May 21, 2020| |
In this tutorial, we will cover schematic design in KiCad. We will also discuss high-speed PCB design with the identification of differential pairs and the marking of high-current traces.
How to Create a Schematic Design in KiCad
We will demonstrate how to create a simple schematic design in KiCad and we will talk about the factors we need to consider when doing a schematic design for a high-speed design. Let’s start with the schematic part.
First, you need to create a new project. Go to File on the left-hand side, click New and Project. Choose the location where you want to save this new project and name it. In this demo, we will name it LED project. Click Save.
As you can see, the two files are created over here. There are the layout file and the schematic file. Let’s open the schematic file first.
How to Place Components on a Schematic
For this demo, we are considering a simple schematic design for a battery, a regulator and a LED. This schematic will be used to light up a simple LED.
Click this symbol on the left hand side and a window will pop up. Search for a battery among the components. We will select a single-cell battery, as you can see over here. Simply click the screen to place the battery on the schematic.
Let’s repeat the same action to find a resistor. Select the one you want and place it on your schematic.
Let’s now choose a LED and two capacitors and regulators.
You can see over here an unpolarized capacitor, which is basically a ceramic capacitor, and a polarized capacitor, which is for electrolyte. We will take the unpolarized one for now.
How to Create a New Library
Now we require a regulator, which we will create. This is very simple. Click Create, delete and edit symbols on the top. A window will pop up. On the left top side, click File and select New Library. For this example, we will name it LED Project and save it to the library table Project.
Watch our tutorial on how to create a symbol library in KiCad.
Now go to File again, select New Symbol and search for the library that you just created. Name the new symbol, such as, in this case, LM7805, which is a five-volt regulator.
You need to create a symbol for every component. Every component has its own data sheet and its own serial part number.
You need to find the data sheet for the part number. You can see on this screen, this is the data sheet of LM7805. You need to see the pin configurations between this data sheet, which is registered by the manufacturer.
In this case, there are four different packages. These packages depend on the manufacturer part number or MPN . so you will have to decode the MPN which will give you the correct package. For now, we will require these three pins: input, ground and output.
You can see that there are more connectors, such as Tab. They all are ground.
How to Add Pins to Symbols
In this case, you will have to create one more pin to have an additional pair on the board. The ground will have two pins, which are Pin 2 and Pin 4. Pin 1 will be input and Pin 3 will be output.
Let’s start and move the components aside. We will use the Add graphic rectangle to symbol body option on the right side. But before that, right click and select End Tool. You have to select the grid first so right click again on the screen and make sure your grid is set to 50 mils. Now, you can click this box and draw. Right click and select End Tool. Right click the border of the rectangle, select Edit Rectangle Options, Fill with body background color, and click Ok.
On the right hand side, click the A1 symbol, Add pins to symbol, and drag it to the screen. Here the pin name will be input and the pin number one. Place this over here.
The next pin is output and pin number three. Right click and select Rotate Clockwise. One more time, Rotate Clockwise. Then, you need two more pins which will be ground. The first one will be Pin 2 Again, right click and select Rotate Clockwise. You can also rotate by pressing R on your keyboard. And repeat the same action for Pin 4.
This is completed. Pin 1 is input, Pin 3 is output, Pin 2 and 4 are ground. Let’s save this by clicking Save all changes on the left hand side. Once this is saved, you can import this in your schematic design file. Go to the main schematic file and on the right hand side, select Place symbol. A window will pop-up where you can find the LED project.
You can see LM7805, the symbol we created. Select it, click Ok and place it on your schematic. You now have all the symbols required to proceed. You can then place these components properly and make the connections. Let’s zoom in a little bit.
How to Connect Symbols in KiCad
On the right hand side, select Place Wire.
Now, over here, click this symbol and then click on the other side. This is how you can form the lines to connect your symbols.
When you are done, you can right click and choose End Tool. You can see this small square over here on the line, it means the line is not connected. So select this line and delete it. Again, select Place Wire and make the connection. Repeat everywhere it needs to be done.
