LED indicators for address pins?

Discussion in 'General Electronics Chat' started by matthew180, Aug 24, 2010.

  1. matthew180

    Thread Starter New Member

    Aug 24, 2010
    3
    0
    Greetings,

    I have a simple bank-switch circuit and I'd like to add LED indicators for the 5 bank select pins. The design is very simple, let me see if my ASCII art is up to par:
    Code ( (Unknown Language)):
    1.  
    2. --+--<A0 to A12>----------->[27SF020]
    3.   |                            ^
    4.   |                            |
    5.   |                       <A13 to A17>
    6.   |                            |
    7.   +--<A1 to A5>---->[GAL16V8]--+
    8.                       ^    ^
    9. -----<we*>------------+    |
    10. -----<rom*>----------------+
    11.  
    The GAL is programmed to function just like a 74379 which is out of production and hard to get (the 379 also only has 4 outputs, but I can get up to 8 with the GAL.) The rom* is a clock enable and will be low when the computer is addressing the 8K (A0 to A12) in the range I'm dealing with. The we* signal acts as the clock to the GAL and will latch A1 to A5 which selects 1 of 32 8K-banks in the 27020.

    Anyway, all that to ask: can I drive some LEDs on the output of the GAL to indicate the selected bank? Both the GAL and the 27020 are 5V TTL versions, but I can't manage to entice the source/sink current values out of the datasheets. I've been looking, but I'm not finding the values that I think I should be seeing.

    I have ordered 5 low power SMD LEDs that draw 2mA at 1.8V, and I am planning on using a 3.32K current limiting resistor that should keep the current between 1mA and 1.5mA depending on the actual Vhi output value of the GAL's flip flop (I'm not sure if it actually goes to VDD, but on my bread board I was reading about 3.8V actual.)

    Seeing as the GAL is "TTL compatible", and TTL is supposed to fanout to 10 devices, I didn't think the LEDs would cause any problems. Basically I'd like to do this:
    Code ( (Unknown Language)):
    1.  
    2.             [27020 A13]
    3.                  ^
    4.                  |
    5. [GAL]--<Q0 out>--+
    6.                  |
    7.                  Z  3.32K
    8.                  |
    9.                  V  LED (1.5mA max)  
    10.                  -
    11.                  |
    12.                 GND
    13.  
    Any insight would be greatly appreciated.

    Thanks,
    Matthew
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try a Google search for "GAL16V8". I don't know if that part was specific to Lattice Semiconductor or not, but Lattice has discontinued that line. The Lattice datasheet I found did give source/sink current and voltage specifications.
     
  3. matthew180

    Thread Starter New Member

    Aug 24, 2010
    3
    0
    The GAL is the Lattice part, but only some of the parts in that line have been discontinued. I'm specifically using GAL16V8D-25LJN and GAL16V8D-15LPN. The circuit is built and working, except for the LED indicators which I have not added yet.

    I also have the datasheet, but I'm not sure which current value I'm looking for. The obvious value Ioh (high level output current) has a value of -3.2mA. I guess it is the negative value that makes me unsure if that is what I'm looking for. If so, then it would seem my 1.1mA to 1.5mA would be acceptable and should not cause any problems since the only other load is the SST27SF020 address pin which has an Ili (input leakage current) of 100uA...

    Am I reading that right?

    Matthew
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you just invert the LED display logic?

    The GAL16V8D-x outputs will sink up to 16mA.

    You'll need to reference the high side of the LEDs to 2.4v, of course. An NPN transistor used as a voltage follower with it's base at ~3.0v-3.1v would work.
     
  5. matthew180

    Thread Starter New Member

    Aug 24, 2010
    3
    0
    Well, board space is limited and since the indicators are for human convenience, it would be nice to have an "on" LED represent a binary "1" instead of "0". Also, non-inverted LEDs will spend most of their time in the off state since higher numbered banks are rare.

    So was I reading the datasheet right? The current for a high output is only about 3.2mA?

    Matthew
     
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