EEWeb Pulse - Volume 98 by EEWeb Magazines

Thilo von Selchow President & CEO of ZMDI

Electrical Engineering Community EEWeb.com

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TABLE OF CONTENTS

Thilo von Selchow

PRESIDENT & CEO OF ZMDI How ZMDI’s energy efficient products have saved 50 million tons of CO2 from entering the Earth’s atmosphere.

Featured Products

Memristors: The Fourth Circuit Element

BY ALEX TOOMBS WITH EEWEB How these unique components promise to change modern thought on electronics and the semiconductor industry once they can be fabricated effectively.

The Raspberry Pi — Part 4 BY KYLE OLIVE WITH EEWEB This fourth installment of the series walks through the process of setting up a simple server for your Raspberry Pi.

VHDL 101 — Part 3 From Logic Gates to Adders BY PAUL CLARKE WITH EBM-PAPST How all HDL languages can bring something unique to programming and why looking at logic and adding circuits is essential to this process.

RTZ - Return to Zero Comic

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INTERVIEW

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EEWeb PULSE When did you start at ZMDI and what was the company like when you started? I joined as the president and CEO of ZMDI back in 1999. The main objective when I started was to reposition the company from a stateowned enterprise to a privately held semiconductor company. One of the problems was that there wasn’t a clear consensus about the difference between a proto-type versus a final product. When I first arrived at the company, they were talking to me a lot about the products they had, but I later found out that those were actually proto-types. That was a big surprise for me, and it was something that we needed to work on, which ended up taking several years. The second main obstacle was re-defining the marketing and sales divisions so as to develop a world-class infrastructure. This was something that was essential to build in order for us to be successful. Everything else, in terms of technical competence, was outstanding. ZMDI’s slogan is “Pink is the New Green.” Could you tell us a little more about that? In this slogan, “Green” represents energy efficiency. The company has a core mission to develop products that enable our customers to create energy efficient semiconductor solutions.

Green energy is the cornerstone of our organization. This is what we stand for and what we want to be measured against. The other side of it—the “Pink” side— is something we adopted when we started out almost 50 years ago. Pink is our corporate color, so we felt that “Pink is the New Green” was the right slogan to position us in this market. We see energy efficiency as the megatrend, not only today but well into the future, if as a society we really want to combat global climate change and make better use of energy. What kinds of energy-efficient products are you making? We have a large focus on sensors. Sensing conditions, such as temperature or voltage/current levels, is often essential for energyefficient applications, which often require regulation and/or actuation. This combination can help optimize energy efficiency. Second, we have a long history in low-power portable medical devices, so we see lowpower ICs as a core competency and it has been for over two decades. We were the first ones to develop a 0.9V hearing aid several years ago. We also see that energy efficiency begins at the chip level—that is crucial, as that plays a large role in all portable devices. Third, I would

“Green energy is the cornerstone of our organization. This is what we stand for and what we want to be measured against.”

say that digital power is also a key portfolio that we have developed over the past four years. There are only a handful of players in the world that can do this, and we are one of them. You could say that roughly only 10 to 20% of competing products are adequately energy efficient in our application areas; e.g., data storage, base stations, and other important energy-saving benefits. What are some ways that your products have impacted the environment? Our products have truly impacted the environment, and we track this. We have a CO2 counter on our webpage so that you can actually see the amount of CO2 that has been saved by using our products. I think the number now is around 50 million tons of CO2 savings since 2004. It’s counting up as we speak: every time we sell parts into an energy-efficiency application it’s counted up on the webpage—a live ticker. Our products have brought us two awards from Frost & Sullivan and the Green Apple Awards for energy savings. What industries do you focus your products on? We have a long tradition in automotive and industrial, and approximately 55% of our revenue is in those areas. We are focusing a lot of our resources on

