Week 6, Electronics Design

Objectives

Group assignment:: Use the test equipment in your lab to observe the operation of a microcontroller circuit board (as a minimum, you should demonstrate the use of a multimeter and oscilloscope); Document your work on the group work page and reflect what you learned on your individual page
Individual assignment:: Use an EDA tool to design a development board that uses parts from the inventory to interact and communicate with an embedded microcontroller

Group Assignment

Design a Development Board

For the PCB design, I built an expansion board for the XIAO that will drive LEDs for my project as well as be adaptive for other projects.

KiCad

I have designed PCBs in the past. I have used Express PCB and I have also used Fusion. So for this project I decided to explore KiCad to expand my horizons.

I started my build by working through the Getting Started tuturial on the KiCad website. It walked through the build of a simple LED and battery circuit to explore the different tools and workflow of the software.

The first step was to setup the schematic. This tutorial used a CR2032 battery, a 5mm LED, and thru hole resistor.

Once the schematic was complete, the board layout tool was used to place the components with real footprints. I drew a board outline on the 'Edge.cuts' layer to define the outer shape. Then I moved the battery to the back copper layer, and added traces to connect all of the components. Then I used the fill block to create a ground plane on the back side of the board.

Once the board was laid out, I was able to do a 3D preview to see what the board would look like. It was helpful to see it rendered to verify that it looks as expected.

Then I took it one step further and submitted the board for quote from a board house. I output the gerber files and pushed them up to PCBWay. I input the board dimensions and left the default values for the rest of the settings but chose purple for the solder mask. Then I uploaded the files. The first time I pushed them, the audit failed because I chose the wrong file. Then I re-expored and zipped the files and re-uploaded. The board then passed the audit and I was able to view the rendering of the PCB on the website to confirm it uploaded correctly.

PCBWay order screen showing the board passed the audit and is ready to order.

XIAO Test Board

Next I decided to build a PCB for the XIAO board. To start, I wanted to recreate the setup that I simulated in week 4 which had addressable LEDs and standard single color LED.

XIAO Library

Since KiCad does not come loaded with the XIAO libraries, I had to download and install them. I was able to find the libraries on Github and downloaded them. Then I used the "Manage Symbal Libraries" tool to associate the XIOA board local folder to the library.

Schematic

Then I went to the schematic and placed the XIAOESP32C3 in the workspace. I then added a WS2812B component and rotated it to match the VSS and VDD with the 5V and GND pins of the XIAO. After reviewing WS2812B info on Adafruit they recommended putting a 470Ohm resisitor on the data pin, so I placed a resistor on the board and placed it near the D10 pin. They also recommended putting a 1uF capacitor every few pixels, so I added a capacitor across VDD and GND too. Then I updated the preferences for each component to have 1206 with hand solder footprint and assigned the values.

Then I added a standared LED. I placed an LED component on the left side of the board near pin D3 and added another resitor component. I used the Digikey Resistor Calculator to find an appropriate current limiting resistor. I am using a green surface mount LED so I used the specs from the data sheet to populate the calculator and it revealed that a 65 Ohm resistor would be appropriate. So I placed another resistor on the board and gave it a 65 Ohm value and 1206 footprint.

I then used the 'draw wires' tool to make all of the connections between the XIAO and the components. I connected 5V from pin14 to the addressable LED, GND from pin 13, and pin 11 data line to the resistor and then to the Din pin. Since there is only 1 LED Dout was left unconnected.

I then hooked up the passive LED and resistor on the other side of the board.

I finished the layout tool by running the design rules check. The errors listed were mostly around unconnected pins which are not an issue.

Board Layout

I then transitioned to doing the board layout. I opened the board layout file and used the 'Update PCB from Schematic' tool to place the components on the board. The initial layout had all the connections but the wires were all twisted, so I rotated and moved the components around to make a pleasing layout.

Then I used the 'Rout Single Track' tool to make the traces between all of the components. To create the board shape I clicked into the 'Edge.cuts' layer and used the polygon tool to create a pentagonal board shape around all of the components.

I then explored the 3D rendering of the board in the 3D viewer tool to verify it looked like what I expected.

Fully Featured Board

That board design was fine, but I wanted to take advantage of more pins on the XIAO. So I went ahead and updated the design to have more IO that would be helpful for later on. Specifically, I added ports for I2C periferals, 2 analog connections, 2 digital connections, a port for a longer addressable LED strip and an onboard button.

Schematic

I copy and pasted the previous schematic and board files to create a new design. Then I started adding the requisite components. I started by adding .1in 3 pin headers for the analog pins. I connected them to 3.3V power and ground and to pins A0 and A1. Then I added 2 4-pin ports for the I2C connections and hooked them up to the SCL and SDA pins. I used the Adafruit pin order to more easily match up with their perifferals. Since they run off the same wires, I was able to double up the pins directly.

Then I added the button and a resistor. I chose an SPST button from the component libray from the lab. I used the generic schematic symbol in KiCad and then used the footprint that matched the B3SN. I wired the button to 3V and GND with a 10k resistor to ground to cut the current, then dropped one leg from the ground leg to pin D6.

I moved the LED to D7 to be closer to the LED. I thought this would be a more interesting place for the LED and would keep it out of the way of the analog pins. I kept the same LED component and resistor as the first board.

I used D8 and D9 for the generic digital IO pins. These were tied to power ground and the appropriate pins, and I used a 3 pin connector.

I added a 3pin connector to extend the addressable LED ring to make it possible to add a longer strip. This will likely be important for my final project. I wired the data pin to the data out from the onboard addressable and took power from the 5V rail.

Finally, I added a 2-pin connector as an alternate 5V power input should that be required for longer addressable LED runs.

Board Layout

For the board shape, I wanted to create a pentagonal design to distinguish between the 5 different types of IO. So I placed the XIAO at the center and put the button and the green LED below it. On the left side I placed the analog pins and then put the I2C connections at 45 degrees to it. On the right side, I placed the addressable LED and the addressable LED connector on the upper right. Then I placed the digital pin connectors at a 45 degree angle on the lower right. Finally, I put the alternate power input on the upper right as it was close to the 5V and GND pins and easy to route.

This layout also gave me the ability to route all of the traces in such a way as to be able to keep them all on the top side of the board.

It was at this point that I realized that I had a big problem. While this layout is fine if I was going to order the PCB from a board house, it is unsuitable for making it in the lab. The traces all have to be on the bottom for the header pins of the XIAO to be effectively soldered to the board. However, I have surface mount components on the top. So there is currently no way to guarantee that the connections would work properly.

Board Layout Update

To make it easier to make the board in the lab, I made some key updates. I moved the traces running to the headers on the bottom of the board. That way I could solder the XIAO headers to the bottom of the board and be sure that they would make good connection to the periferals. Then, I added vias in key areas to move power, ground, and signal wires from the bottom to the top. The vias will allow me to connect the top and bottom of the board by soldering a header pin on both sides of the via to make the connections. Then I updated the trace widths for all the traces from .2mm to 0.5mm to make them more substantial.

During this update I found a mistake on the connection from the button to the XIAO and was able to fix that too.

Then, to give the board a unique shape, I drew the pentagonal design in SolidWorks. Then I exported it as a dxf and imported the dxf into the 'Edge.cuts' layer on the board. This gave me the ability to control the shape of the board in a nicer way than using the drawing tools inside of KiCad.

Updated board design with traces moved to the bottom of the board and vias to connect to the surface mount traces.

Then I rendered the PCB with the rendering tool. I also changed the soldermask color to purple to make it more interesting.

Files

Tutorial PCB

XIAO LED Board

XIAO Development Board