Good Beginners SRAM IC and Counter

Discussion in 'General Electronics Chat' started by ajm113, Nov 15, 2011.

  1. ajm113

    Thread Starter Member

    Feb 19, 2011
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    hehe, I'm getting more parts then I can chew from dissembling old PSUs and now I want more. ;) If anyone is looking for anything shoot me a PM and I'll get what you want.

    Anyways I'm looking for good I/P ICs. One counter and one SRAM and I was wondering if any of you can give me suggestions.

    What am I building? Nothing too special, but I'm wanting to build a 4-bit computer, so I'm going to need SRAM and a counter to store instructions and perform actions. I'm looking for SRAM that can maybe hold 64 bits or more and I'm also going to need counter(s) more then likely a 8 binary one.


    How will it work? (Encase better ideas)
    The 555 timer will act as a clocking device for the counter. When the output of the 555 timer is high the counter will count up by one. The counter's output pins will go to the SRAM/EEPROM to look at a address. When the memory IC spits out a value in 4-8 bits on a bus. My PIC microcontroller will read the value and perform a action such as turning LEDs on off as the display.

    All I'm hopping for this little machine to do is be programmed to do simple tasks such as a calculator, count down timer, and what not and expand from right there and put it in a metal box and have it sit on my desk and have something to show my friends or family just for kicks one day. :)
     
    Last edited: Nov 16, 2011
  2. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Sorry for the double post, but I'm kinda surprised no die hard fans of TI or Microchip aren't jumping into this thread. =/ Was I not making much sense?
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Might want to take a peek at what you are getting into:

    Here is a nice example

    You'd probably be better off writing a 4 bit computer instruction set based on a PIC, something like the BASIC Stamp for an interpreter.
     
  4. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Thanks for the reply!

    I dont think my message was too clear on how my computer is going to work... I went ahead and edited it so it was more clear, sorry about that.

    I've mentioned I will be using a PIC as a CPU as in I'll be using it as my "interpreter" for when the counter picks a address to read and spits out the value on a bus where my PIC will be reading. All the EEPROM/SRAM will be doing is storing instructions the user enters in and the PIC will do the work. I do want to save space on my desk of course and not build a giant box with tons of logic gates. ;)

    All I want to know is where can I find a 8 bit counter or SRAM/EEPROM. Microchip seems promising for the SRAM/EEPROM since I use their PICs all the time, but I was wondering what everyone thoughts were.
     
    Last edited: Nov 16, 2011
  5. davebee

    Well-Known Member

    Oct 22, 2008
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    I read your post, but wasn't sure it was serious, because I would have thought that an experienced PIC programmer intent on designing a computer would have enough experience to be able to select RAM and counter chips.
     
  6. thatoneguy

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    Feb 19, 2009
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    The link I posted showed what main components need to be built. I/O is the hardest, such as saving/reading from SD card and LCD display, that part you could offload to a PIC.

    The rest is a lot of bus lines, I hope you are very good with wire wrap.

    Try building an ALU in logisim using only 74xx parts from the 74xx library.

    Have you had CPU design and Compiler design in college classes or self taught? They will come in useful, since you'll be making your own instruction set.

    Somebody else here started one and gave up, search forums for 4 bit computer. Turns out to be essentially just as complicated with half the address and data lines, since the adder, bus tranceiver, and other ICs are all designed for 8 bit operation.
     
  7. ajm113

    Thread Starter Member

    Feb 19, 2011
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    eeehhh... I wouldn't say experienced, I would say still a beginner since I've been using BASIC as my primary language for using most of my PICs. I haven't had time to get into assembly and work on another project I had going, but had to put down because of real life work and a lot of other stuff going on, so I'm just looking for small projects for now for kicks. I'm currently aiming for the Air Force so I dont want a long term project at the moment.

    @thatoneguy I'm currently not going to school for any of this at the moment, I'm just using my experience using software engineering and primary knowledge of electronics design to put this together.

    I have most of how my computer is going to be put together and how the "PIC" will read the bus. The bus it's self will consist of 13 lines. 8 lines are the address of a memory location, 4 is the instruction and the 1 other lines will be used to tell if we are in read or write mode. When in write mode the bus will be blank and the counter will be reset to 0. The user will then flip a few switches and tap a button which will output a instruction to the address 00000000 to the I/O ports of a RAM/EEPROM to record. The counter will go up one and the user can keeping adding instructions. When switched to read mode, the counter gets reset to 0 and kicks on the 555 timer for clocking to start spitting out instructions each instructions on the 4 lines where the PIC will read and perform a action. I.E Turn LEDs on or off, trigger a event to another circuit and what not.

    P.S The resetting of the counter is controlled by the PIC when a read/write switch state has been changed.

    I believe my method will remove the complexity of most old school home brew computer has to face.
     
    Last edited: Nov 16, 2011
  8. ajm113

    Thread Starter Member

    Feb 19, 2011
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  9. kubeek

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    Sep 20, 2005
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    Ramtron makes nice chips and provides samples.
    I got some serial and parallel FRAMs, which are about as fast as standard SRAMs, but are non-volatile.
    Ideal for playing wiht the stuff without having to upload the code over and over.
     
