Complete Circuit Board Lab & POV Business Card

Step 12 Build a ICSP Jig

In-Circuit Serial Programming (ICSP)
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Microcontrollers can be programmed a number of different ways. The traditional way of doing this is a Zero Insertion Force (ZIF) socket, where the chip must be removed from its it's circuit board in order to program it. Some chips are capable of In-Circuit Serial Programming, and, as you might gather from the name, the chip can actually be programmed while it is still on its board. This is very important for our little business card, considering it will be a tiny surface mount chip that can't be placed in any normal socket. Further, we will have the ability to reprogram it at will without  the need to desolder it every time.

With the PIC, the programmer puts the chip in a high voltage programming mode and it works surprisingly well assuming the rest of the circuit is relatively isolated from the programming pins.

For ICSP to work there needs to be some way of connecting the programmer to the right pins on the PIC. Most through hole PCB designs use a connector soldered to the board, but in our case we need a cheap, flat solution that doesn't need a connector. That's where the programming jig comes in.

The Programming Jig
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I had to make several programming jigs over the course of the project, depending on what stage the prototype was in.The first one simply used a scrap prototype board with SMD pads for the IC and wires to the traces coming out of the appropriate pins. Then I just positioned the chip and hold it down with my finger while it is being programmed, taking care not to short out the leads. I also tried placing a magnet underneath the board to hold the chip in place.

The final solution relies on a clever little mechanical connector called a "pogo pin." It is essentially just a spring-loaded pin that makes a good temporary contact against the circuit board. They are a little fragile, but they work very well. It helps if the circuit board was planned with this in mind, as you need contact pads connected to the proper pins. With ICSP it is important to have the programming pins isolated from the rest of the circuit and connected directly to the PIC, since it could disrupt the programming signals.

Building the Improved Jig
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There are simpler ways of doing this, and certainly lazier ways, but nothing beats a dedicated jig. The one I built uses a hinged transparent lid with pogo pins mounted in the acrylic. The lid locks down with two rows of neodymium magnets and presses the pogo pins firmly down against the programming pads. It also presses the momentary power button, since the PCB layout was changed after the programmer was built; the switch needs to be pressed to connect pin 1 to the programming pad.

The base of the unit uses a scrap chunk of black acrylic and the cover is a a clear piece that helps in lining up the connections when a board is inserted. over the place it needs to bend. The PCB is held firmly in a predictable place with two PCB rails cut to the proper length, and there is a spacer insert that both raises the board up to the right height and stops the PCB in the proper position when it is pushed in. Using a hole saw, I cut a semicircle out of both the pacer and the black frame so that the card can be easily grasped for removal.

For the lid to line up parallel to the rest of the unit it needs to be bent at a slight angle. This is easy to do with a blowtorch. With the lid attached to the black frame on its hinge, run the torch back and forth from about 2 inches away. Take care not to heat it too long or get too close as it will scorch, so practice on a piece of scrap first. When it becomes malleable bend it down over the card guides and until it sits flush against the top of them. Hold it in place until it does not move back, then leave it to cool. Be sure to do this outside or near a window as the plexi will fume as it is heated.

The holes for mounting the pogo pins were drilled into the lid of the unit using a drill press. I made a template for the holes and printed it on transparency film. Slide them in one by one and make sure it lines up with the right pad on the board.  With the lid shut,  press the the pin down to a depth that will push the spring-loaded poker in a little more than halfway in. Then mark the depth with a sharpie and add a blob of solder to either side of the pin. It should not move up and down, add a dab of glue to keep it from wiggling in any direction.

Connect the ICSP wires to the pogo pins with the pogo holders counting from the marked red wire, to these pins on the PIC:

ICSP         PIC Pin

1 .............. 4
2 .............. 1
3 .............. 8
4 .............. 7
5 .............. 6

Once it's complete, connect the programmer, slide in a populated board, and snap the lid down. It's ready to program! Be sure to remove the coin cell battery from it's holder or the programmer is toast; there is no protection on its output pins so any external current will likely destroy it. A simple fix is to put a bumper in the lid of the programmer that would stop the lid from closing if it ran into a battery.

Parts

• iCP01 USB In-Circuit Serial Programmer
• Scrap acrylic or polycarbonate
• Miniature Neodymium magnets (2.5 x 1mm) (x 14)
• Pogo Pin holder (x 5)
• Pogo Pin, round tipped (x 5)
• 2" hinge
• 5-pin header connector
• Circuit board slide-in mounting rails, left and right
• Rubber feet (x 4)
• Assorted screws & bolts

Tools & Supplies

• Soldering Iron & solder
• Power drill or drill press
• 1 1/4" holesaw
• Blowtorch
• #2 Screwdriver
• Hacksaw
• Superglue
• Printable transparency paper
• Tape

Comments

[Barnaby Walters]

That is just plain cool.