#124 Making a Custom Programmer

While looking for a good example PCB project I decided upon a simple UPDI programmer PCB for the well known new ATtiny ICs.
I had a few prerequisites I wanted to satisfy for the project.

• Design must work but also have room for improvement
• small SMD PCB design
• BOM list and prices
• simple schematic using KiCad
• 1206 component size sop-8 IC size
• USB-C
• 2 sided PCB
• all standard sizes for PCB manufacture
• PCB panel example

I wanted the project to cover many of the basic and intermediate PCB manufacturing practices of course the IC is not very sophisticated and there is no impedance matching or equal length traces etc. However it shows how designing, quick and affordable manufacturing can be achieved in South Africa. Designing for manufacturing, thinking of the capabilities of the manufacturer and keeping best practices in mind at least to the best of my knowledge at that specific time.

In the project I keep things as standard as I can using metric measurements and easy milling panel design for the panel (no fancy curved edges or designs) although many interesting looking designs can be made surprisingly simple, it takes experience plus CAD artistic intuition. That being said I wanted this to be a simple and practical example I could use for teaching references later.

Getting to the component choices for a small UPDI programmer I chose the CH340N IC. It needs very little external components and has an internal crystal. Small and lightweight and perfect for my UPDI project. It is missing other pins like the reset pin but in my case this was not an issue. Always double check your IC datasheets and capabilities.

On that note I did receive some FAKE CH340N ICs and I tested them. They only worked on lower baud rates and adding LED indicator resistors to the TX and RX made them unstable. Unfortunately in this day and age fake ICs are very common you need to recognize them and weigh the balance between using them or not. Sometimes there’s a “good fake” so as a maker I may use this test projects or make throwaway projects but the characteristics of the IC are now different so any calculations are incorrect… so it’s a balancing act that no-one can escape. For production always use IC’s from the original manufacturer the stores are easily found online and it’s worth it in the end.

All other components are very basic 1206 (easy to solder) SMD capacitors, resistors, TX/RX LEDs, power indication led, a diode 16-pin USB-c connector with 5k1 sense resistors keep the PCB copper balanced on both sides also saves etching chemicals for the manufacturer I also added some example fiducials and the copper stops 1mm from the edge of the PCB creating a small protection layer all around.

Since there is a USB-C connector I chose not to include any ESD or fuse but I would add these components to a commercial variant.

Also this v1.0 has the TX/RX LEDs on the wrong indication pins. I would change them to the USB side instead.

Overall I think it’s a good introductory example showing a simple design including some errors and fixes.