  10. ajm113

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    Feb 19, 2011
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  11. kubeek

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    Yes they do have DIPs, when I ordered the samples, I got I think two types of smd serial rams and two types of dip parallel ones, IIRC two pieces each.
     
  12. kubeek

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  13. PC Pete

    New Member

    Nov 11, 2011
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    You might be better off using the PIC to generate the clock, either with one of the GPIO pins or even a timer output (which you can modify to speed up and slow down in code, much easier than turning a pot!).

    Just be aware, the good ol' 555 can cause major hassles with logic and especially RAM because of the output driver. Even the CMOS 555 can glitch the power, so bypass caps will definitely be needed if you do use the timer.

    Sounds like a really interesting project!

    FWIW, I still have a bunch of old SRAM chips (2kx8 and 8kx8, but also some 32kx8), but I'll have to dig around to find them. If you can make use of them, I'm happy to post a couple off to you with spec sheets and so on.

    Although, if you're keen on using 4-bit architecture, there were a whole bunch of 74 series parts like adders, half-adders, PROMs, and even 8x4 and 16x4 RAMs, but you'll have some problems sourcing the memory parts. You should still be able to get counters (7490), shift registers (74168 (I *think*)), latches, and so on, from Jameco, Digikey, or Mouser. You can even try Sparkfun, they have lots of breakout boards you can use to mount components as well, and they do carry some 74 series parts still.

    Good luck, and don't forget to post pictures!
     
  14. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Really? Well good thing I just ordered a bunch of 20mhz crystals on Amazon from good old Hong Kong. :) I know I'm not going to use them all, so I may end up putting half of them on my website for sale for like 25 cents each.

    Well I'm sure I can maybe convert 8bit to 4 bit if I want too, I can still go the 8bit route. I was hopping to stay 4 bit for 4 LEDs, but I guest I can add 4 more wouldn't hurt.

    EDIT: I'll stick with the 4 bit route since my computer isn't going to do much except turn a couple of LEDs on or off and maybe intergate my computer USB with my desktop computer so they both can send messages to each other. I'll have to do soom looking up with the 74 series. :)

    Of course I will thank you for the information!
     
    Last edited: Nov 17, 2011
  15. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Oh which reminds me, the crystal will be very effective and everything, but this the first time I'm using a crystal hands on. Do I just connect one of the ends of the crystal to the counter? Or will I have to do something in the PIC in order to do so?

    Thanks, dont mean to turn the thread off topic, but I may as well ask since I'm guesting it's a very easy answer.
     
  16. thatoneguy

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    Feb 19, 2009
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    If you want to go old school, order some Magnetic Core Memory

    If it was good enough for the Space Shuttle, it should be good enough for your coffee table computer.

    You would also get MAJOR geek points for using active MC memory!
     
  17. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Naw, it's not good enough in fact I need better, faster, and smaller. :)

    I'll get the 7490 counter, but the FRAM I'm still trying to get someone to see if they can sell them to me.
     
    Last edited: Nov 18, 2011
  18. PC Pete

    New Member

    Nov 11, 2011
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    The 4-bit latches were 7475s, there were some 64-bit RAMs, but my TTL manual says they're "preliminary". You can try 74189, they're 3-state outputs which are much easier to interface to modern MCUs than the earlier 7489s (which had open-collector outputs, a pain in the butt IMHO).

    And there are 4-bit magnitude comparators (7485), 4-bit counters (74161 are pretty easy to set up and you can clear the counts at any time, no fiddly setup), and more besides. You can probably google "TTL series" and get something useful...

    WRT your crystal question : no, you can't just wire up the crystal, it needs to be "driven" by a signal so it starts to oscillate in the correct mode. You'll either need to look up TTL crystal oscillators (you can use 7404 inverters with the crystal in series between two inverters, which is very robust), or use a dedicated CMOS crystal oscillator (most binary counters in the 4000 series have crystal-compatible inputs). You're probably better off with a RC oscillator, as that will allow you to single-step your way through execution...

    Don't forget to divide your clock signal by 2 (using a 7474 flip-flop) to give you perfect 50/50 clock symmetry. It halves the clock frequency, but many clocked TTL devices needed good clean clocks to work properly.

    Don't knock MC or bubble memory. The coolness factor is outta sight! Definitely makes up for the fiddly setup (usually with +12 and -5V supplies) and slow readback.

    I'd start with some digital memory first, then... expand your horizons! Who knows what you'll come across!
     
  19. kubeek

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    Like I said, don´t buy it if you don´t need lots of them, order free samples from the manufacturer.
     
  20. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Thanks PC Pete!

    Okay I ordered some FRAMs for free and I bought 4 74161, 2 7474, 2 7404, and 2 IRF510, MOSFET. The Mosfet has nothing to do with my computer, I just wanted a part I didn't have encase of a future project. ;)

    I'm gonna give these guys a try and see what I come up with. ;) If I have any problems connecting anything up I'll let you guys know. :) Of course there will be trial and error, but hopefully I get this guy running and make it hopefully do some really cool things along the way.
     
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