Screenshot of ZMDI’s Fuel and CO2 Savings Ticker Retrieved 5/7/2013

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INTERVIEW mobile devices like smart phones and tablets as well as high-end consumer devices. These are the four key markets that we are focusing on now. What’s the motivation behind ZMDI’s standard product line and how it changed over the years? The most important part is always thinking about how the customer can benefit from the products we offer and how they can improve their product’s performance and be successful. We call these products “platform products,” and they offer a unique opportunity for our customers to take advantage of our existing know-how and improve their intellectual property. Their finished parts can then be used to test their applications, and so the technical risk for them to work with us is very low. With that, we’ve been able to build the application know-how into our parts, which then can be built into products. the customers can benefit from that. What are the advantages of ZMDI being a fabless semiconductor company? In terms of being fabless, I’d say that the major effect of it was really enabling what we refer to as: “focus, focus, focus.” We focus on our core competency, which is technology; i.e., the product itself and its product definition. This is key, and becoming fabless allowed us to allocate greater financial investment in our product development, marketing, development kits, and application engineering. We feel so strongly about the necessity for this type of investment that we are currently allocating 30% of our revenue into product development, which is a relatively big number compared to others in the industry. What sets ZMDI apart is the fact that

“The most important part is always thinking about how the customer can benefit from the products we offer and how they can improve their product’s performance and be successful. “ we have been a high-end analog company for over 50 years. We have done a lot of analog integration, which is something that is also unique. We really strive to get the customer the best product for the best price, which also distinguishes us from our competition. Could you tell us about your investments and initiatives that you’ve taken since 2012 to grow the company? A portion of investments have gone to brand building. The other portion has been devoted to doubling our sales force in the past two years— especially in the U.S. and Asia. On top of that, we have invested in additional application labs in all of our major regions. We have one in Silicon Valley, one in Boston, one in Seoul, and one in Munich. Whenever our customers within a region have expressed a need, we have listened and built labs to help our customers design their products. How have customers responded to these application labs? The application labs are very effective for our customers. When the customer finalizes a technical solution and

then the moment comes when they want to design the solution into their application, then that is when the customer needs good technical support and guidance. Our teams have been great in helping the customers overcome the last little hurdles in their product design. This is why the investment in the application labs has been great for ZMDI. What is ZMDI doing in terms of expanding its distribution? When we started to develop an additional portfolio of standard products three years ago, we started small with our distribution—but I would estimate our distribution now is around 5%. We have generated 120 new products in the last three years, and we have around 70 new products coming out this year, so with this increase, we expect our sales through distribution to grow considerably. Our aim is within three years to see our fulfillment via distribution to be at 25%. We see our distributors as partners, so we need to give them the right support levels and make sure there’s no internal competition so that they can thrive. When, collectively, we have the same mindset in place and the right incentives, then it will be a great area for us to grow. Visit www.eeweb.com

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20,000

CO2 Savings (Tons) Cumulative

60,000,000 50,000,000 40,000,000

15,000

30,000,000

10,000

20,000,000

2013

2012

2011

2010

2008

2009

0 2007

0 2006

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5,000

CO2 Savings in Tons

Fuel Savings (Mill. Liters) Cumulative

25,000

2004

Fuel Savings in Mill. Liters

Energy Savings (Cumulative)

Years Fuel savings and greenhouse gas (CO2) reduction achieved through ZMDI products in the automotive segment.

What is your vision for the future of ZMDI as it relates to product development and your global footprint? The vision is pretty simple: we want to become #1 in our core segments, which are sensing and digital power. What does this mean for product development? Currently in Europe in terms of product development, we have six major development offices. We are also starting to do a lot more customizing in the U.S. and in Asia, and we are seriously considering expanding our design resources on other continents as well. We also recognize the new trends that arise within the industry—design work can be outsourced as well. That’s a little bit new as a business model, so when a company becomes fabless, it needs to be able to define products well enough (and have superior application know-how) so that it can give its customers the right products.

