Obviously any development project is going to hit bumps in the road as it progresses, there are going to be all sorts of things that you need to take into consideration and prioritise when working on such a project, just as you do in any corporate role where change programmes are running. The various types of change we are going to see are :
- Design problems - things that are wrong in the current design
- Design changes - new requirements, ideas and changes to scope
- The day-to-day activities, what works, what doesn't work, where we have got to, since this is a part-time, fun project, not the daily 9-5 day job.
So, how to record all of these ?
My preferred way of handling such change is to keep a journal, in a word document, recording all the things I've been doing, my thinking and progress, this is better for me than keeping a log book, as that would be in the way on the bench and I can search for things quicker in an electronic document.
I also keep a spreadsheet of all the problems I've found (a defect tracker), their cause, resolution and any workaround that may be possible. This also has a change log for each printed circuit board (PCB) that I've created, so that when I come to make another one, I know what modifications I need to make to it, to bring it to the current working version.
There is also the challenge of the circuit design and board layout, which will evolve over time. The best way I've found of working through these is to have the major version (i.e. v1.0) as the design that covers that boards that were actually manufactured. Since this is the second iteration of the design, the PCB's are v2.0, or v2, rev 1. The workarounds made to the current boards (also known in the industry as bodge wires) then form the next minor version, so v2.1 or v2, rev 2. Any design updates that can't be made to the current board and require a respin of the board are then the next version up, so v2.2, or v2, rev 3.
This means that I can look at what I had manufactured, what I'm currently working with on the bench and what I plan to make next when I send for the next board version. This is easier than trying to remember the context and detail of each issue, at some point in the future.
The source code also takes a similar approach. The current release of code is what works against the v2.1 / v2 rev 2 boards, but I'll add into the code comments TODO_NEXTBOARD or #ifdef TODO_NEXTBOARD with information about what needs to change to support the next code version, this can be pin layout changes, new functionality that needs adding, etc. It all just helps aid memory when you need it.
So, returning back to the previous point of the Pi's power, this is not defect number 1 though, since I found and recorded some defects during assembly of the boards, such as pin 1 not being present on the connectors, and the polarity of the polarised capacitors is not present and supply problems of some of the Analogue to Digital Converter (ADC) chips, which, at the time were impacted by the semiconductor shortages after COVID.
In general, the solution to working around defects in the boards is to think creatively about how you can fix it, can you cut a track using a scalpel to make a couple of mm gap, but not going deep enough to cut into the next layer inside the board, which is really close to your current layer Consider that there are 6 layers in the 1.6mm board thickness, so less than 0.4mm below where you need to cut. This is also why many people put the signal layers on the top and bottom of the board, leaving power planes internal, but professional boards will have many more layers and many of them will include signal traces inside the board. At that point, the chances of respinning a board is much higher if things go wrong in design.
Its also possible with surface mount and through hole components to solder another component on top of an existing, since they share the same 0603 footprint. This is handy to adjust resistor values, or to put a decoupling capacitor over a voltage sensing resistor divider. Older leaded components can also be used to bridge across the board when things are not adjacent.
You will also see blue, yellow and white "bodge wires" on the boards, these are 30awg wires (0.254mm diameter), with a single solid core and a heat resistant outer insulation. Mine happens to be Tefzel wire or wire-wrap wire. Even with such a small diameter, soldering these onto a leg on a surface mount IC is challenging, particularly with the smaller footprint devices. The way that works for me is to tin the wire, tin the leg, then put it on top, apply heat and come off, repeating where necessary. Routing of the wire is then done in straight lines with 90 degree bends and the wire is held down with little dabs of cyanoacrylate glue (super glue) to stop it from getting caught and coming off. It also looks a lot more professional. In some cases, small holes may be needed through boards to get to the opposite side, but great care is necessary to figure out where to put those holes so that you don't hit any important trace inside the board.
In this case, a bit of creative thinking by flipping the new transistor over, bending its legs a little and soldering a bodge wire to one of the pins allowed me to get the replacement component in place. The log was updated, the circuit diagrams were updated on the as-is v2.1 version and the next v2.2 version, ready for next time.
The reset problem was solved by a carefully placed wire link across the top of the ULN2003 transistor array and some carefully placed surface mount resistors down to the pins that needed pulling up. Its not pretty, but it works.
Now the Pi's power can be controlled properly and things can be reset when needed, so on to the next problem