Check that the processor footprint has the appropriate power and ground connections. Solder the processor to the board (or plug it into a socket if you’ve used one), remembering to ensure that the system is powered down before you do so. If your processor needs to be externally programmed with your code, make sure you do this before putting it into your circuit. Power it up and check the processor’s clock with an oscilloscope to confirm that it is oscillating. You should see a nice sine wave of the appropriate amplitude. Check the voltage levels you measure against those stated in the datasheet. If the oscillation doesn’t have the right amplitude (perhaps due to a poor connection or a partial short), it may not be able to drive the processor.
If the system you are building uses a microcontroller with no external ROM (such as the example presented in this chapter), the first test software you will write will simply waggle an I/O line of the processor. Observing this with an oscilloscope will allow you to see if your system is executing code correctly. If you included a status LED in your design, turn it on! Seeing a status LED blink on for the first time on a machine you’ve designed and built yourself is sure to bring a smile to your face.
Once you’ve confirmed that the processor is operating under software control, you can begin to add in the other hardware and software components of your application. A word of advice, though: don’t get too adventurous at any stage of the building process. If everything suddenly stops working, it’s much easier to find the cause if you’ve made only one change or addition. Take things a step at a time.
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