This doesn’t necessarily mean you have to do all the design work inhouse, so we started to qualify external design centers that can support us so we can develop more products in a shorter period of time. We think this is very effective new setup. On a different note, could you tell us a little bit about the ZMDI Open Chess Tournament? Why is this important for ZMDI to promote chess as a sport? It actually started through my relationship with Gary Kasparov, former World Chess Champion, who is still considered today to be one of the top chess players in the world. We became involved in this European chess festival, and on the basis of that, we were able to attract the Chess Olympics back in 2008 and Gary was a part of that. We built on that basis and grew the chess festival and not I’m now sitting on the board of The

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American Foundation for Chess. This foundation is geared towards putting chess into schools—particularly in 2nd and 3rd grade classes. We are working on some studies, and it seems that those kids that have been trained in chess early on in their lives perform better throughout different aspects of their lives. I find this fascinating. The main strategy of the game is to think a few steps ahead, which is an extremely important skill to learn early. I really enjoy supporting this initiative and it’s a great honor to be on the board of the American Foundation for Chess. ■

INTERVIEW

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ne of the oldest circuit effects based upon exposure to varyin Berkeley professor, theorized that thi circuit element back in 1971, terming t derived from the term memory resistor relate electric charge and magnetic flu proven difficult, and while we can rel inductors very well, we still are unab on a wide scale.

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TECH ARTICLE

s is the alteration of resistance ng charge. Leon Chua, a UC is effect could be harnessed as a that device a memristor. Memristors, rs, are two-terminal devices that ux. Fabrication of memristors has liably make capacitors, resistors, and ble to cheaply produce memristors

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Efforts from several companies and research organizations including Hewlett-Packard (HP), the University of Michigan, and HRL Laboratories appear to be close to effective manufacturing of memristors. Memristors promise to change modern thought on electronics and the semiconductor industry once they can be fabricated effectively. Multiple memristors can be used to create devices that act like switches, and can create new types of nonvolatile memory. Difficulties remain in characterizing how memristors operate, as well as in finding ways to manufacture them as reliably as we can other circuit elements and devices. Leon Chua was able to predict the existence of devices like the memristor largely through extrapolation. Published in 1971, Chua’s paper entitled “Memristor – The Missing Circuit Element” focused on his explorations into the behavior of nonlinear resistors, capacitors, and inductors. He observed that relationships existed for voltage and current in resistors, voltage and charge in capacitors, and magnetic flux and current in inductors. This led Chua to believe that there was a theoretical device out there that linked flux and charge. A depiction of these circuit element relationships is shown below.

How a Memristor “Remembers” As can be seen in the bottom right corner of the graph above, the relationship for a memristor shows that the magnetic flux is directly related to the charge on the device, multiplied by a value M. That value is denoted the memristance, and is the slope of the function defining the device. It is a quantity similar to a variable resistance and carries the same unit, though it generally varies with charge. This is the inherent property that makes memristors so valuable and so unique. No matter what combination of the other fundamental circuit elements you assemble, you cannot make a circuit that “remembers” like a memristor can.

Memristors and Ohm’s Law Often times, Ohm’s law is explained in terms of water flowing through a series of pipes. Current is represented by water, and the diameter of the pipe is constrained by resistance, while voltage is the force motivating the water to flow. It gives a good introductory view of how these circuit variables interact. In this situation, a memristor is represented by a pipe that changes diameter depending upon the direction and amount of water flowing through

Memory + Resistance = Memristance The property of an electric component that allows it to remember its last resistance before shutting off.

Figure 1: Fundamental Circuit Variable Relationships (courtesy of Wikimedia user Linear77)

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TECH ARTICLE

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2nm Figure 2: Titanium Dioxide Memristor

it. Current flowing in one direction nonlinearly increases the resistance of the device, while current flowing in the opposite direction nonlinearly decreases the resistance of the device. Controlled variances of the current through the device can result in nonvolatile storage of information. While capacitors and inductors lose energy gradually after a power supply is removed, memristors maintain their resistive state long after they’ve been disconnected from the terminals of a battery.

though they are nonvolatile and generally take up less space. Theoretically, storage much more dense than current hard drives or flash memory could be made, and it wouldn’t require power to keep information stored. In addition to the many applications these technologies could have for persistent storage in consumer electronics, memristors appear likely to help the fields of neural networking, signal processing, and controls, among many others.

