Not a 14X2 but....


Senior Member
Now that I have your attention, this is not a new PICAXE model or product from Rev Ed. Rather, it is an attempt to get the most PICAXE-punch from a 14-pin-DIL-like package.

In the past I have developed several projects based on the PICAXE 14M. The 14 pin PICAXE is great for small applications. However, to get the full potential from the 11* programmable pins of the 14M, 256 bytes of available program space which also doubles as EEPROM data storage, can be severely limiting. (*12 programmable pins if SerRxd is used.)

Another idea to get a more powerful, small-footprint PICAXE was suggested here a few months ago, where part of a 20X2 was physically carved off to leave 14 pins. However, the penalty paid for this approach is that some of the 20X2's most powerful features were lost: hi2c, hSerial and hspi.

My design offers the ability to configure the module as a close-to-drop-in equivalent of the 14M. At the other end of the scale, the 14-pin module can be configured with the 20X2's hi2c and hSerial at the same time. Another configuration option allows 9 ADC channels. Not bad for 11 or 12 programmable I/O pins! It goes without saying that the 20X2 on this module can run at 64MHz, a far superior speed than 8MHz on the 14M.

The PICAXE 20X2 SMD chip is physically about 75% of the size of a 14-pin DIL package. The prototype PCB that I developed is a double-sided through-hole-plated board that measures just 22 x 15.5mm. Actually, the PCB is larger than necessary as I included an on-board 100nF decoupling capacitor and a led/330r resistor combination connected to the Serial Out/A.0 pin. In a separate project I had another PCB that was about 22mm wide fabricated in China. I added the artwork of this board to the end of the other design and the composite board was manufactured with a score line between the PCBs that allowed them to be separated easily. So for less than A$1 each, I got 25 of the prototype PCBs made. The apparent thickness of the PCB in photo #1 indicates the "package" is smaller than it first appears.

The different hardware options would usually be configured when fabricating the module by bridging 6 SPDT solder jumpers.

Q. Why would you not just use a 20X2 SMD PICAXE when designing a project?

A. Most of my work with PICAXEs is just a low budget hobby. I rarely fabricate PCBs, especially for prototypes. This 14-pin DIL module allows the development of quite sophisticated projects on stripboard("veroboard"). It also allows more flexibility and program space for my existing 14-pin PICAXE projects that have outgrown their original, humble 14M.



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Senior Member
Assembly of the 20X2 to 14-pin Adapter

Very fine indeed. Could you share your schematic? How did you attach the 14 pins?
The 14-pin base is a machined-pin dil IC socket like this one from Altronics. (It's contact resistance is actually 30m ohms ~milliohms~ rather than the advertised 30M ohms!) It ensures the base, once reassembled, is fairly robust and the pins are correctly positioned.

An alternative base, which I have not seen for many years, was made of fibreglass and had gold plated pins. These were firmly set into the board and the tops stood about 1mm above the surface. These would be ideal if you can find them.

Top Assembly and Solder Jumpers:
I used the solder paste and electric frypan/skillet method to assemble the upper side of the board. I then selected the combinations of functions/pins that I wanted and created solder bridges on the 6 solder jumper pads (SJA to SJF) using a soldering iron and solder.

Prepare the base:
I used a matrix pin insertion tool to release (push) each of the pins out of the plastic former. My matrix pin tool is home made from a small piece of aluminium rod with a 1mm hole drilled in one end. With all of the pins loose, I then filled each of the hollow ends with melted solder so that they were slightly convex. Each of the 14 pads were very lightly tinned: use solder wick to remove any excess.

Soldering the pins:
I then soldered 1 end pin to the board, making sure that it was both central to its pad and perpendicular to the board. Care must be taken at this point as the hot pad could easily come free of the board. Then, using the plastic former to accurately position the opposite corner pin, it was soldered too. With the former removed, the remaining 12 pins are placed in their respective holes and the former is placed over the 2 already-soldered pins. Holding the board in a PCB vice and using a fine conical tipped soldering iron, each pin is pressed into position as its base is heated. The plastic former sits 1.2mm off the board, to give you an idea of the gap available for soldering. It is important not to use too much solder or heat: I managed to do the job first time with any problems.

Hindsight with the design:
If I were to make another batch, I would make a couple of changes to the board. The first would be to position the solder jumpers (SJs) on the underside of the board near the edges for easier reconfiguring.

The other change would be to include pin C.6/leg 4 in the SJ options. Pin C.6, while needed to be included for 'drop-in' equivalence, has very modest value when compared to other pin and function options. But I have 25 tiny PCBs made. On the other hand, their cost was minimal and I might get some offers to get rid of some...


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