8 bit CPU: Soldering the boards

The boards arrived, so did parts. Time to put it all together.

I've chosen to make boards using SMD parts, soldering those might sound a little scary. What I remember from previous experience (I've soldered a couple of small boards before), it is easier than one could think before trying.

Tool-wise it also does not require anything fancy. Good tweezers, small soldering iron, flux pen. Solder wick could be useful, but I did not need that very often. It helps to have brightly lit workplace.

Soldering SOIC chip on the board so that it is connected properly is quick and easy. It takes a bit longer if one wants to align it perfectly. Solder one corner pin, check alignment, re-melt and correct until you're happy, then do one in the opposite corner. Finish with the remaining pins. 0805 resistors and capacitors also are quite easy - solder one pad, check, re-melt. Once happy - solder other one.

What I struggled a bit with was LEDs. Again - it is not hard to put them on board and solder (they also are 0805 size), it is hard to align them. When LEDs are placed in a row, it is very visible if they are even a tiny bit off. Later I invented a technique that helps get more precision - taped a thick paper on the board and used it as a guide.

My boards have components on both sides, soldering first one is fine, but becomes tricky when you flip the board over. Boards are designed to be plugged into breadboard, so I used headers as a support. Once I was done with SMD parts - soldered the headers last.

The most difficult thing to solder was the PLCC socket for my EEPROM writer. I managed to do it, but not without damaging the plastic part. Will have to do some research how to do it properly when time will come for Control Logic or Output Register boards.

Here's a real-life register board:


And when flipped over:
 
 
I have to admit that not everything turned out faultless. First - I did forgot to order the 74HC283 adder chips for ALU. When those arrived, it turned out that I've built a board using wrong footprints. I used a bit wider footprint, but luckily I could still solder the chip.

Turned out, I've also made the same mistake for 3 chips in Flags register. Then there was more embarrassing fault - I had forgotten to route a pin. Still, the signal there was just a GND, that I could obtain by deliberately making a solder bridge to neighboring pin.

My EEPROM writer did not turn out to be perfect as well. I did not check the limitations imposed by Arduino board itself and placed a thru-hole chip over the USB connector. The problem is that chip's pins touches the connector's grounded metal casing. At first I tried to place a layer of insulating material (paper, electrical tape), but pins were too sharp and kept puncturing it. In the end I ground them flatter and then placed the tape.
 
All in all the boards are functional, but only Register board turned out to be faultless. I'll definitely redo the Flags, as it was just too rushed, but I think I'll keep the rest.

Comments

Post a Comment

Popular posts from this blog

8-bit CPU: ALU and Flags

Image
Now that I have a pair of devices that can store a byte, I can proceed to do something more interesting. Build a device that add them together, for example. In other words - let's build an ALU. The design follows same basic idea as Ben's - two 4-bit adders, XOR gates and an output buffer. I'm using HC logic here, so I used 74HC283, 74HC86 and 74HC245 chips. Since I've built only one Register and did not want to build another, I connected my Program Counter board as register B. If I keep the counting function disabled, it serves the purpose just fine. It took a lot of wires, but I'm just getting started, let's build Flags register as well.  There were no surprises with Zero flag - 4 NOR and 3 AND gates does the job - just following the Ben's design. Another latch (74HC173) chip to store it, done. The fun starts with the Carry flag: I wanted to support a carry-in signal for the ALU. It is very useful if one wants to perform the addition or subtraction for wid...
Read more

8-bit CPU: Clock module

Image
The parts are finally here an I can have some fun! Since nothing runs without clock, I decided to start with that module. It's probably not the most fun one, but I definitely need it, before doing something else. I was planning to follow Ben's design to some point. I had some TS555CN timers at home, so I decided to try those first, before opening the package with the new ones I ordered. Wired up the 555 in basic astable configuration with capacitors, potentiometer and decided to test if it works. So I connected a LED to the output and powered it up. It worked. Well, sort of. LED blinked and I  could adjust the frequency, but then suddenly it started to blink very fast, then again back to normal. What's even stranger I could trigger or suppress the effect by touching either chip or capacitor (just top of the chip, not the leads). I did accidentally connect my power backwards on first try 😓, so I figured it caused some damage or something. Replaced the chip with one of my n...
Read more

8-bit CPU: Memory

Image
Time for another important part of the computer - memory. Here I think I have to come up with my own design. Ben's version has only 16 bytes, uses nowadays-exotic 74189 memory chips and there are DIP switches for programming. For the memory I plan to use 62256 SRAM chip. It can store 32 KiB of data, I initially plan to use just 256 bytes, so I'll connect remaining address pins to GND. I'm also not in a mood for any DIP switch based programming, so I'll add an EEPROM instead. There are several ways how computer can utilize such setup: split the address space into program and data memory. This is how Ben's 6502 breadboard computer works. Some additional logic gates are used for chip selection, the main issue however is that it is not very flexible. use Harvard architecture , where program and data memories are completely separate. Reading and fetching instruction are two different operations: former accesses RAM, latter - ROM. This is how AVR microcontrollers (heart ...
Read more