[BUG] Dev Kit Power Management Insufficient

[ZAdamMac]

There is a design flaw in the Revision B Rear Expansion Board (REB); alternatively in the dev kit overall, depending on the desired resolution.

The REB presupposes that it will always receive "sufficient" power from the two AA batteries held in series. This is nominally true for the purposes of running the MSP430fr5994 and most of the accessory components. However, the SHARP LCD requires a minimum input voltage of 3V.

This power input voltage is driven from the GPIO pins on the MSP430. The 430 has power management, but no voltage regulation, and digital out is always limited to Vcc, which is the voltage recieved from the input pin, ie the voltage of the battery pack.

When driving this with alkaline or carbon batteries, with a nominal voltage of 1.5V per cell, this is fine and we get enough voltage to drive the display. This was how the power circuit was validated before the boards were milled.

However, when driving with NiMH rechargeable batteries, each cell has a nominal voltage of 1.2 V and even in series we fall ~0.5V below the threshold to drive the display.

For the purposes of the Dev Kit this is not necessarily an issue; in fact, the flatened curve makes it faster to test things like battery degredation and handling low battery conditions. However, for the final toy design and maybe even as an experiment with the dev kit, it would be nice to fix this problem, by either:

• Implementing some kind of power management between the battery package and VCC which would hold the 3V3 rail up around 3 V, potentially requiring a re-imagineing of the low-voltage detection circuit (And likely a rev C of the Rear Expansion Board), and;
• Implementing similar voltage conversion for at least the LCD-Power (and possibly other LCD driver pins) as a shim module between the controller and the display, which has its own limitations.

A decision on how to fix this problem is due by February 28, 2022

[ZAdamMac]

To preserve it for the record, the resolution will be option 1: full regulation of the voltage before supplying it to the 3.3v Rail.

[ZAdamMac]

I want this to be resolved before completing the 0.1.0 firmware release.

I also think that this will give us the ability to indicate a low battery condition through the regulator IC.

[ZAdamMac]

This was validated in breadboard today using the TPS60204. My local branch of the repo has an eeschema file with the necessary changes to the breadboard schematic, but I don't have time today for footprint golf so I won't finish and upload the design for the new PCB until later.

Until then this issue remains open.

[ZAdamMac]

The new eeschema and pcbnew files are finished, and I've rendered new gerbers for the new boards. However, I'd still like to order them in and actually assemble one to validate the new circuit before I upload the files to the repo and close this issue out.

[ZAdamMac]

This was tested and corrected ages ago; the new hardware files are more or less sufficient.

However, hardware data is going to be broken out into another repo soon so I'm just closing this outright at this point.