PURPIC, the wearable PICkit2 clone using PIC12F508 programmer

PURPIC, the wearable PICkit2 clone

Step 1

An extremely brief history of the PICkit2

Microchip launched the PICkit2 in 2005. It is a hugely popular device that programs and debugs a wide array of PIC microcontrollers and EEPROMs. Even though it has long been superceded by the PICKit3 and the ICD3 programmers, there are still a variety of features which make the PICKit2 quite useful (and cheap) for specific applications.

In addition to working with Microchip’s MPLAB IDE as both a programmer and debugger, Microchip also created a nifty standalone software for the device which is very easy to use. There is a Widows GUI version and also a command line version of the programming software. The engineers even added a “programmer-to-go” functionality, which allows the PICKit2 to burn chips with no computer at all. So even though the PICKit2’s MPLAB debugger is painfully slow, all those other features are quite useful for low volume batch programming on a budget. So even though I celebrated the PICKit2’s 7th* birthday by buying an ICD3, it will never make my PICKit2’s obsolete.

Step 2

An extremely brief history of PICKit2 clones

Step 3

Enter PURPIC – Video in Action

The 4 main features of my PURPIC clone are

1. Integrated li ion battery charger. Whenever the device is plugged into a USB port, it charges the battery with a controlled charge rate of 85mA.
2. DC-boost circuit provides 5V from any lithium battery or a possibly out-of-spec USB port/hub. The boost circuit is good for over 500mA, so it can deliver the goods.
3. The Program button is ported out to the ICSP header, to allow access to Programming-to-Go via your programming interface of choice.
4. Even with the added circuity and the extra large toner transfer-friendly vias, the area of the PCB is still 25% smaller than the PICKit2, making the programmer light enough to stick on the end of a pogo pen interface or to even wear on your wrist, utilizing the Programmer-to-Go function.

I haven’t done extensive testing, yet, and I’ve already found one shortcoming. The real PICKit2 detects a short circuit between Vdd and ground, shuts off power, and displays a Vpp error. When connected to the computer, my clone doesn’t detect a short fast enough. It will brownout and lose communication with the computer. Or worse, it may even trip the USB port to shutdown, which (if you have a battery connected) leaves the device locked up in brownout, with output still (trying) to power the short.

*Edit: I found a simple solution for short-detection is to add a 5 ohm resistor in series with the Vdd line on my pogo pen. Yea! All of the Vpp errors are now detected. A 5 ohm series resistor is obviously not an ideal solution, because it will result in significant voltage drop if your programmer is ever called upon to drive a significant load, so some careful selection and experimentation with different FETs might be warranted. I checked the specs on the P-FETs I’m using, and they have much faster rise/fall times and a significantly lower RDSon resistance than the one in the dual FET that the schematic calls for. Of course you still have some extra power draw from the DC boost, which is unavoidable. But I’d think a slower Vdd  P-FET could alleviate or solve the problem.

*Edit: I updated the PDF images. The new revision has a space for a 0805 series resistor on the output of the Vdd P-FET. If you were to end up having short-detection issues, you could put a small resistance here. If not, you could just bridge it with a 0R resistor.  I also updated the PURPIC layout to include a pad big enough for a 10uF ceramic cap (probably couldn’t fit the better-at-this-frequency tantalum variety, but I’m using all ceramic, and everything looks good even without this cap, at all) on the source of this P-FET as per the original schematic. So now the PURPIC layout is quite faithful to the original schematic other than a missing decoupling cap on the opamp, an extra optional cap footprint or two on the +5V rail, the smaller Vpp pump cap substitution, and the space for the series resistor on the Vdd output. (I did not update the PICZYL pcb with the Vdd P-FET 10uF cap pad; I never changed this version’s Vpp pump cap from the 47uF tantalum to the 22uF ceramic, so there’s no space for this.)

Here are some links of the action:
Here I’m wearing the programmer on my wrist
http://s18.photobucket.com/albums/b103/klee27x/?action=view&current=Moviepp.mp4

But in reality, it’s light enough to just stick on the end of a pogo pen.
http://s18.photobucket.com/albums/b103/klee27x/?action=view&current=PP2006.mp4

Step 4

Schematic and board layout

PICZIL v1.1 back.pdf10 KB

PICZIL v1.1 front.pdf9 KB

PURPIC v1.1 back.pdf8 KB

PURPIC v1.1 front.pdf12 KB

Step 5:

PIC 12F508 annotated source code; if anyone has a problem assembling as posted, please comment