You now need to place the ground terminal. Click the Ground option on the right hand side and click on the screen. The Choose Power Symbol window will appear. Search for GND, select the symbol and place it on the schematic. Select Place Wire again and connect the negative to ground. Right click and select End Tool.
How to Annotate Symbols
The connections are done for the part of this schematic design in KiCad. Don’t forget to save the file. As you can notice, there is a question mark next to the U symbol. It means you have to annotate this symbol. On the top, there is an option called Annotate Schematic Symbols. Select it and choose Use the entire schematic and Keep existing annotations. Then, select Annotate.
Now you can see that the question mark has turned into a number. The tool automatically gives numbers to all the components. Again, don’t forget to save.
The schematic design is complete but you need to assign footprints. We show you how to in another video since you might have to create a new footprint. The footprint information will be given in the datasheet.
How to Generate a BOM in KiCad
This is how you can create a simple schematic. You might also have to generate a BOM, or Bill of Materials, which tells you which components are in your schematic, as well as the similar types of components you have and a complete list. On the top, there is an option called BOM. Click it and this window will pop up. Select bom_html_grouped_by_value.
In the Command line section, you can see .xls. Make sure that you have this extra .xls before you select Generate. This will generate a file in the folder of your project.
You can see over here, you have the reference, the quantity, the value and the part. This is your complete list, the complete data of your schematic design that indicates which components you will request.
How to Create a High-Speed Schematic Design in KiCad
Now, we will show you the basic factors you need to consider while designing any schematic. Let’s close this schematic and open one of the schematics we previously created.
This is the schematic design of a microcontroller that is interfacing with a FTDI chip and a USB. USB is basically a high-speed signal, so it normally operates from 12 Megahertz to 14 Mbps. For such cases, you need to be careful when designing the layout.
As you can see, there are certain power lines here and here. These lines can carry power up to 1.5 amps. You need to design the specs to meet this requirement of 1.5 amps. You can also see that this firewall is coming over here, so you can mark this particular part as a power line, but not this part, which is also connecting the same line.
Here, the voltage is 5 volts, so the current will be 5 milliamps. It is not of much importance. This can be a simple trace of 10 mils or 8 mils
This connector is marked as power pin because 5 volts are directly going to it and we don’t know where this will be connected. It will depend on the person who will be using this. By marking the pin as power pin, we indicate that the trace needs to be sufficient to carry 1.5 amps of current.
Impedance and Differential Pairs in KiCad
Let’s move on to another sheet.
This sheet has the FTDI chip which converts the input serial signal into a USB differential signal. The other yellow rectangle is the USB connector. Here, we are using a USB 2.0 which can operate from 12 megahertz to around 240 megahertz. You could also use a USB 3.0 which can operate up to 5 gigahertz.
Normally, we use differential pairs. Differential pairs are nothing but two lines carrying the same data in an inverted form with respect to each other at a particular point of time. You can see that we have DIFF_DN_90R and DIFF_DP_90R. These are the two pins that bring data into your IC from this connector. This is a negative line and this one is positive.
For the USB, you need to a controlled impedance. Basically, the impedance varies at very high speeds, like in megahertz, so you need to take care of certain factors, like impedance matching.
For instance, this IC over here has input impedance at these pins of 90 ohms as defined by the USB standard. The differential tracks should match this impedance. In order to do so, there are certain rules you need to follow, such as the width and thickness of the track.
There is also crosstalk, which is nothing but interference on one line due to another trace which is close by. You need to maintain a specific distance between these two tracks. The width of each track should be as per the requirements. The trace width and spacing for a certain impedance may be obtained using impedance calculators.
The length matching of these differential pairs is important because these two lines are time-dependent or delay dependent. A signal comes from a connector at a particular time and reaches its destination at a particular time. The signals from both these tracks should reach at the same time. If the lengths are different, and since both signals on both differential pairs operate at the same speed, this can generate a false signal at the output.
This is the power track which operates at 500 milliamps. 500 milliamps is considered high for a USB. We normally put less power, less current. And this is how you get started on your high-speed schematic design in KiCad.
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