In the early stages of product development, it is common to constantly be re-programming PCB's for both firmware development and building prototypes. To quickly flash boards, a simple development fixture can be designed to program boards and run early tests.
This is part 1 in a series that covers FixturFab Development Fixtures. In this first part, we cover designing a development fixture to be 3D printed for the SparkFun RedBoard Artemis using the FixturFab Automated Design Service.
Development Fixture series
- Designing a 3D Printed Development Fixture
- Assembling a 3D Printed Development Fixture
- Test Instrumentation for Development Fixtures
- Designing a Laser-Cut Development Fixture
- Assembling a Laser-Cut Development Fixture
This guide goes over uploading an Eagle PCB file and selecting the test points needed for flashing the board and running tests. Then we demonstrate how to generate the CAD files for 3D printing the fixture.
To follow along with this tutorial you will need the design files for the SparkFun RedBoard Artemis.
You may also want to install Eagle to view these files.
PCB Test Features
The SparkFun RedBoard Artemis is a development board based on the SparkFun Artemis module which contains an Apollo3 from Ambiq as its core IC. This is an ARM Cortex-M4F IC. The development fixture will use test probes (pogo-pins) to interface with the 11 test points and several other nets on the RedBoard Artemis.
After downloading and unzipping the Eagle design files, open the RedBoard-Artemis.brd and RedBoard-Artemis.sch files. From the schematic file (.sch), we can identify the 11 test points and select other nets that we want access to.
Three test points for the JTAG interface to flash the ARM microcontroller. This interface needs Vdd and GND. There are multiple through holes for these on the bottom on the board that we can treat as test points.
- TP1 - SWDCK
- TP2 - SWDIO
- TP3 - RESET
Two test points are provided for the serial com on the processor.
- TP5 - RXI
- TP6 - TXO
The Artemis module has 6 additional IO pins that did not fit the Arduino header footprint of the RedBoard. They were given test points though that can be exposed with this fixture.
- TP8 - SDA3/MISO3/RX1
- TP9 - SCL3/SCK3/TX1
- TP11 - MOSI3/RX1
- TP12 - SCL1/SCK1/TX1SCCLK
- TP13 - SDA1/MISO1RX1SCCIO
- TP14 - MOSI1TX1PDMCLK
After visually inspecting the bottom of the board and confirming in the .brd file, 4 pads were exposed for the USB C connections. We can treat these as test points and manually add probes to access these nets. We will also use Vusb to power the DUT in this fixture.
There are four holes in the board that we can add guide pins for to mount the DUT in the fixture.
Designing the Development Fixture
Starting at fixturfab.com, click either the Create Fixture button in the navigation bar on the top right or the Start Designing button in the middle of the screen. If you are not signed in, you will be asked to log in or create an account.
Create the Project
On the Create page, enter the Project Settings. We will name this fixture SparkFun Artemis Development Fixture, keep the PCB Thickness at the 1.6mm default, and select the RedBoard-Artemis.brd file to upload. You may also enter an optional description for the test fixture.
Click the Create Project button which uploads the PCB design file to be parsed for test features like test points and holes in the DUT for guide pins.
On the Design - Fixture Type tab, select the Toggle Clamp Development Fixture.
Configure the Fixture Settings
All the information collected from the PCB design file will then be displayed on the Design - Fixture Settings tab. This page allows you to modify the PCB size and thickness and configure the test points and mounting holes to be used in the fixture.
The first section of this page shows the Fixture Project Settings. Verify that the PCB width, PCB height, and board thickness is correct.
Configure Test Points
The next section shows the Test Point Configuration. Modify the table with the test points we outlined in the PCB Test Features section of this post.
- Three dedicated Test Points for the JTAG interface
Vdd, 5V, GND
- Multiple Vdd and GND pins help reliability. A rough guideline is one Vdd per 1A of expected current draw.
- Add access to 5V for powering the board.
- Note these are through-hole, instead of SMD test points.
Serial COM test points
Artemis Module test points
The USB C pads were not parsed as potential test points by FixturFab so we will add them manually with the “Add new test point button.” Before grabbing the coordinates, set the grid to mm.
- Vusb (1.905, 44.577)
- D- (1.905, 42.799)
- D+ (1.905, 41.021)
- GND (1.905, 39.243)
The four USB C pads were conveniently exposed by SparkFun on this board. Even though they were not parsed as test points, they can still be accessed via a pogo-pin. With FixturFab you can add manual test points for any exposed pad, all you need are the coordinates.
Configuring the Guide Pins
The next is the Guide Pin Configuration. In this table, verify that the four mounting holes were identified.
Mark the other rows as disabled. A UI update is coming to improve handling this many potential test points!
Leave the Pressure Pin Configuration, and Hole sections empty (they are not used by this fixture type), and click the Next button on the bottom right to generate a preview of the development fixture.
Review the Generated Model
The Design - Configure Fixture tab displays the various settings for the test fixture along with a 3D render of the design. Make adjustments here such as moving the DUT or clamp to align the fixture.
For this fixture, no settings need to be modified. To generate the final CAD files, click the Generate Fixture button at the bottom of the page.
Download CAD Files
To download the CAD files you need to purchase the fixture by clicking the Purchase Fixture button. If you're testing FixturFab's capabilities or following along with this tutorial, send an email to email@example.com with the subject RedBoard Tutorial, and we'll unlock the files (for free) for your project.
After submitting your payment, the CAD files are available to download as both STL's (for 3D printing) and STEP's (for CNC machining). Future releases will soon provide the DXF drawings, a BOM, and CAD files specific to laser-cutting.
In the next part of this series, we will cover downloading the CAD files, The BOM for a 3D Printed Development Fixture, printing the Fixture Base and Probe Plate, and assembling all the mechanical fixture.