This page provides suggestions and resources for designing the PCB for this project.
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Table of Contents
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Summary of To-Do List
Note about Design Software
Altium Designer was used to create the initial PCB schematic for this project - you can apply for a student license here (using your student email account to create an account). If you are new to Altium, there is an excellent tutorial on getting familiar with the schematic and board designs.
Links to Footprints and Schematics
Note from Magdalena - I have included all the components that I used for the PCB project in the big SSMachMo folder under CAD. In the event that there are problems with the footprints, here is where I found the footprints and symbols for the major components (all other components were found in the Manufacturer Component search tool):
- Processor
- Full CAD for this part (symbol, 2D footprint, 3D footprint) were provided through Mouser (here). Don't forget you need to download Altium Library Loader from SamacSys to port parts over from Mouser to Altium.
- Hygrometer
- 3D footprint from SnapEda (here), made by Sensirion engineers so these dimensions are highly reliable.
- There was not explicit 2D footprint or symbol, so I adapted them from a similar DFN-4 package part from Sensirion on SnapEda, the SHT3 (here). This is also made by Sensirion engineers, and the dimensions are supposed to be the same for the two hygrometers, but I am suspicious of the spacing on the 2D footprint for this part because when I use the dimension tool, it looks to be 2.5mm for the lengths of the sides as opposed to the 1.5mm for the sides of the SHT40. I have sloppily corrected this in Altium, however I highly recommend changing this footprint - perhaps by the time you read this, the 2D footprint has been released online, or perhaps you can make a better one from scratch.
- I also manually changed the pin order to reflect the change from the SHT40 to the SHTC3. I high recommend double-checking this before you proceed.
- Accelerometer
- 2D and 3D footprint harvested from SnapEda (here). I made the symbol using the symbol wizard in Altium.
- RTD
- Placeholder for a 2-SIP connector package. The pitch is currently 2.54mm, which will likely need to be narrower for the final version.
- Microphone
- Found in "Manufacturer Component Search" under IM69D130V01XTSA1.
Helpful Design Diagrams
Processor
For design guidelines for the MCU and Bluetooth antenna, be sure to also read [1], [2] and [3].
Pin diagram (image from [4]):
References
- There is a lot of documentation on this subject, too much to capture in a few select links. This SiLabs KBA has an extensive list of Silicon Labs white papers and resources on the subject.
Hygrometer
Pin diagram (image from [5]):
Typical application diagram (image from [5]): The big takeaway here is to remember the pull-up resistors for the I2C bus (like any I2C bus). They recommend 10kOhm resistors. (currently implemented)
Accelerometer
For design guidelines for this sensor, be sure to also read [6] and [7].
Pin diagram (image from [8]):
Pin description (image from [8]):
Typical application diagram for SPI (image from [8]):
Extra insight from the manufacturer taken from the additional caps and resistors from the evaluation board (image from [9]) (not currently implemented):
References
- Design Guidelines for the KX134-1211
- Design Guidelines for Soldering and Mounting KX134-1211
- Accelerometer Evaluation Kit
Microphone
Pin diagram (image from [10]):
Application notes (image from [10]): (currently implemented)
References
RTD
This temperature sensor is functionally a resistor. There is no pin diagram. Our chosen model is 1kOhm at 0°C.
For an RTD, the most reliable way to measure the voltage is using a Wheatstone Bridge - you can find an online calculator like this one to optimize your bridge (image from [11]) in conjunction with an RTD resistance calculator (this is possible because RTDs are standardized):
To get a ROUGH ESTIMATE of the maximum temperature you might be measuring, I recommend considering the skin temperature of a motor at ~100°C [12] and put on an appropriate margin to compensate for the thermal paste keeping the RTD in the sensor node and the lack of air circulation inside the device.
References
Radio
This PCB design will need to incorporate a radio somewhere inside for the Bluetooth stack to be able to transmit.
Recommended RF matching network (image from [4]):
References
- There is a lot of documentation on this subject, too much to capture in a few select links. This SiLabs KBA has an extensive list of Silicon Labs white papers and resources on the subject.
Debugging
To be able to develop the software on this board (as well as flash your firmware and install a bootloader in the first place), there will need to be some kind of interface between this board and another computer, such as a USB hub or a Segger J-Link port. This could either connect straight to a computer, or it could connect to the SLWSTK6021A Evaluation Board that we already own and have used to connect to radio eval boards.
References
- AN958: Debugging and Programming Interfaces for Custom Designs - Essential reading material to solve this problem. Compares different approaches and makes very good recommendations for providing a debugging interface.
- AN0002.1: EFM32 and EFR32 Wireless Gecko Series 1 Hardware Design Considerations
Power Supply
All of the components that will be on this computer, including the processor, all sensors, and all other peripherals, can take in power from the 3.3 V voltage rail. Care will be needed to ensure that this power remains constant to the processor (at the expense of the other peripherals, if necessary), that no power can be back-channeled into the battery, and that there are no resonances from parasitic capacitances near the data lines.
The battery has not been chosen yet. This might require creating a spreadsheet to track all the predicted current draw and then comparing it with the current draw of a first prototype for this device. Care might be needed to cut down on parasitic or power-hungry processes in the device firmware.
References
- AN0948: EFM32 and EFR32 Series 1 Power Configurations and DC-DC - This is crucial reading for designing the power source with our chip.
- AN0002.1: EFM32 and EFR32 Wireless Gecko Series 1 Hardware Design Considerations
Final Resources
- At the end of your design, you can create a technical support ticket and have Silicon Labs engineers review your PCB. This has been recommended by several employees on the Silabs Community Forum.