Digital Systems. Role of the Digital Engineer

Transcription

1 Digital Systems Role of the Digital Engineer

2 Digital Design Engineers attempt to clearly define the problem(s) Possibly, break the problem into many smaller problems Engineers then develop a strategy for designing a solution If the solution includes the design of a digital system Engineers proceed through a detailed and systematic approach to devise a system that would solve the problem Digital system design includes the development of a set of Boolean equations that describes the circuit Today engineers use a hardware description language (HDL) such as VHDL or Verilog to program CPLDs, FPGAs, or ASICs

3 Not-too-recent-past Engineers would take the set of Boolean equations and sketch a circuit, using basic logic gates They would draw the circuit by hand, using a template with digital symbols Today we use powerful computer software and hardware, such as computer aided drafting (CAD) systems, integrated development environments (IDEs), or integrated software environments (ISEs), to develop our logic designs and draw our circuits

4 Not-too-recent-past The next step would be to test the design: The engineers (or technicians or technologists) would build the circuit using a wire-wrap or breadboard Once built, the engineer or technician would test the circuit using a detailed set of input test parameters Depending on the number of inputs, this could be an exhaustive set or a representative set and take hours or even days to complete

5 Not-too-recent-past Rarely did a circuit pass the first tests troubleshooting began There are many places in which an error could occur: in the design, in the IC, in the connections, in the timing, does the power supply provide enough current?, etc. When trouble was found in the design, the engineers went back to the drawing board (literally) to begin redesigning the circuit to remove flaws

6 Not-too-recent-past When the circuit passed the tests and was declared complete and operational, the printed circuit board (PCB) would have to be designed Again, this process was performed by hand, a very long and tedious process Finally, the integrated circuits (IC) were placed on the PCB and soldered into place The finished product had to be tested once more before it was shipped to remove any accidental short circuits, open circuits, and faulty ICs.

7 Not-too-recent-past You may be thinking: All this is ancient history, all this happened before I was born! This may be true for many of you, but not all As late as the mid 1980 s, these practices and procedures were still in place, but things were beginning to change

8 Change Begins In 1970 a group of electrical engineering professors and graduate students at Berkley, led by Don Pederson and Ron Rohrer, began working on a program called CANCER (Computer Analysis of Non-Linear Circuits Excluding Radiation) 1,2,3 eventually evolved into a program called SPICE (Simulation Program with Integrated Circuit Emphasis). By the 1980s the first commercial version of SPICE was released by MicroSim for the PC: Called PSpice (PSpice is now owned by Cadence Design Systems).

9 Big Changes With SPICE, PSpice, and the advent of PCs into the home, office, and universities, analog and digital circuit design was changed forever Now circuits could be simulated, debugged, and perfected before construction began Saved a lot of time and effort Today circuit simulation is standard practice in circuit design

10 PSpice Almost every semiconductor company in the world, as well as a majority of electronic system companies, uses a version of SPICE, or one of its derivatives, for the design of their products. The design of virtually every integrated circuit relies on the simulation of at least some of its constituent parts at the device level, and most of these simulations have been, and are being, carried out using SPICE-related programs. Bruce A. Wooley, (2004)

11 HDLs 1980 s: Hardware Description Languages (HDLs): VHDL and Verilog development The development of VHDL, [V (Very High Speed Integrated Circuit) Hardware Description Language], was initiated in 1981 by the United States Department of Defense (DoD) to address the hardware life cycle crisis Because technology was advancing very rapidly at that time, designs were not documented properly, and there was a wide range of incompatible simulation tools DoD decided that a standard had to be created: thus, VHDL Gateway Design Automation was also developing Verilog around the same time that VHDL was being developed.

