28-Pin AVR Development Board Evolution
Using a development board has its advantages, especially if you are new to microcontrollers. They are generally easy to set up and get working, and you can have your first "hello world" LED blinking in no time. But not all boards are equal, and definitely not all boards work right out of the box. I have mixed feelings for this 28-pin AVR board from Olimex, but since I bought it, I vowed to make it useful.
The original board contains both the bare necessities and a few nice additional features. It has the IC socket, a removable 8MHz crystal, and 10-pin ISP connection which is all you really need to get started. The board also has a supply jack and built in regulator circuit which can be changed from 5V to 3.3V with a jumper. There is a reset button and LED connected to PC5 with a jumper.
Probably the nicest thing about this board is the built in MAX232 chip and external components including the DSUB-9 port necessary for a computer serial connection. This can be a source of frustration, however, as the the Rx and Tx pins are not initially connected, nor are there jumpers for an easy connection. External wires must be soldered to the board for this to work.
The other great thing about this board is the large amount of open space for soldering additional components. The problem with this is that building a circuit onto this board makes it worthless for any further testing of other circuit designs. That is probably why they label it as a "prototype board." I hope to remedy this with a few simple modifications.
The folks at Olimex were nice enough to supply a schematic as well.
For starters, the power circuitry is pointlessly elaborate. I understand that using 3.3V is desirable in some situations, but not on this type of board. Similarly, the voltage monitor and reset IC (U3 - ZM33064) seems pointless since there is no easy way to control this circuit using a battery pack due to the jack used, and this circuit is only good for 5V circuits anyway making the 3.3V circuitry even stranger. Also, why the bridge rectifier? A typical wall wart has that included already. Lastly, why not include a cheap LED indicator to show that the board does have power?
After finding a power supply with the perfect sized jack for this board which outputs 17V DC, I decided to clean up the power circuitry on the board. The new system has 12 and 5V regulators as well as an indicator LED, nothing fancy.
The next big step was to take out the standard socket and add in a zero insertion force (ZIF) socket. These are pretty useful when a chip will potentially be replaced in a board very often. The new socket takes up more room on the board, so the sockets for the crystal will also have to be moved. Also, headers along every I/O pin were added as well as 5V, ground, and 12V power buses. A potentiometer is also added to the REF pin through a jumper such that a custom reference voltage for the ADC may be established.
Finally, a row of headers was added for both the SPI and TWI buses as well. The SPI bus is separated from the I/O pins through 4.7k resistors, and the pull up resistors on the TWI bus are separate by jumpers in case this bus is not in use. Jumpers were also added to easily connect the TX and RX pins to either the MAX232 chip or another serial device.
The test LED connected to PC5 is still available through use of a jumper. The really tricky thing about the actual fabrication was fitting everything where it needed to be. After removing unnecessary components, I tried to reuse the solder pads for something else, especially the power system. There are a lot of tiny wires on the solder side, and once I am 100% done adding and tweaking things, I will probably coat it in a thin layer of hot glue to prevent shorts or any wires from getting ripped off their solder pads. I am very happy with the final product, and it has come in handy as a starting point for many designs involving a data bus. Although, all things considered, it would have made much more sense to just build a development board completely from scratch instead of buying this board given how much work I put into it, but I didn't entirely know what I was doing when I did buy it, so I am just happy I am still able to use it.