I design circuit boards (
PCBs) with programmable devices - things like
MCUs,
PLDs and
FPGAs - for a living. It's almost always necessary to provide a connector so that an external programmer, debugger or something as simple as a serial port can be hooked up to a board.
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| Programming, Debug and Serial Interface Hardware |
In the early stages, this connector enables development and debug. Later on, it allows programming and configuration in the production process. The connector is often a simple header with one or two rows of pins on a 0.1 inch (2.54mm) pitch. Device manufacturers specify various pinouts to match their own programming and debug hardware. So far, so not very exciting...
Recently, I needed to design a board with an
Atmel AVR MCU. It had to be no larger than 60mm x 14mm. There was no way I could fit Atmel's standard 2x3 header, so I started looking at placing small test pads and producing a companion board with
pogo pins to break these out to a full size header. This is often a good way to go - the companion board can take on other duties, and maybe evolve into a full blow test jig - but it's another design task, and I was short of time and trying to keep costs down.
Enter
Tag-Connect with their own clever take on this problem:
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| 10 way Tag-Connect to IDC cable ("no legs" version) |
Tag-Connect make cables with various standard connectors at one end, wired to a "plug of nails" at the other - a small array of pogo pins, together with rigid alignment pins.
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| 6-way and 10-way Tag-Connect plugs, and a retaining clip |
You place a corresponding array of round pads on your PCB, and locating holes to match the alignment pins. The "plug of nails" can then be pushed into the board with no danger of misalignment or rotation, allowing the relevant signals to be brought out to a programmer or whatever.
If you need to hold the plug in place for a while, there are a couple of options. For short periods of time, a clip (shown in the above image) can be used to hold the alignment pins.
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| 10 way Tag-Connect attached to target board and retained by clip |
If you want to work for longer, a more secure option is to use a version of the plug with "legs" that clip into additional holes on the board.
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| Tag-Connect version with legs for longer term attachment |
Be careful about your PCB layout - Tag-Connect provide advice on this subject. And remember that the socket pins on the retaining clip may touch down on the back surface of your PCB; keep tracks and vias out of these areas to avoid mysterious and intermittent short circuits!
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| TC2050 land pattern with leg holes, showing escape routing |
So the Tag-Connect approach saves space as well as the cost of purchasing and fitting a physical connector. And because not everybody has a Tag-Connect cable, or knows your pin-out, it can also add a little bit of security-through-obscurity.
Thanks to Chris Monslow at
Kielowatts for pointing me at Tag-Connect.
