Raghu:
I have many times built add-on boards that interfaced to the eval boards through the connector on the side. The added length does not create a problem when using this as a development vehicle. The add on board can be made with whatever types of parts and packaging technology that makes its creation the most straight forward and stress free as possible. If high speed is at play (i.e. say greater than 20-22MHz) I would tend to make these addon boards as a 4-layer board to control trace impedance and aid in good power distribution. This board would also incorporate any buffers and isolation for interfaces to the board that have length, noise, or voltage issues.

Making the add-on board relatively low tech keeps the cost down and it is even feasible to spin it once if needed to cover an oversight or shortcoming. (More than once I have simply re-worked the add-on board to take care of some problems).

The add-on board also allows you to get most, if not all, of the code for a product written and developed before you have to commit to the actual product circuit board. That is of course that you place provisions for all of the product specific I/O onto the add-on board. On many projects I have done I build the add-on board and get going writing the code, maybe even being able to demonstrate it to the customer. At some point in parallel a separate engineer (or even outside contract service) can start the product specific PC board layout and be focused on the packaging specific to the product. More than once this process has yielded high tech multi layer PC boards filled with surface mount components that worked and shipped on the first revision. I always keep that add-on board in a safe place and use it to perform future code changed or debugging. With this scheme it is often unnecessary to ever have to debug code in the actual target product.

Another thing I like about the add-on boards is the fact that I can re-use one eval MCU board among several development projects.

Now to get to your questions about using the 2x5 headers on the eval boards with ribbon cables. Personally I would generally recommend not using these in a product environment due to the risks of noise and uncontrolled impedance. For quick test setups I have used them successfully with cables up to about 10 inches (250 mm) long without trouble when they are used to simple low speed I/O but to expect to see quite a bit of undershoot and overshoot on the signals at the end of the ribbon cable. The fast edges of the push-pull drivers on the MCU will actually create more problems than signal attenuation or loss of bandwidth in the cable transit.

I would also question using the actual SiLabs eval boards in a product environment as well. All the EVBs that I've seen use two layer boards and whilst they are designed well and operate good unto themselves one has to wonder about the signal integrity of the pins at the MCU when you hang a lot of I/O onto the port pins. This would apply equally to the use of the 2x5 headers or the connector on the edge of the EVB. If you look closely at the pinning layout of the edge connector or the 2x5 headers you will note that very little regard has been given for bringing a controlled impedance GND plane out through this connector and onto an add-on board.

One last comment regarding the creation of add-on boards. In a couple of instances I have made piggy-back type boards that mated with one or more of the 2x5 port headers on the EVB and and connected added additional circuitry to suit a specific need. You can see an example of that in the following picture at my website.


Michael Karas