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help pls!! memory address decoder design

Discussion in 'Electronic Basics' started by [email protected]!!!, Feb 2, 2005.

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  1. the question is
    Design a decoder for the following using a "138 decoder, NAND gates, and
    iinverters":
    a 1K memory is connected to a 16 bit address but. the first location of the
    memory is to be at 4000H. DO NOT use partial decoding. draw a decoder
    circuit that gives an active low for these memory locations.

    this is a review quiz i got today in my first microcontroller lecture.
    my prof just gave us a super simple solution that totally confused me...

    1K = 1024
    A0 ~ A9 change
    A10 ~ A15 decoded

    i understand that 1024 = 2^10
    so A0 ~ A9 is used for address.
    but what does he meant by "A10 ~ A15 decoded"???

    thnx for any help...
     
  2. You need 10 bits to address each individual byte in the 1k memory. Since it is
    likely that the intended use is to have each byte of the 1k memory to occupy
    successive bytes in the memory space, the low order 10 bits of the address are
    implied. That's the A0 through A9 that you agree with.

    What this also means, though, is that you need to break up the 16 bit address
    space (65536 bytes) into 64 distinct 1k sections (that's the assertion from the
    instructor about the "do not use partial decoding." He/she wants it fully
    decoded to the exact 1k section and doesn't want the memory mapped anywhere
    else. This means you need all 6 of the other address bits to decode out the 64
    different 1k regions. And you only want exactly ONE of them to be hooked up so
    that a low is presented to the 1k RAM at the right 0x4000 start address.

    That is, you need to decode this: 0100 00xx xxxx xxxx

    A '138 decoder will only decode a 1-of-8 circumstance and that, by itself and
    without using the enables, wouldn't get you there. Hence the NAND and INVERTER
    options allowed for you to use -- most likely something going to those enable
    inputs on the '138. (I see a single inverter required, in addition to the
    '138.)

    Have you read and understood the '138 data sheet? Do you need to have anyone
    explain the solution the instructor provided in more detail?

    Jon
     
  3. thnx for your reply

    now i understand the 4000H part.
    from 0100 0000 0000 0000 to 0100 0011 1111 1111 is the memory im lookin
    into...
    if the question changes to ,say , a 2K memory connected to the same but
    starting at 5000H.
    i'll be looking at the memory 0101 0xxx xxxx xxxx, which starts from 0101
    0000 0000 0000 and
    ends at 0101 0111 1111 1111. am i right?

    for the decoder part. is he looking for a active low output when the address
    of interest is fed into the input of the decoder??
    isn't that a encoder?

    for '138, if i feed it with a 011, i should get a output of 0000 1000 which
    means im selecting the 011th ouput of the decoder. that's why 138 can also
    be used as a DMUX which means make only the Nth, N is the input, output
    active.

    so let me go back to the question. if it's asking for a decoder, im taking a
    number in binary,N, which represents the Nth byte in the starting from
    4000H. I'm gonna decode it then produce some result.
    if i want to access the 1023th byte my memory, according to the function of
    a decoder, i am getting a 1024-bit output which the MSB high and all other
    low.

    something is just not right here...
    please point it out...

    thank you very much
     
  4. Well, a 1k memory requires 10 address lines to access each address. So the
    adress lines A0~A9 need to be connected to the memory chip. The first
    address you need to decode is

    0100 0000 0000 0000

    and the last one

    0100 0011 1111 1111

    or generaly

    0100 00xx xxxx xxxx

    So you need to provide a chip select for the memory:

    CS = /A15 + A14 + /A13 + /A12 + /A11 + /A10

    This function does not fit in a 138 so you have to add some other logic.

    Google for the LS138 or HCT138 to find out the possibilities of this chip.

    Be warned that, although it is not mentioned in the exercise, in practice CS
    lines are often activ low.

    petrus bitbyter
     
  5. Lord Garth

    Lord Garth Guest

    CS is to be active low. The 6 upper address lines are not OR'd together.
     
  6. Andrew Holme

    Andrew Holme Guest

    You want 0100 00xx xxxx xxxx

    The 138 has 3 enables: G1, G2A, G2B.

    G1 is active high.
    G2A/B are active low.

    You could feed the top 3 address lines to the enables and the next 3
    address lines to the ABC inputs:

    G2A = A15
    G1 = A14
    G2B = A13
    C = A12
    B = A11
    A = A10

    This way, output Y0 would be your RAM chip select:

    Y0 = 0100 00xx xxxx xxxx
    Y1 = 0100 01xx xxxx xxxx
    Y2 = 0100 10xx xxxx xxxx
    Y3 = 0100 11xx xxxx xxxx
    Y4 = 0101 00xx xxxx xxxx
    Y5 = 0101 01xx xxxx xxxx
    Y6 = 0101 10xx xxxx xxxx
    Y7 = 0101 11xx xxxx xxxx

    You don't need any extra gates.
     
  7. You are right if you consider the plus sign an OR which is usually done. I
    also use to write the * for AND and the + for an OR but the other day I had
    to work with a text that used the + for an AND and a | for the OR. Very
    uncommon but that's why I was mistaken. So you have to read :

    CS = /A15 * A14 * /A13 * /A12 * /A11 * /A10

    petrus bitbyter
     
  8. Lord Garth

    Lord Garth Guest

    I think we lost the OP....

    I thought you might confuse the OP which is why I pointed it out. I think
    some
    text use the asterisk for negation rather than the / most of us use just to
    mix
    up our heads some more!
     
  9. thank you guys very much....

    altho the notations are a bit confusing, i'm good with it...
    i guess sooner or later im gonna mess up with the notations...

    still remember the words my C intructor told me when he reject my code...
    "No matter how good your code is, as long as it doens't follow my rules,
    it's not completed. Here im the boss, you work for me. Just like in the
    industry!!"
    i was just using a differnet indent style...
     
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