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Logic family selection

Roger Arrick
Prodigy 40 points
Other Parts Discussed in Thread: SN74HC00

Need advice selecting a logic family.  Requirements:

* Through hole parts.
* Standard 74xx devices readily available.
* As fast as possible using 2-layer boards.
* Power draw not important.

Application: Custom CPU.

Leaning towards ACT.

  • 0
    TI__Guru* 96681 points
    Hi Roger,
    My recommendation for logic family would be the HC family (SN74HC00) - it's CMOS based logic, and comes in PDIP packages.

    If you want something fast, I would recommend using an FPGA instead.
  • 0 in reply to Emrys Maier
    Prodigy 40 points
    It looks like ACT is substantially faster than HC/HCT.
  • 0 in reply to Roger Arrick
    TI__Guru* 96681 points
    The two families are directly compatible (they both work at 5V), so you can use ACT anywhere we have devices for it and HC for any missing functions (since ACT isn't as large of a logic family).

    I'm not really sure what you are trying to achieve here. When you say 'fast' referring to a discrete component CPU, what type of speed are you trying to achieve? There are many processors available that will leave any discrete CPU in the dust, and will cost orders of magnitude less (for example, any of the MSP430 line of microcontrollers).

    Note that the speed of the individual chips will not be the limiting factor of a discrete CPU -- it will likely be limited by the parasitics from the board, traces, and packages.
  • 0 in reply to Emrys Maier
    Prodigy 40 points

    Ok, so even with the slower HCT in DIP, the main speed limitation would be board layout?

    My understanding is ACT is 10X faster than HCT.

    Are you saying that it wouldn't help improve speed of the circuit?

    If that's true, why would a selling point of ACT be speed if it couldn't be used?

    I'm not trying to be practical on this project and just throw in an off-the-shelf MCU, reasoning is for all my other projects :)

  • 0 in reply to Roger Arrick
    TI__Guru* 96681 points

    If you pull up datasheets for a D Flip-Flop from each family (I chose the 374 function), at 5 V the HC device is rated to ~60 MHz (typical) operation, and the ACT is rated to ~160 MHz (typical) operation.

    What you're talking about isn't using one or even just a few devices (which is usually how these devices are used), you're talking about using hundreds of them in conjunction, with many inputs tied together and traces/wires running everywhere (a CPU is pretty complex, as I'm sure you know).  I would be surprised if you could get a discrete computer running past 1 MHz.  I have seen some similar builds in the past, and they typically operate in the low kHz range.

    The speed of the device will certainly make a difference in many applications, but in this one, you will run into many other bottlenecks.  I wish you luck in this project -- it's a big undertaking!

    If you want to see a more direct comparison of ACT and HC, you can check out or Logic Guide, page 11 has some details on both families listed out.

  • 0 in reply to Emrys Maier
    Prodigy 40 points
    I'm going to guess about 50-100 chips for an 8-bit risc machine and 16 bit address bus . Basic instructions of inc/dec, rotate/shift, add/sub, and/or/xor, compare, move, call, conditional jumps/calls, stack manipulation. Looks like I can get those in less than 8 states. That doesn't include a uart/baud generator, but does include an old-school front panel to access registers, flags and memory. How about a contest? :)
  • 0 in reply to Roger Arrick
    TI__Guru* 96681 points
    I look forward to seeing your results - unfortunately between family, school, and work, I have very limited time for my own hobbyist electronics these days :)