Of course there’s the minor problem that USBasp is designed for ATMega8/88 and to run at 12mhz but since it can run on an ATTiny85 I figured it couldn’t be that hard to port it to the ATTiny84, since both my business card and USBasp uses vUSB.

Turns out: It wasn’t bad at all. And in case my USBasp dies some day, my bizcard is now self replicating.

So. If you’re missing a programmer for ISP or TPI and you only have an ATTiny84 lying around, the code I just put on github should do the trick.

The downside, compared to my USBasp clone, is that it doesn’t support 3.3V, but the upside is that it’s actually more handy because of the micro usb connector, so I no longer need a USB extender cable or long jumper wires. And of course the BOM is hardly more expensive than the price of a chinese USBasp clone.

Also – I used it to finally upgrade the firmware on my old USBasp clone so that it now supports TPI. Expect to see some projects featuring the ATTiny10 soon

Have anything to say? Please leave a comment.

I’m not saying I have the final answer as for what is right for you, but here are the ways I’ve tried and the experiences I’ve gathered.

1) Program your chip before soldering it in

When working with though hole IC’s it’s easy to program them on a breadboard before soldering them into your project(or a socket in your project) but when switching to SMD, this becomes a problem since sockets are no longer an option and this takes away the option to reprogram the chip without desoldering it.
Pros: Good for THT IC’s in sockets
Cons: Bad for SMD IC’s

2) Add an ISP-header to your project.

Since you’re reading this page and you haven’t turned away yet, you’ve probably seen the 6 pin ISCP header on the Arduino UNO. It’s the AVR for programming the ATMega328p directly, without using the bootloader and programming it over serial.

Pros: Easy to use, standard layout
Cons: Takes up board space and you’ll need to do extra soldering

3) Use a test probe clip

If you program the same kind of chip a lot, these clips are handy. You just connect jumper wires and clip it onto the IC. I have one of these for SOIC-14 and it worked great the first couple of times, but after a while I’d gotten the pins bent a bit and it got harder and harder to make a good connection.

Pros: Doesn’t take up board space, also good for testing
Cons: Not always easy to clip on correctly, wears out over time, not good when IC’s are hand soldered and might have excessive solder(like some of mine – whoops).

4) Pogo pins and pads or vias

I saved my personal favourite for last. This is an alternative to the “bed of nails” method where you make another board that fits on top of your own and makes connections the right places.

These needlepoint pogo pins are easy to make a connection with, even if your programming pads are relatively small. I haven’t testet it, but I expect you could use this method with something as small as an uncovered via. The next time I design a board that needs to be programmed fast I will probably just make circular pads half a mm in diameter 2.54mm(.1″) apart, which means it will be easy to put traces or maybe even components between them.

Pros: Easy to connect, hardly takes up any board space

Cons: You have to hold the pins down while programming

For my PCB Business card I used this method and it works great – even though the ISP pads on the board are waaay bigger than they have to be. I simply soldered the pins onto female dupont jumper wire terminators and then soldered them into a perfboard. That way I have something to hold on to and the spacing between the pins is easy to get right.

Did I forget a smart way of programming an AVR? Questions? Please leave a comment.

This is my business card. It works as a USB-HID device, takes input from a 2.4Ghz transceiver and it also comes with some flashy RGB LED’s and a prototyping header. Oh.. And it has my contact information.

Spoiler: This is also on Github if you just want to skip to code and PCB files. If you want to know more, read on.

Why?

After seeing a couple of PCB business cards on hackaday.com, I knew I had make my own. But how do I make it stand out? What should it do? How expensive can I get away with?

For this little project I decided to squeeze as much as possible out of a cheap ATTiny84. I chose the T84 because it’s dirt cheap (<1$), I’ve been through it’s datasheet more than just a few times, and I wanted to see exactly how much it’s possible to squeeze into the 8192 bytes available.

Turns out the answer is: A lot!

I have a lot of individually addressable RGB WS2812B’s LED’s – aka NeoPixels – spread around my home(and about 850 left on a reel) that I’ve been controlling with prototypes made up of ATMega32u4’s and ATMega328p’s with 2.4Ghz nRF24L01+ tranceivers and a Raspberry Pi with the same tranceiver so I can control it from my phone. To justify spending more than a few cents per business card you need it to actually do something useful. Controlling RGB-LED’s was a start.

