3 CLASS="western">4) Define a known good state</H3>
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<P>We now look at every storage element in the design and
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<P>We now look at every storage element in the design and
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define a safe state for each element of either 1 or 0. Don't
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define a safe state for each element of either 1 or 0. Don't
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cares are not allowed. If you cannot pick a value then one will be
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cares are not allowed. If you cannot pick a value then one will be
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assigned for you. This task is best performed after the system and
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assigned for you. This task is best performed after the system and
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board designers have defined the known good state for the PCA.
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board designers have defined the known good state for the PCA.
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They will define the state for all of the pads, the ic design team
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They will define the state for all of the pads, the ic design team
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must define the states for all internal nodes.</P>
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must define the states for all internal nodes.</P>
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<H3 CLASS="western">4) Assign storage elements to triggers</H3>
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<H3 CLASS="western">5) Assign storage elements to triggers</H3>
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<P>Once we have a list of all storage element we list any and all
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<P>Once we have a list of all storage element we list any and all
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triggers that will force them into a safe mode.<BR>A typical list
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triggers that will force them into a safe mode.<BR>A typical list
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would list all the flipflops in timer module u12.r567 would be reset
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would list all the flipflops in timer module u12.r567 would be reset
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by:</P>
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by:</P>
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<UL>
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<UL>
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timer. Once this step is complete it will provide a map for the
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timer. Once this step is complete it will provide a map for the
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reset distribution tree that you will need. The best way to
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reset distribution tree that you will need. The best way to
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distribute the reset over a large design is to use what is called a
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distribute the reset over a large design is to use what is called a
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"synchronous reset tree".<BR><BR><BR><BR><BR><BR>
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"synchronous reset tree".<BR><BR><BR><BR><BR><BR>
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</P>
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</P>
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<H3 CLASS="western">5) Select between synchronous or asynchronous
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<H3 CLASS="western">6) Select between synchronous or asynchronous
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reset system</H3>
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reset system</H3>
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<P>At this point it is easy to see if we need a synchronous or
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<P>At this point it is easy to see if we need a synchronous or
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asynchronous reset system. If your trigger is asynchronous then you
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asynchronous reset system. If your trigger is asynchronous then you
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must design a asynchronous reset system. If the trigger can occur
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must design a asynchronous reset system. If the trigger can occur
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without a clock then you must be able to reset the system without a
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without a clock then you must be able to reset the system without a
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synchronous reset system or you may also choose to design a
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synchronous reset system or you may also choose to design a
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asynchronous one. The async one will enter reset one
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asynchronous one. The async one will enter reset one
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cycle before the sync one but they will both exit on at
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cycle before the sync one but they will both exit on at
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the same time. <BR><BR><BR><BR>
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the same time. <BR><BR><BR><BR>
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</P>
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</P>
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<H3 CLASS="western">6) Select reset style for each flip/flop</H3>
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<H3 CLASS="western">7) Select reset style for each flip/flop</H3>
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<P>We now need to select a reset "Style" for each flip/flop
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<P>We now need to select a reset "Style" for each flip/flop
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from the four possible reset styles.<BR><BR><BR>
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from the four possible reset styles.<BR><BR><BR>
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</P>
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</P>
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<UL>
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<UL>
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<LI><P STYLE="margin-bottom: 0in">Synchronous
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<LI><P STYLE="margin-bottom: 0in">Synchronous
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