It's been quite some time since I last updated this, been a bit busy lately getting my 13.5kg combat robot up and ready for its first competition in mid April. But anyway back to the PQ60 EPS! I have been working on this in the background, namely looking at the new PocketQube (v1.1) spec (available at http://pq60.info/) and seeing what I need to update on my EPS to bring it into line.
The big difference between the PocketQube v1.1 spec and v0.2 spec is the pin assignments of the 60 pin connector:
- Ground pins are no long spread out and are next to their respective power supplies
- General Purpose In/Out (GPIO) pins are now grouped together instead of being spaced out
- USB has been completely removed
- Power pins have been grouped together and have detailed current capacities.
Overall I think these changes are good, they've simplified the pin arrangement massively. I hope to now re-design my board to fulfil the full PQ60 spec and supply the required 800milliAmps on each of the two main power buses as well as the 400milliAmps on each of the six switched buses. However I disagree with the need for switched power buses.
I agree with the need, it is important that the On Board Computer (OBC) can turn off current consumers, known as load shedding, however I feel that it would be better served by having this managed by a local I2C controlled switch on the load's PCB. This could be done with a dedicated microcontroller that allows for load telemetry/control or even a barely smart load switch like the Texas Instruments TPS22994 or the Linear Tech LTC1710 both take up ~10mm2 of board space and ~20microAmps.
In addition to the spec related update I also went through the footprints to check that I hadn't made a mistake which would make the first boards really hard to get working. I had, nuts! My inductor footprint for the Linear Tech LT3105 energy harvesters was wrong, really wrong. The correct footprint was much larger and will require a redesign of the EPS.
Whilst I continue with the changes to the EPS I've been thinking, while EPS is very important, but it's also important to have communications and command processing so I may expand this entry to add a radio board and a computer board for a full PQ60 solution.