Design Rules

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General Comments

The design, verification, and implementation of an FPGA-based digital system requires you to manage a considerable amount of detail. The computer tools are not at all forgiving. Apparently minor mistakes can stop you in your tracks. You need to work in a careful and meticulous manner.

There are infinitely many incorrect methods to implement a design; the number of correct methods is very small.

Therefore, you should strive to be creative in the paper-and-pencil phase of your design (creative within the bounds of sound digital design practice, of course), but do not get creative once you begin writing Verilog descriptions. The design entry phase should be nothing more than a translation of your existing design into a usable format for computer simulation and hardware implementation. The only acceptable place for creativity during Verilog entry is in how you express the behavior of a particular register-level component such as a counter. All else must be considered a template!

When you faithfully adhere to the design rules below, you will experience significantly lower frustration levels, and you will also produce cleaner designs. On the other hand, if you choose to stray from the standards and invent your own methods, then plan to spend a great deal more time on your project than is necessary!

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Sequential Circuit Verilog Structure

All sequential circuit ‘always’ blocks must be structured as follows:

always @ (posedge MasterClock or posedge MasterReset)
   if (MasterReset) begin
      // Set value of register when
      // asynchronous master reset is asserted
      ...
      ...
   end
   else begin
      // Set register values for normal operation (responses to
      // level-sensitive inputs).
      // Helpful to read each assignment line ("<=") as
      // "on next clock edge, register ___ gets the value ___."
      ...
      ...
   end 
bulletNote that signals that you test during the “normal operation” part specifically do not show up in the sensitivity list.
bulletThe entire behavior of a device (register, counter, shift register, etc.) must reside in exactly one 'always' block
bulletIt is permissible (and even likely) that you will create a circuit description that tests a control input to determine whether to set an output to the same value that is used during asynchronous reset. Keep the concepts of “asynchronous reset” behavior and “synchronous initialization” totally separate, even if they lead to the same result (the device output goes to zero, for example).
bulletYou must not attempt to do anything else for a sequential ‘always’ block than what is shown above. Common attempts include multiple clocks, multiple resets, level-sensitive resets with edge-sensitive clocks, clock only, etc. If you choose to stray from this standard, you likely won’t see any problems in functional verification, because the simulator is much more forgiving than the synthesizer. It is much better at the outset to write your description in a way that is expected by the synthesizer.

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Master Reset

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The design must have exactly one master reset signal

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The master reset signal must asynchronously force all flip-flops in the design to a known state

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The master reset signal must not be used as an input to any combinational circuit

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The master reset signal must not be used as part of the "normal operation" section of a sequential 'always' block

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Master Clock

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The design must have exactly one master clock signal

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All flip-flop clock inputs must connect only to the master clock signal

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No "gated clocks" are permitted anywhere in the design

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The master clock signal must not be used as an input to any combinational circuit

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The master clock signal must not be used as part of the "normal operation" section of a sequential 'always' block

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Datapath / Controller

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Controller and all datapath elements must operate on the same master clock edge

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Each datapath element must be described by a single 'always' block or 'assign' statement

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When selective operation of a device is required, then synchronous enabling signals must be used (i.e., no gated clocks permitted anywhere in design)

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Verilog Identifiers

All Verilog identifiers must be structured as a three-part name of the form

vT$D

where

v = Verilog type,
T = signal type or category, and
D = descriptive phrase.

Verilog Type (v):

Symbol

Interpretation
i input port
o output port
r registered (but not an output port)
w wire
p parameter

Signal Type (T):

Symbol

Interpretation
G Global system resource such as master clock and master reset
C Control point; created by system controller, used by datapath
S Status signal (or condition signal); created by datapath, used by system controller
D Data or information
M coMmand signal to controller that originates from an device external to the system
E Externally-controlled device; created by system controller, and used by device external to the system
Q Associated with state register in the controller
N Numerical constant (used with parameter)

Descriptive Phrase

The "description" portion of the identifier must be constructed according to the signal type T as follows (use uppercase letters to improve readability):

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C, M, or E - action sentence (verb, noun). Examples or input and output ports: oC$InitializeTimer, iC$SetTimerToZero, iM$BeginProcess

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S - statement of fact, or statement of state. Examples: iS$TimerIsDone, oS$KeyIsUp, oS$TankIsEmpty

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D - state the essential information embodied in the device, instead of simply referring to the device itself. For example, suppose you have a counter to keep track of elapsed time (the counter is registered, but is not an output, so use 'r' as the Verilog type). Use rD$ElapsedTime as the name of the counter, instead of rD$Counter or even rD$Timer. Note: it might take you a while to get used to this naming convention, but it pays big dividends in terms of clarified thinking about your design!

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Q - state the primary activity performed in this state. Examples: pQ$WaitForKeyPress, pQ$InitializeAllDevices, pQ$SaveMeasurement  

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 ECE333: Digital Systems (W 2002-03)
Department of Electrical and Computer Engineering
Rose-Hulman Institute of Technology


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Last updated: 03/10/05.