12 HDLs HDLs are a programming language used to describe the behavior and structure of electronic systems Particularly suited as a language to describe the structure and behavior of digital systems The designes are then used to develop ASICs (Application Specific ICs) Conventional digital circuits FPGAs (Field Programmable Gate Arrays) CPLD (Complex Programmable Logic Devices)

13 FPGA Group of 8 logic cells Memory block Interconnection wires

14 HDLs Software exists that allows you to: Describe a system using either VHDL or Verilog Simulate it Synthesize it Download it to a development board We will be using such a system in this class: Xilinx ISE (Integrated Software Environment), version 7.1 along with the Spartan 3 development board The Xilinx software not only allows you to describe a circuit using VHDL or Verilog, it will also allow you to draw the circuit so that you can test a circuit even if you do not know an HDL

15 Design Process Digital systems engineer will design circuits using either: software (not necessarily CAD software, but similar) to draw the circuit An HDL to describe it They will then use a development board and some type of Programmable Logic Device (PLD) to test and troubleshoot their design Often use a test-bench program to analyze and test the design Document that there are no errors within the logic. CAD packages are used to develop the PCBs Automated machinery will assemble, solder, and test the resulting board before it is shipped.

16 PCB

17 Design Process Basic Design Loop

18 Troubleshooting Dr. John Wakerly, says: It s next to impossible to be a good designer without being a good troubleshooter. Successful debugging takes planning, a systematic approach, patience, and logic: if you can t discover where a problem is, find out where it is not!

19 Troubleshooting Debugging a design is part of the design process The digital systems engineer must be able to: test their design, account for any problems, correct the problems Finding the problem can be difficult and frustrating work a lot of patience is needed (you also need to know when to take a break and walk away for a short period this should be long before you reach the urge to throw the computer or development board across the room!) A good place to start in any troubleshooting exercise is to determine which parts of the design are failing and which parts are functioning properly.

20 Troubleshooting Before you can troubleshoot, you must understand what the circuit was designed to do Good practice: break the problem down into smaller problems For example, you could look at just the input stage of a circuit and test to see if this section of the design is functioning properly

21 Troubleshooting Be systematic: I have seen many technicians assume that because the output of an IC on a PCB is incorrect, that the IC is faulty and must be replaced They replace the IC only to find out the problem is still there. Before an IC is replaced: Disconnect the faulty output from the circuit This step isolates the IC so that you can discover if: the IC is truly at fault, if another IC further down the line is the problem, an open or short circuit is to blame

22 Troubleshooting Another useful tip: Look at the complete schematic when troubleshooting, Determine which outputs are failing, Then trace this signal back through the system to the inputs. Go to the signal s halfway point between the input and faulty output Measure the signal to determine if the signal is good at this point effectively cuts the job in half If the signal is correct at this point, then the problem lies somewhere between the middle and the end of the circuit If the signal is faulty (assuming you have properly isolated this signal), then the problem lies between the middle and the beginning of the circuit Repeat as necessary

23 Troubleshooting You may also have more than one problem! This can really complicate troubleshooting especially if you have assumed that there was only one fault in the circuit.

24 Break the problem into smaller problems! In designing, the engineer (or engineering student) should not try to develop the complete system in one sitting design small sections of the circuit and test them before moving on to the next section When you are sure that a particular section (or you can think of it as a block in a functional diagram) is operating as designed, proceed to the next section The same approach can be used when developing software or circuit descriptions written in an HDL. Break the problem into smaller problems! Then, test, troubleshoot, and correct the problems before moving on to the next section.

25 At which level will you design?

26 Professionalism I believe (sometimes) that engineering students think that documentation is a four-letter word Documentation is important for several reasons: Companies require it! You must document every characteristic and operating parameter of a digital system design You may be required to pass your part of a design over to another engineer (possibly to software engineers) also to other departments and eventually to customers You may be required to enhance or update a design at a later date Without complete and thorough documentation, you would have to go through all those painstaking hours of development again, re-inventing the wheel, and costing time and money to the company, and quite possibly costing you a job.

27 Professionalism When you complete your studies and land your first career-position you will be a professional You may have the title: engineer, technologist, programmer, designer Whatever the title of the position, you will be a professional.

28 Professionalism This will require you to communicate with others: other engineers, your managers, customers You must communicate in writing and verbally in a technical and professional manner One way in which people can improve their writing and speaking abilities is to have opportunities to practice, obtain critical feedback, and then try again. You will have several opportunities in this class to write and to make presentations

29 Summary The goal of this course: Students should be able to design, test, troubleshoot, and implement combinational and sequential digital circuits in a professional manner and by using medium scale integration (MSI) ICs and programmable logic devices To meet this goal in the timeframe permitted you must have the prerequisites for this course You can meet the goals of this class if you have the prerequisite knowledge and are committed to learning and applying the information discussed in this class