8-bit CPU: Display (part 1)

In his series, after building the clock module, Ben started to work on registers. Those are important parts of the computer, but feels a little underwhelming again. So I decided to go for output display instead.

The module requires a clock for multiplexing its output to four 7-segment displays. The clock should eventually be independent from the main clock and run much faster. But in the meantime other parts of the computer do not exist yet, and I might like to run it really slow. So I just placed the 555 on the board, but did not even bothered to wire it up. I'll just use signal from my freshly finished clock module. I haven't really used it for anything yet!

Ben used 74LS76 JK flip-flop to manually build a 2 bit counter. I just went for 74HC163. It's a 4-bit counter, but there's no harm in using only two outputs. Wired counter to inputs of (one side of) 74HC139 2-to-4 demultiplexer - that should allow us to cycle the displays.

I was anxious to see if I'm on the right path so I added LEDs to the output of the demux, connected clock module and powered it up. I was not a surprise that it worked as expected, but still was an exciting moment. It's my first build using logic chips, that does something more complicated than blinking a LED.

With that sorted out, I could proceed and move the 7-segment displays from my Arduino prototyping board to the right side of this breadboard. Luckily I had not to prepare the connecting wires again, the layout stays the same, I'm just moving them to another board.

Next I placed an AT28C64 EEPROM on the board and tried to figure out how to fit in current limiting resistors. Damn, that 28-pin EEPROM is huge. I could not locate 28C16, so I went with a bigger one (still smallest I could get).

The good practices call that for LEDs connected in parallel, there should be a current limiting resistor for each LED. This is also case with 7-segment displays. The technique ensures that each segment gets same current (and has the same brightness) regardless if other segments are lit or not.

So in this setup (with common-cathode displays) it calls for a resistor on each positive side of the display - on the 8 wires coming from EEPROM. That was the configuration I connected them in my previous Arduino build.

If I placed my chips end-to-end, without any gaps between them, I could probably squeeze them in. But otherwise I managed to find a place for just 4 of them. Well, it looks like I'll have to deviate from the good practices. There are 4 wires coming from displays and going into 74HC139 demultiplexer. It's not ideal, as now segments when showing digit '8' will be less bright compared to when displaying '1'. But well, it's a prototype, it'll have to do. I'll just keep that in mind for PCB version.

In one of my next builds I found out the hard way, that trying to squeeze in parts tightly on breadboard is not such a good idea.

While wiring up EEPROM, I decided to deviate (again) a bit from Ben's design. He used A8 and A9 address lines for selecting the display (ones-place, tens-place, etc.). I decided that I'll use A0 and A1 for that.

It won't change how the device works in the end, but will make a lot neater data layout in the EEPROM. In my case, each mapping for number will consist from 4 subsequent bytes. In Ben's case - they are scattered all over the place.

On the other hand - address lines A0 to A7 are conveniently placed next to each other on one side of the chip, but A8 and A9 are placed on the other. This adds a little annoyance when wiring up the input lines. Well, no design is perfect!

Hardware-wise I'm almost done. The 555 is not connected yet, but I'll just use the clock module for the time being. There's a bigger problem though - the EEPROM is blank now and no matter how I connect the address lines, It just shows "8.8.8.8."

I can, however, remove the EEPROM and connect some jumper wires to get funny symbols on the displays.


If you look very carefully, the picture does not match the story exactly. I forgot to snap a picture at that time.

I'm using those 2 incoming address lines to get different symbol on each display, proving the basic principle. But to display normal digits, I have to program the EEPROM. For that I have to build an EEPROM burner first.

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