Week 6
Electronics Design
Printed Circuit Board Design in Eagle
I redrew the example board from scratch in Eagle, adding a blue LED with a 100-Ohm resistor in series along with a push-button switch. To figure out what resistor value is needed, one must look up the typical voltage drop across the LED Vd and its maximum current Im. Then, given that the attiny44 provides 5V output on its pins, we need a resistor whose value is greater than

(5V - Vd)/Im

in order to keep the current through the LED at less than Im during operation.

In drawing the board, using the "net" feature to connect repeated, common terminals such as VCC and GND was very convenient. This involves typing "net," drawing the connection to a particular terminal, then using "name" to rename the net to a common name, e.g. VCC. The program will then ask whether you actually want the new connection to be connected to every other instance of VCC, to which you answer in the affirmative. Finally, it is helpful to label your net so you know what is connected to what.

When using "net" or "wire," to connect terminals of components that you put on the board from existing library files, sometimes I was not careful with where I clicked and did not produce a connection that Eagle recognized. The way to make sure the connection has been made is to go into the board view and check that there is a new, unrouted yellow line.

On the schematic I decided to put one zero-Ohm resistor to help me get my VCC lines everywhere I needed it to go. Otherwise it became a nightmare trying to avoid crossing wires and also keeping enough clearance between traces.
The resulting board drawing and the product from milling on the Modela are shown above. In the board view in Eagle, I used 17-mil clearance between traces as a design rule check. To check your board for clearances, type "drc" into the command window, and then click the Clearance tab. One high level solution to create more space to run traces is to redraw components from the library to have smaller pads, but in my case I just decided to "cheat" and use a single zero-Ohm resistor to allow me to cross a trace. This is R3 in the board drawing.