HP & Memristors

Fabrication of Memristors

According to an IEEE Spectrum article, The Mysterious Memristor, about the history of the memristor, Chua’s publication in 1971 remained largely untouched for close to three decades until HP researcher Stanley Williams and other employees noticed strange aberrations when characterizing some devices they had made. HP has filed for and received several patents for two-terminal “scale crossbar switches configured to perform a logical function in response to a sequence of pulses that encode logic values in the nanometer scale crossbar switches as impedances” (US Patent 7203789). HP has seen some uses for devices that can be fabricated from the memristor technology that they are currently working with. These crossbar latches can be used similarly to transistors,

There are several approaches now to fabricating memristors, with the charge largely being led by HP’s research laboratories. The most well-researched version is the solid state memristor that was inadvertently discovered by Stanley Williams of HP. Solid-state memristors are fabricated out of titanium dioxide with electrodes on either side, taking advantage of thin film characteristics. Interestingly, a paper by F Argall et al published in SolidState Electronics in 1968 already discussed the “Switching phenomena in titanium oxide thin films,” three years prior to Chua’s publication on memristor theory. An example schematic of how TiO2 thin-film memristors are fabricated is shown above.

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Titanium Dioxide as a Memristor Although it is not a traditional semiconductor like silicon or gallium arsenide, titanium dioxide is a semiconductor and is used in organic solar cells, among other applications. It has some interesting properties that help it as a memristor but hurt its case to be used widely as a transistor. Namely, current applied through a titanium dioxide device (as shown above) causes ions to follow drift, and the device enters a mode called hysteresis— a crucial element for a memristor, as the definition of a system under hysteresis is that its current state depends not only upon its present but also upon its past. Charge stays at an upper bound and M becomes constant while current flows or does not flow through it, staying that way until the flow of current is reversed. The “pinched” hysteresis curve for a memristor (such as the device in Figure 2) is shown below, with voltage on the x axis and current on the y axis.

TECH ARTICLE

memristive systems. These systems take advantage of changing magnetic flux to alter the resistance of the device by altering concentrations of electrons of certain spins, changing the overall magnetic field alignment of the device.

Conclusion Memristor technology is well understood in theory, but currently impractical. The ability to store information without any charge is impressive, and while we have nonvolatile memory, there is technically still a chance that information can be lost over many years. Additionally, smaller-scale processors could be made where transistor alternatives are desired by using crossbar latches made from memristors. Further still, current flash memory densities can be far surpassed when memristors are able to be reliably and effectively manufactured.

About the Author Alex Toombs is a senior electrical engineering at the University of Notre Dame, concentrating in semiconductor devices and nanotechnology. His academic, professional and research experiences have exposed him to a wide variety of fields; from financial analysis to semiconductor device design; from quantum mechanics to Android application development; and from low-cost biology tool design to audio technology. Following his graduation in May 2013, Alex will be joining the cloud startup Apcera as a Software Engineer. ■ Figure 3: Pinched Hysteresis Curve Typical of Memristive Systems (Courtesy of Wikimedia user Blm19732008)

While these devices function in a fashion that illustrates memristance, they are impractical in their operation and do not scale well. Current speeds reported by HP are around 1 Hz, whereas transistors are currently available at hundreds of gigahertz commercially. Titanium dioxide memristors do not take advantage of magnetic flux as Chua predicted that they would, though there are other competing technologies like ferroelectric or spin-based

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VCC ISEN

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ICOMP

GND

ISL6730

VIN

ISL6730A, SKIP

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100

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TABLE 1. KEY DIFFERENCES IN FAMILY OF ISL6730

February 26, 2013 FN8258.0

VERSION

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ISL6730B

ISL6730C

ISL6730D

Switching Frequency

124kHz

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At this point in the series, youâ&#x20AC;&#x2122;ve set up Arch Linux ARM on your Raspberry Pi and you are ready to turn it into something. In this article, youâ&#x20AC;&#x2122;ll set up a Raspberry Pi as a simple local network server that can serve up web pages and act as an FTP server. This article will only explore setting up the server for use on local networks, not through the internet.

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Do so by inserting the lines reading:

127.0.0.1 localhost.localdomain localhost 1 localhost.localdomain localhost to be:

127.0.0.1 localhost.localdomain localhost webpi 1 localhost.localdomain localhost webpi To ensure your setup is complete, restart your Raspberry Pi, and then attempt to ping your hostname locally using the command ping webpi.

