74HCT193 and Clock speed

Discussion in 'The Projects Forum' started by Enliten, Feb 10, 2009.

  1. Enliten

    Thread Starter Member

    Feb 10, 2009
    Hi, im currently working on a couple of small projects but have a question regarding the 74HCT193 counter chip. I want to be able to connect a 25.175Mhz clock crystal to the counter but cannot seem to get my head to work out if that would be too quick for it.

    The datasheets seem to say that the HCT can run at 9ns. My head tells me that this would be fine as 25.175Mhz it would clock at around 40ns, or am i wrong.

    I use the HCT as its good at sinking and sourcing current which can be helpful.

    Im intending to link the 3x193 together as i need to count to 800 and 525 then reset. Im intending to use multiple AND/OR gates to link the output of the clocks to the reset line so that an output of 799 then reset back to 0 (counting from 0 - 799, 0 - 799 etc). Does this sound right. You know when you have it all planned out in your mind then you start doubting yourself ;)

    Oh on a side note, does anyone have any suggestions for good books on Logic gates etc, mine is very old and only covers the LS chips in basic. Im also wondering if there is a good logic design/simulation package. Ive been using CEDAR, but it doesnt have much in the way of logic gates, it would be nice to have a package with alot of the 74 series included.

    Many THanks

  2. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    At 4.5V supply voltage, the min. spec is 20MHz at room temperature and typical is some 43MHz so you are likely to get away with 25.175MHz at 5V supply voltage and room temperature.

    see the image where the cursor points to:
  3. Enliten

    Thread Starter Member

    Feb 10, 2009
    Thanks for the reply. I wonder how they get these values (other than trial and error?) Since the 9ns would suggest a better handling. I guess its suck it and see. Someone suggested using ACT instead of HCT?