BasicPI

First attempt on block diagram for the 3D Sensor Hat. The main components are FXOS8700 (Accelerometer and Magnetometer), FXAS21002 (Gyroscope), NEO 6/7/8 (GPS) and BME280 (Temperature, Humidity and Pressure). I am also attempting a TFT connection here, but this is secondary and can be ditched if needed. The rest is classic – Raspberry PI/SPI on backbone, CAN as secondary control bus, USB/SWD and STM32F405RG to drive it all.

Final 3D of the Sensor Hat. The most noticeable here is that I start using the 2×5 pin SWD header. The rest is as described before.

• MCU STM32F405RG
• 1 x High Speed SPI for backbone
• Raspberry PI Hat format
• 1 x CAN for control bus.
• 8 x 2 x Analogue/Digital IO Signals.
• 3 x I2C ports
• 3 x SPI ports
• 1 x Power port
• 1 x USB port
• RTC w/Battery
• SPI Flash

Just added the package for CR1220. The actual package look at bit different, but this has the correct footprint and size. This looks like a large component, but it is not – it is more an indication of how small and dense these Hat’s are. I will attempt to update the next revision of XPortHub with this as well.

CR1220 is a small battery that in this case support RTC VBAT. I The battery have some size on the PCB, but it is all empty space underneath so PCB routing in that area should be easy – I also added the external power connector back. What I will do a bit later is to print a PCB copy on paper and add the actual connectors as an exercise to verify that it’s not to dense. I can move those 2 led’s into the card etc. I could also move leds in between USB and Power connector to avoid that they get to close.

The picture above show the green lines that needs to be routed. This looks straight forward – not to many dense ratnests. I probably need to turn J16, J17 and J16 around to minimize crossed signals – but, lets see. I am still on only 2 layers so I prefer to route as much as possible on top layer and use the bottom as ground plane.

Another concern is the 6 pin SWD connector I use here. I used a 2×5 pin earlier and wanted to simplify that since some of the pins was never used. But, a 2×5 pin is easy to get female and male headers for. The only solution I have for 6 – pin is to either use a 2×5 pin adapter or add a JST Micro connector. The intention was to use the JST Micro connector, but it was unpractical to connect/disconnect + it added 10cm wiring on SWD that made it challenging getting the SWD signals working. I have to think about this, but I am considering moving back to the 2×5 pin header and specialized adapters similar to the one below.

This is an SWD adapter I drafted a long time ago and never ordered. The adapter Connect between the 1.27 Pitch on the PCB and a 2.54 Pitch header that makes it easy to connect ST-Link. It show the male header, but I could modify this to connect directly to the small ST-Link SWD adapter and it should be straight forward to connect this even if a Hat is in the middle of a stack. The original design added 5V and a RX/TX, but I want to simplify this for GND, 2 SWD signals + Boot + Reset + 3.3V. I actually only need 3 pins for a SWD alone, but I need 3 extra pins to support Boot and Reset through the adapter. I have never needed Reset, but I have used Boot a few times.

I have still not routed this, but schematics and component location is getting in place. I have to make a package for CR1220 battery holder for RTC + I would like to make the proper packages for the connectors as well. I dropped the extra CAN and RS485 due to lack of space + I have other cards for those. I also ditched SD card due to lack of pins, but I still have the SPI Flash.

I also have to think a little about what Power I feed out on the ports. I am currently using 3.3V, but I want to add a jumper to select between VIN and 3.3V for the Hat. Ideally I should have done that per port, but that would require 30 jumpers. It is possible I can manage separate for AD, I2C and SPI. Many of the available breakout boards uses 5V due to Arduino, but that said our AD ports do not have 5V tolerance.

STM32 have a lot of 5V tolerant pins, but analogue pins that I use on AD ports are 3.3V. So giving 5V out then you only can receive 3.3V signals might be a conflict. I could add a level shifter, but I actually think I prefer to stay with 3.3V. As always – work in progress.

My backlog of designs I want to do and PCB’s I need to work on is getting huge so I decided to be a bit more systematic and make sure I do proper doc on each design. The “doc” I use is a small document I call “annotated schematics”. It basically take the schematics, block diagrams, 3D models etc and add some notes to them in a paper. If I do a Revision it will add a list of changes to the previous revision etc. This doc only takes a few hours, but it is worth gold a few weeks down the line then things have been forgotten.

The backlog of designs will continue to increase. The main reason for this is because I find it quite relaxing to sit and work with schematics/PCB routing in spare moments. SW is a bit different as it is more complex and require Focus.