Note: Depending on the quality of your router, you may be able to skip this step. Try pinging webpi before editing your host file on your client machines If you haven’t already set up the “ sudo ” software and a separate non-root account on your Raspberry Pi, and you plan to have it accessible to the public on a network, I would recommend you do so. You don’t want people to have root access to your Raspberry Pi over the network; they could seriously mess with your setup or steal your files. There are plenty of good resources online for setting up sudo. A setup guide is available here for Debian. We’re using Arch Linux ARM, but the process is mostly the same (note: you have to use pacman on Arch instead of apt-get to install the software). Also, if you’re not familiar with command-line text editors like vi or nano, you can find many quick references online. One such reference for Vi is located here.

Hostname Setup You’ll need to define a hostname that you will use to refer to your Raspberry Pi Server. You’ll want to edit your hostname configuration file, located at /etc/hostname, using any text editor. For example you can use:

vi /etc/hostname

Now, get your local ip address by running the “ifconfig” command. It will most likely be of the form 192.168.XXX. XXX if you’re using a local home network. Make note of the ip address. If that is successful, hop onto another computer on the local network and attempt to ping the local IP address of the Raspberry Pi that you have noted down. If it works and you get a response, add that IP address to your hosts file on the machine you’d like to access the server from. If you’re using Windows, you want to edit C:/windows/ system32/etc/hosts, and on Linux you want to edit /etc/ hosts. Add the line:

<LOCALIP> webpi Try to ping again by using the “ ping webpi ” command. Hopefully it will work. If you’re stuck on this step, have a look at your router’s settings to make sure it isn’t interfering.

Apache Setup

You’ll then want to change the file to contain the name by which you want your Raspberry Pi known. In this example, my Raspberry Pi will be known as “webpi”. Any time you see the name “webpi” for the remainder of this article it should be replaced with whatever you’ve decided to call your server.

Now that we can access our Raspberry Pi on the network, we only need to setup a webserver to be able to serve web pages. A very common webserver is Apache. Install Apache on your Raspberry Pi by using pacman:

Once you edit the hostname, you should also edit the hosts file to let the Raspberry Pi know that it should refer to itself with that name. This time we want to edit /etc/hosts .

Before we start the webserver, we may want to configure Apache. To do so we must write the configuration files that Apache will use to run the server. Navigate to /etc/httpd/ conf. This is the folder that Apache stores it’s configuration files. We’ll want to edit httpd.conf, which is the main

You want to add 127.0.0.1 and ::1 as webpi.

pacman -S apache

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configuration file for Apache. The default settings will usually work for most applications. If you want you can go through and change settings as desired. For example, you may want the DocumentRoot (the folder the web server points to when serving files to a client) to be pointed to a different folder. Once that is completed, you can run httpd . This will start the HTTP Daemon which will serve web pages up to clients. To ensure it’s working, hop on another computer on your local network and enter webpi into the URL bar. You should get a page titled: “Index Of /.” At this point, we have a basic functional web server. If you haven’t changed the DocumentRoot of Apache then putting the following HTML into an HTML file somewhere in the folder /srv/http will make that webpage accessible through a browser by navigating to:

http://webpi/filename.html

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Setting up an FTP Server You may also want to set up an FTP server to allow you to easily upload and download files from your Raspberry Pi. After setting up FTP access to your Pi on the local network, you can use it as a central storage location in your home or workplace. Attaching an external USB hard-drive to your Raspberry Pi can allow you to store a lot of files in a way that’s easily accessible to anyone that has access. The easiest way to set up an FTP server is to install OpenSSH. Do so using :

pacman -S openssh Restart your Raspberry Pi and you should now be able to access your Pi’s filesystem using any FTP client. One graphical client is FileZilla . You can connect to your Pi using the following credentials:

hostname: <Your Pi’s Hostname or local IP Address> login: <Your Pi login> password: <Your Pi password> port: 22 If everything is working the login should work, and you’ll be able to upload and download files to and from your Raspberry Pi.

