Outlining the PCB

Thursday, Dr. Phillips, Prof. Battat and I met from 10am to 3pm to focus on the design for the printed circuit board (PCB) to be used with the final prototype. We didn't even look at the gEDA software but, rather, discussed the needed components and their layout on the board itself. Once we found a room with a sufficient amount of white boards, the team dived into drawings and note-taking.

Over the past few days, in debugging the breadboard circuitry, we have discussed greatly about the impact of impedance and capacity with long wires. We tried to eliminate such unnecessary distances with grouping families together, i.e. all the MUXes in parallel rows. We also looked into the issue of single-point grounding between digital and analog electronics. On the breadboard, the addition of a MUX to the same voltage supply greatly affected the abilities of the counter and could distort the counting completely. Additionally, we discovered that the MUX and 4-bit counter had different tolerances for voltage supplies (5 V and 3.3 V). Hence, we could use one large voltage supply split between two different regulators or two different voltage supplies with respective regulators. We chose the latter as it would ensure there were no unexpected shifts in the distribution of power or "domino" effects that ripple through the entire circuit.


In regards to the grounding issue, Dr. Phillips suggested separating the PCB into two frontiers: digital and analog. Since we plan on using a double-sided PCB where one side holds all the components and the other is a single sheet of copper for GND*, we could use a moat-like design: a single bridge connected the GND plane for digital components to the GND plan for analog components. (*Note: A single plan for GND helps eliminate inductance by conducting large current flow from components without significant voltage drops, meaning the GND connection for all the components are at the same potential.)

I was given the task to re-draw the schematic on paper and in gEDA this weekend. So far, I've drawn a very detailed 4 x (4:1) schematic and a simplified version for a single PCB. Finding the components in gEDA for the integrated circuits will most likely prove to be more difficult and, for now, I won't worry about the footprints until later.

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