Introducing picocart - it works

[JasonACT]

I'm not quite sure why the memory test was passing, it must be very close timing, or I had a bug in the '612 chip's 16 bit register code...  Changing the code to be simpler 8 bit registers works now:

 

 

 

Actually, the 8 bit code is doing quite a bit less, no need for bounds checking, so two less bit-mask ops.

[speccery]

It never ceases to surprise me how fast time goes... Already half a year no update in this thread. And this update is off-topic, but an update of my recent activities. As background, I've been very busy with work and I guess somewhat tired due to the dark season we're going through in my neck of the woods. Not much energy to work on retrocomputing. Also the hobby seems to go in cycles for me, I've playing with my synthesizers and trying to make progress with my piano playing in the meantime.

 

With that, this update is going to be somewhat off-topic but related to the pico cart one way or the other. Firstly, I've been curious to know if I can manually assemble boards with QFN packaged chips. Two chips interest me in particular: the RP2040 (powers the rasberry pi pico) and ICE40UP5K (small FPGA). In order to test this, I ordered already ages ago (I think maybe already in 2022) boards from PCBWAY with Minimal Design Example by the Raspberry Pi people. No changes on my side at all, a simple two layer board, minimal design for the RP2040. I finally today got around seeing if I can build these by hand. I assembled two boards, and got both working. The first prototype build took something like four hours... A big chunk of that time went into finding the PCBs, components, going through the schematics etc. I don't even want to mention all the mistakes I made - but let me say that I did the big one and despite double checking managed to install the RP2040 chip 180 degrees off first...The other problem I had was that I my micro USB connectors were not the same as in the design originally and it shorted VBUS (the 5V power input) to ground. Once I figured out this was the problem, I was able to fix this easily by just moving the connector a bit. Easy with the hot plate, which I used my hot plate to bake the boards, and that greatly speeded up both the assembly and the numerous iterations of assembly bug fixes. 

 

The second board assembly only took about 35 minutes, starting from having the components and the bare PCB, to the board passing a simple test program - the LED blink, although there is no LED on the board so I observed the blinking with an oscilloscope.

 

Here are the finished boards:

 

Here are some pictures of the build process. First after applying paste (in a bad way - too much paste mostly, and not enough in some spots):

Then most components placed on top of the paste - including the RP2040 misoriented  by 180 degrees...

And finally the board after first bake - the RP2040 still in the wrong orientation. I did not realise the chip was mis oriented before applying power... That meant the first chip became toast, but I replaced the chip and put another in place and the board worked  If you open the picture and look closely, you can see many solder balls next the QFN chip "pins", there was too much solder paste. Removing the balls turned out to be very easy, just apply flux and give it a touch of the soldering iron, with a pretty sharp tip. The QFN packaged chips don't have any real pins, they just have the contacts exposed at the very edge of the plastic.

Despite the numerous problems with the first board I was surprised how easy the QFN packaged chips were to work with. The key is to order a stencil when ordering PCBs, and let solder paste, flux and surface tension do the job. This board also uses 0204 sized capacitors and resistors. I have never so far dared to use these in my own designs. They are very small, virtually inseparable from pecks of dust... But using the stencil and solder paste they were surprisingly easy to work with, you basically just drop them where they need to go and baking takes care of the rest. Except when there is too much solder paste, and they float a bit...

 

Anyway with this experience I'm probably going to design my first FPGA board using the ICE40UP5K. Perhaps I will do a revision of the picocart which would include both the FPGA and RP2040. I think the original picocart design is still the way to go for most DIY people (when I have time and inspiration to finish the firmware) as it is a much simpler build that using the plain RP2040 chip.

 

Then a completely off-topic thing, before the holiday season I completed one of my simple dream electronics projects and built a small mechanical keyboard, pictured below. It is my own design, the Raspberry Pi Pico board to powers it. The board has two quadrature encoders (with push button functionality), 15 Kaihl low profile hot-swappable mechanical switches, and 15 neopixel RGB LEDs, one for each key switch. After building the board and developing some software, I realised that I can run the QMK firmware on it. I also added a screen as an afterthought. The current QMK based firmware supports 4 key layers, one of which is a MIDI layer enabling notes to be played with the keys. Depending on the layer the encoders can be used to control multimedia features such as volume or adjust RGB backlighting effects. I still need to do more work with the firmware, very many ideas where to go from here. Also this design would benefit from not using the Pico board but rather the RP2040 chip directly on the board. I built four of these so far, gave two to my brothers as xmas presents and one to my son  Not sure if they appreciate the effort though  but fun project - and very tactile