Configuring the Services to Autostart Last but not least, make sure that your server services are configured to automatically start when the Pi is powered on (this way you don’t have to turn them on manually every time the power goes out). In order to do that, use the following command:

systemctl enable httpd.service systemctl enable sshd.service The previous two commands tell Arch to enable httpd and sshd, our web and SSH/FTP servers, as system services, and they should now automatically start on boot. You’ve now set up a simple web server that you can use to make pages accessible to anyone on the network. While this isn’t useful on it’s own, it allows you to set up a numerous amount of web applications. There is a ton of open source stuff online, everything from open source scheduling software to project management tools. Having these applications available for free from any computer on your local network is a definite asset. (That being said, for many of these tools you will need to install a full Web Server Stack. For more information see: http:// en.wikipedia.org/wiki/LAMP_(software_bundle))

Conclusion Now you’ve turned a Raspberry Pi with Arch Linux ARM into a local network server that you can use to store information, data files, and more. Check back on EEWeb in the future to start learning how to develop some cool applications for a Raspberry Pi. ■

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VHDL 10 Part 3

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Paul Clarke Electronics Design Engineer

VHDL is all about creating logic. As a descriptive language, all the HDL languages bring something unique to programming â&#x20AC;&#x201C; making a true parallel logic circuit. As we take baby steps into VHDL, we will start Part 3 by looking at logic and working through to adding circuits.

m Logic Gates to Adders Visit www.eeweb.com

EEWeb PULSE Click the image below to read Part 2 of the VHDL series:

VHDL has something called ‘signals’. These are wires within the FPGA fabric that we can assign values or logic levels to and then they are used to carry them around. We generate these just before the ‘begin’ keyword in the architecture of the code. Here, we have generated four signals called Blue, Green, Red and Yellow. It is then very easy to see just after the ‘begin’ keyword where we generate the circuit — we assign each colour signal with the button value NOTed. Simply adding a NOT in front inverts the logic value. So then, when we press a button, the color signal is intuitive again (to us humans anyway!).

It’s very intuitive when using the logic keywords. In our first chunk of same code you can see how we The most basic logic we will all know and understand can AND, OR, NAND and NOR the signals together, is the primitive gates: AND, NAND, OR, NOR, XOR, feeding the result to the LEDs. XNOR, and let’s not forget NOT. The use of these is easy in VHDL, in that you just have to use the terms as they are. Looking back at our basic design, you will remember that we have four LEDs driven by four switch inputs. The switches had pull-up resistors so that normally we have a logic 1 input. This means pressing the button will generate a logic 0. This is not intuitive, but is a common way of doing things. However — for now, to make it easier to understand, we need to invert the button signals. Let’s add NOT gates to each of our incoming signals.

The next example chunk of code rounds this example off by showing how you can use the XOR and XNOR keywords just the same. However, you can see that we have introduced a more complex logic circuit. In VHDL you can use brackets just like you would in a math equation. By using them like this you can give priority to which signals are calculated first. Can you generate the truth table for LED 3 and 4 from the code given? As you can see, it’s quite easy. Let’s start trying to do some more interesting things with our logic. Many people may remember early digital electronics showing the example of adding

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bits together. Can we do the same in VHDL? Well yes — let’s start with a half adder. The rules are quite simple here, we use LED4 as the LSB, and LED3 as the MSB of our binary answer. With two signal inputs, our answer can be 00, 01, or 10. LED3 is only on when both buttons are pressed, so we use an AND gate. LED4 is only on in an XOR mode of the two buttons, so we use one gate here as well. Now our LED output will count how many buttons we are holding down at once. However, we have more than two buttons in our system!

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This next section is one that often confuses people a lot in VHDL — how to add, count and use numbers and logic together. What I will show you is not the best VHDL code, but I will demonstrate the different ways of handling bits, bytes, and numbers. Let’s consider the task at hand. We need to take four logic inputs, turn them into values or ‘one’ or ‘zero’, add them together, and then take our result and turn this back into logic that can drive our LEDs.

A full adder allows for three inputs. Normally this is two logic inputs and a carry, but we can use all three in this example. The logic for this full adder is a little more complex. The logic can still be written out, but it’s now harder to see what it’s doing. This full adder works well and we could just write the code for two full adders so we can add up all four inputs. But, I have to say, that this is all getting to be quite a lot of work. Suddenly, using primitive logic gates seems like a bad idea — we need something else. What we really need is a plus (+) sign like that used in computer programming. Well, VHDL can do this too. It’s not quite so easy, but let’s see how we can get there.

