A custom development board for the Parralax Propeller 1 microcontroller.
The Propeller 1 by Parallax is an unusual, 8-"cog" (not quite core) microcontroller first released in 2006. It has since been replaced by the Propeller 2, but for a few years I've had some of the delightfully chunky DIP-40 versions of the chips sitting in one of my drawers. I've tried out the chips a couple of times, and while they're fun, the breaboards get quite busy: as well as the chip itself, it needs a 3v3 regulator (they aren't 5V tolerant), a USB serial converter to program it with, an EEPROM for storing programs, and a reset button.
A DIP40 Propeller 1 beside an Arduino Uno. It's huge!
There are official dev boards for the Propeller which include all of this, but they're relatively expensive and not that easy to get hold of where I am, and besides, why buy a board when I already have the processor? So I designed my own, and this repo contains the files for it.
Setting out, I wanted this board to:
- Contain all of the necessary parts for a fully-functional Propeller development board: a USB serial connector, 3v3 power, an EEPROM, and a reset button.
- Make use of the big, chunky DIP-40 chips which I already had, instead of requiring me to buy additional chips in a smaller format.
- Be as small as possible on the breadboard, to give plenty of space for LEDs, buttons etc.
- Use USB type C. A lot of the official dev boards use micro B, get with the times!
The board was designed in Kicad 5.1.9. To keep the board as compact as possible the layout takes a stacked approach: most of the components are soldered to the PCB in a narrow section across the centre, and then a DIP40 socket is soldered with its legs either side of this. The DIP40 Propeller chip can then be inserted into the socket, keeping almost everything underneath it and within the footprint of the P1 chip. A set of 0.1" headers is then soldered just inside the footprint of the DIP socket so the whole board can be inserted into a breadboard. This gives a board about the size of a stick of gum even though it uses the large DIP40 chip, so in keeping with the engine-themed names that Parallax has for the Propeller and its accessories, I decided to call this the DIPstick.
You can see this in the image below: the bottom PCB shows the upper side of the board, with most of the components in the narrow centre section. The outer holes along the edge are for the DIP40 socket, and the inner rows are for the 0.1" headers. The only components sticking out of this footprint are the USB C socket and some LEDs/resistors to the left.
The upper image shows the underside of the PCB which doesn't have a lot on it, but does have a Bunny wearing a Propeller Hat.
Whee!
The schematics for the board are in this repo. The major parts are:
| Component | Function | Reason for choice |
|---|---|---|
| Silicon Labs CP2102N-A02-GQFN24R | Allows serial connection and programming over a USB cable. | Readily available from major wholesalers, fairly cheap, not made by FTDI. |
| ON Semi NCP708MU330TAG | The Propeller needs the 5V from the USB connection stepped down to 3v3. | Small footprint, only needs 2 capacitors to support it, and outputs an ample 1A of current, plenty for all of the dev board components and some LEDs on a breadboard. |
| Parallax P8X32A-D40 | Microcontroller | The whole point of the dev board, an interesting 8-cog microcontroller which does a few hardware things differently to most other microcontrollers. |
| Microchip 24LC256T-I/SN | Used to store programs between reboots (the Propeller's internal memory is volatile). | Cheap, widely available, fairly generic. |
| Panasonic EVQ-P7C01P | Button for resetting the Propeller | Small, and has a convenient Kicad footprint! |
| Murata CSTCR5M00G53-R0 5 MHz resonator | Helps the chip keep time properly. | Doesn't require additional capacitors like a crystal, small but still hand-solderable. |
| Adafruit 4458 USB Type C SMT / THM Jack Connector | USB connector | Physically a USB C connector, but wired for USB 2 connections so it has fewer pins than USB 3 connectors. Has solderable lugs for physical support. |
The boards were ordered from Aisler, were hand-soldered, and came together quite nicely (ignoring some hardware flaws in v0.1...). You can see the completed stack below, with the P1 chip removed in the image below that.
It's fairly chunky, but honestly works well. I love the small footprint of the whole thing, despite the size of the P1 chip. I'm not actually sure I could have made it any smaller even if I'd bought the smaller 44-pin QFP package, because that would have to be soldered to the PCB with the rest of the components, instead of being "stacked" over them via a socket. The Propeller Flip, the smallest official dev board, is 51x18 mm in size while the DIPstick is 59x18 mm, so there isn't a lot in it. All-told, the components cost ~£15.50 including VAT, while the PCB cost approximately £4.26 (for one, although they had to be bought as a pack of 3). That theoretically gives a cost for one board of just under £20, although in reality it comes out at more because I got 3 PCBs and plenty of spare components. The cheapest official dev board which is comparable (the Project Board) is ~$30, or about £22, so that's pretty comparable.
The pair of LEDs beside the USB socket are the TXT and RXT LEDs of the CP2102N, and show whether there is data being transmitted or received by the CP2102N. It's worth noting that this function isn't enabled by default, you have to use Silicon Labs' Simplicity Studio to enable it. It's fairly straightforward, although you do have to create an account and jump through some hoops to get it.
Overall I'm happy with the board and it functions well. The one thing I'd change about it is the reset button: it is poking out the underside of the rear of the PCB, and actually doesn't stick out more than a millimetre or so. This actually makes it quite hard to push with a finger, you really need to use something to poke it. For convenience I think it would be better to extend the board a handful of millimetres further and use a larger button to make it easier to press. Alternatively, the power LED to the right of the USB connector could be moved under the PCB (you'd still see the light as a downlit glow) and the reset button could be placed there instead.
This doesn't really stick out far enough to be easily pressed.
That said, I'm happy enough with the whole thing that I'm not going to worry about a version 0.3. The button issue won't get in the way of learning how to use the board, and in a worst case scenario the reset pin is broken out on the pin headers, so I can bridge that to ground with a jumper wire or button on a breadboard. I'll keep it in mind for the Propeller 2 DIPstick...