ROM as instructor

Discussion in 'General Electronics Chat' started by Robin Mitchell, Oct 28, 2009.

  1. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    Hi guys,

    Some of you may have seen my ALU schematic on my 4 -Bit computer. As you can see, it uses many logic gates to instruct the alu to do diffrent functions.

    Now, is it possible to have a chip of some sort that you personally program bit by bit so when a valid instruction is sent it gives an output. For example;

    I send the code 1001 to the chip and it selects the artihmetic unit

    Thanks!!!
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    It's called a microcontroller.

    You could look into a demux like the 74LS138
     
  3. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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  4. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    It was a commonly used part for address decoding. It's also known as a 1 of 8 selector. The truth table is on page one. A really big but less common version was the 74LS154
     
  5. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    I get the table now, but i dont understand that the output i want is low and not high. Does this mean that every output needs an inverter?

    Example:

    Input output
    001 1111111110

    I want:

    Input output
    001 0000000001
     
  6. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Yep that's it.
     
  7. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    WHAT WERE THEY THINKING :mad: :mad: :mad: :mad:
    AHHHHHHHH!!! THEY ARE SO IRRITATING

    Can i just enable all the enable pins?

    Oh, who else here hates it when they do stupid pin layouts. For example, the RS flips flops 4043 have the inputs in random locations!!!

    Oh, by the way blueroomelectronics, love the avatar!
     
  8. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    You could use a PAL or GAL like they did in the 1980s

    also be careful when mixing logic families. TTL is 5V and CMOS has low output drive.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you want to "Roll Up" some of the chips, but still stay with DIPs, take a look at the 16GALxx type Programmable Logic Arrays

    If you have a ROM Burner, it should also work with them. I use palasm, an assembly type language for describing what the pins "do", but there are other options as well.

    This could simplify the 4 bit ALU without using a uC. Have you looked at the Home Built Z-80 and 6502 systems made with Wire-Wrap?
     
  10. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    Yes i have, too complex!!!

    Where can i find a range of microcontrollers that can have like 4 bit instruction and many outputs?
     
  11. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    The ancient Intel 4004 is a 4 bit micro.

    4 bit micros never had a large market share. I thought you wanted to avoid microcontrollers?

    Hint look at an Apple 2 vs Apple 2e motherboard. The 2e used a custom IC to replace scads of TTL on the Apple 2

    Idea. You might want to look up a Sinclair ZX80 clone. There was a bit of hardware that used software tricks to get rid of glue logic.
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    Going way back, I can only think of the Univac 1206 and 1208 computers, and they had 6 bit instructions. But they also had lots of I/O channels.

    Just over 10,000 transistors (PNP) and could be considered as all NOR gates (1206) or NAND gates (1208).
     
  13. t06afre

    AAC Fanatic!

    May 11, 2009
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    If you have problem use a ROM. They are easy program. I am sure they have some at your school
     
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