The first new type we will add is the Std_Log_Vector. This is a bundle of signal wires — unlike Std_Logic which is just one wire. We can only convert Std_Logic_Vectors and non Std_Logic in VHDL to numbers, so our example code will first pack the color inputs into a single Std_Logic_Vector called “Bits.” You will First, we must learn a little more of VHDL. As in see that each ‘bit’ within the Bits can be accessed computer programming, values come in different separately. For example, we assign bit 3 with the types. There are different sizes, 8bit, 16bit, etc. They value of the inverted button logic of blue. Our ‘Bits’ can be signed or unsigned, long or floating point, signal, however, needed to be defined a little differtypes. These rules of computer programming also ently — you will see that after the Std_Logic_Vector apply to VHDL, and using these types needs care. type we have “(3 down to 0).” This tells VHDL that there are a number of bits called 3,2,1, and 0 and So far we have only seen the Std_Logic type. This is the order they come in. (for our purposes) nothing more than a single signal wire carrying a logic 1 or 0. It has no numerical We now have our signals in the Std_Logic_Vecvalue, however, adding these signals together would tor type, so now we convert them into numbers. mean nothing to VHDL. VHDL does have another We need two steps to do this. First, we must turn type called a ‘integer,’ however — a real number the bundle of wire into a format that can become a with value and scale. number. Numbers come in signed and unsigned Visit www.eeweb.com

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Numeric Standard Conversions Type conversions

Function calls Numeric_std.

Signed

Signed()

To_integer()

Std_logic_vector() To_signed(,length) Standard.

Std_logic_1164.

Std_logic_vector

Signed()

Integer

Unsigned()

To_integer()

Unsigned()

To_unsigned(,length)

Std_logic_vector() Numeric_std.

Arrays

Unsigned

to go via two conversions. Here we generate another new signal called ‘answer’. This, however, has the type Std_Logic_Vector, and is 3 bits wide. The conversion this time is done using ‘to_ unsigned’ and the value 3. This value is the number of bits you want to use, to carry the result. Then we use the ‘std_logic_vector’ function to perform our last operation and generate the logic 1’s and 0’s in the answer. The final stage is to assign each bit to one of the LEDs.

This looks like a lot of work, but when it comes to adding two large numbers totypes, so first we use a type conversion of called ‘unsigned’. In our example, we only pass one of gether, or lots of numbers together, this is a lot easier the bits at a time. Our example could pass Bits as than using primitive logic gates. This is hopefully a whole, and that would indeed be valid, but would an example you can refer back to as a reference for create a unsigned type of its own. We then use the type conversion. ‘to_integer’ function to convert our unsigned value into a real number we can use — these can then be There is one last thing; If you build the above you added together. Remember, in this example we are will get an error. This is because you need to include only adding signal bits together; converting larger the library that allows the ‘+’ sign to work. You will need to include the numeric library at the head or values works in just the same way. your code. This is done by adding the line “use IEEE. Our sample code, as I suggested, uses just the ‘+’ NUMERIC_STD.ALL” at the top of your file. sign to add the values together. We need somewhere to store the result, however. Here we use ‘count’ to If you would like more help converting types from store this — ‘count’ is of an integer type. This was one to another, the image above shows the paths to declared in a similar way to our other signals us- take to get from one to another. ing the ‘integer’ keyword and the ‘range’ keyword. ‘Range’ allows us to tell VHDL the size of values it Next time round we will look at more of the structure is going to get — after all, these could be signals in we can apply to our code using the ‘if’, ‘case’ and the FPGA fabric and they need to know how many ‘where’ keywords. ■ bits to use to hold the number. Telling it the range of 0 to 4 means it will put aside two bits for us. Our » CLICK HERE result is then stored or assigned to this ‘count’ signal. Now that we have a result, we need to go back the other way and get these LEDs to show us the answer. As the LEDs are Std_Logic we need (just like before)

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