Attiny84 √

WS2812B’s √

nRF24L01+ √ (though i suspect pretty much everything out of China is actually SI24r1)

I also added a 20 pin header that makes it easy to use this board for prototyping with the 14pin ATTiny’s. Easy access to every pin(and a few extra power pins) means you can easily solder sensors etc. directly onto the pins – or add male header pins and push it into a breadboard.

How about USB?

One of my annoyances when designing prototypes has always been that it’s really hard to make anything USB-capable(not to speak of compliant) for less than 5$. It is possible though, if you don’t follow the usual path. OBDev’s V-USB (with a little bit of help from Adafruit’s TrinketHidCombo library) made it possible to squeeze some  USB capability into this project without adding much more extra hardware than two zener’s and three resistors.

Software

I have to be honest. I like the Arduino IDE for prototyping. Slapping a couple of libraries together doesn’t usually result in the most efficient code but when you want to put something together that showcases a lot of features fast – as in: a few hours – the vast amount of libraries for interfacing with every popular IC makes it hard no to take the easy route.
Everything didn’t work straight out of the box though. The RF24 library needed pin definitions changed for the ATTiny84(since the USI’s DO/DI are reversed compared to MISO and MOSI and this hasn’t been fixed in the current version) and Adafruit’s TrinketHidCombo library also needed some tweaking to work with the T84. I also went with the Adafruit NeoPixel library  for controlling the WS2812b’s, even though it’s relatively huge for a tiny chip, but somehow I managed to fit the basic functionality into the 8k of flash anyway. It does however, use every single byte available and the debug functions won’t fit unless something else is disabled.

I thought about using the smaller FAB_LED library for driving the LED’s to leave more space for HID-stuff, but it doesn’t really play nice with the 10ms USB-polling when controlling more than one LED at a time so I went the easy way with the NeoPixel library and limited the HID capability to just pressing media keys (start/pause, volume up, volume down and stop). I pride myself in making software that’s good enough for doing the job without too much “gold plating”.

So for now, all it does is work as a 2.4Ghz USB-dongle for media keys along with controlling flashy flashy LED’s. Pretty good for 5$, gold plated board included, though.

PCB Design

So how do you make a PCB business card look cool? The easy way to make it would’ve been some silkscreen text with contact information etc. but to me the typically low resolution silkscreen text wasn’t good enough so I went with making a big ground plane and putting the text and logo on the solder mask layer. The font in KiCAD isn’t my favourite either, so I went with making a footprint of the text from a bitmap instead.
For the surface plating I could’ve gone with the cheaper options but I really believe the immersion gold(ENIG) makes too much of a difference to skip.
I chose 0.8mm board thickness because it’s enough not to break unintentionally and also fits in a standard credit card slot of a wallet – without components that is.

How do you program it?

At first I thought I would just skip the programming interface and use a test probe clip(like this one from Pomona) to program it.

But since I have a feeling I will be using this as a protoboard many times, I decided to find a better solution. Of course I could just hold down jumper wires to the 20-pin header or solder them on – but in my opinion that would either be a hassle for fast prototyping or ruin the look.
Instead I decided to experiment a little bit with programming pads(J2 if you take a look at the schematic), since I already had some pogo pins on the way from China when I ordered this. I didn’t have exact specs for how big the pads needed to be, so I went with a footprint meant for an SMD male header. When I got the pogo pins I soldered them onto female dupont wire terminators and then onto a perfboard. You can see the final result on the right, together with my overused USBasp. This is a solution I can really recommend, but the next time I make an interface like this I will most likely just use tiny pads 2.54mm(.1″ standard header) apart and as small as possible. With the thickness of the solder mask, the pins slide right into place. The only downside is you need to apply a tiny bit of pressure while programming the device, but I prefer this to connecting jumper wires any day.

If you want to read more about this subject, take a look at my post about programming an AVR without taking up board space.

Want to make your own PCB Business card?

The point of a business card is to share it with other people, so I’ve uploaded this whole project to Github. Feel free to fork it and put your own spin on it.

Leave a comment if you have any questions, suggestions etc. Any feedback – good or bad – is very welcome