To ease myself into this hobby, I began with the SparkFun Beginning Embedded Electronics tutorials. They were a great way to start, and had me well on my way to making my first RS-232-controlled blinkenlight in no time. As part of that tutorial, they told me to install bypass caps near Vcc on all ICs. Okfine, monkey can add prophylactic caps everywhere. But why? The SparkFun page describes their function at a high level, but I've always wanted to know more. Cap monkey want pictures. Cap monkey want to know why he has a pill box compartment dedicated to his supply of 100 or so 0.1uF caps.
This post is what I've been looking for. Details. Pictures. Sample circuits. What more could a cap monkey want?
I started this post back in October when I actually finished the work, but I forgot to post it then. So here it is now.
Three million wires later, my 8-bit full adder is complete.
Here it is with the front panel attached. I use the front panel in lieu of switches and LEDs.
Here's the circuit by itself:
Each pair of columns calculates four bits. Each bit needs nine gates, and consists of two dedicated 7400s (actually 74HCT00s) and a quarter of a common 7400. If we number the ICs in column-major order, bit 0 is calculated by ICs 1, 2, and a quarter of 3; bit 1 is calculated by ICs 4, 5, and a quarter of 3; bit 2 is calculated by ICs 6, 7, and a quarter of 3; bit 3 is calculated by (wait for it) ICs 8, 9, and the final quarter of 3.
Due to layout weirdness, bits 0-3 are calculated by the right pair of columns, while bits 4-7 are calculated by the left pair of columns. I'd love to say the vertical offset is intentional, but it's actually due to a layout mistake or change in plans, the details of which I've since forgotten.
Wiring this thing up was mind-numbing, but it was awesome to see it come to life (I tested each bit after I wired it up).
Here's the schematic. My original intent was to build a 4-bit adder, but I had so much fun wiring(!), that I decided to do it twice, for 8 bits.
At long last, I finished populating the front panel board. Oh what an adventure that was. I also built up quite the list of things to change for v2, if ever there is a v2. Even if there isn't, I learned a lot about what to do and not to do for the next PCB I design.
Before I get into the details, here are some pictures of the completed front panel board:
Here's a video of the front panel controlling a simple 2-input AND gate from a 74HCT08N. The whole purpose of the front panel is to make LEDs and switches unnecessary, but LEDs make an appearance in this circuit for validation purposes -- just to confirm that the front panel is doing what it says it's doing.
Things I learned:
Lots more and better silkscreening. I'm having a heck of a time identifying the correct input and output pins, to say nothing of distinguishing between the +5 and GND pins. I should've silk-screened the pin numbers all along the bottom headers. Silk-screened IC numbers (the manufacturer's part-number -- not the Ux or ICx number) are less useful when they're under the IC -- especially if you're using sockets. I also failed to leave any orientation marks to help me plug in the ISP cable correctly.
Status indicators. There aren't any LEDs on the board, which can make it difficult to tell whether the board is on, whether it's receiving data, or in general what it's doing. Eventually it would be fun to have LEDs on every input and output pin, but in the meantime, for v2, I'd settle for power and one that flickers when data is transmitted or received.
More space for I/O cable connections. I'm still conflicted as to how I'd like to connect the front panel to the PCB. I had originally thought to use ribbon cables with IDC connectors, but those are wide enough that I'd need to either widen the whole board or make a third and fourth row of I/O pins. A further complication there would be that I'd need cables with IDC 1x4 on one end and 8x1 0.1" sockets on the other. That could get expensive, but we'll see -- connector prices jump dramatically once you stray even slightly from the beaten path.
Pin orderings matter for the ISP connector. I used the P6 connector in the schematic capture program, which numbers pins like an IC -- ascending counter-clockwise from the upper left. Unfortunately, the pins are supposed to be numbered in zig-zag style, with 1 and 2 on the top row, 3 and 4 on the middle row, and 5 and 6 on the bottom row. The end result of that was chaos, and a non-functioning ISP connection. I ended up cobbling together a cable to go between the ISP and PCB which unraveled the confusion.
Pin orderings matter for the DB-9 connector. The DB-9 part I used on the schematic used the male DB-9 pin numbering, which is the mirror image of the pin numbering for the female DB-9 that I'd eventually be using. Happily, I was able to get myself out of that situation by mounting the DB-9 on the bottom of the PCB rather than the top. Doing so actually made the design better, as my USB-to-serial adaptor takes the form of a heavy dongle that plugs into the DB-9 port, so mounting it on the bottom of the board kept the dongle from flipping the board over. Unfortunately, getting to this realization required much gnashing of teeth, the purchase of an unnecessary male DB-9 connector, the sacrifice of a female DB-9 connector, and much soldering and desoldering.
