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Thread: Resistors for OE and DIR on 74HC245, plus a couple other noob questions

  1. #1
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    Resistors for OE and DIR on 74HC245, plus a couple other noob questions

    Hello, a couple of noob questions: I am making a circuit board to among other things drive 4 short LED strips using the 74HC245:

    1) If I want OE low and DIR high, can I just connect them directly to ground and vcc, respectively? Or, do I need resistors in there?

    2) I have used a TXB0104 and you had to connect your Vlow to the chip as well as Vhigh. For the 74HC245, does it simply take the IO and raise it to Vcc? That is, am I right in that you don't need to connect the input voltage anywhere, just the IO pins you are trying to shift and your resultant voltage is shifted to Vcc?

    3) In testing with other components and cutting up one of those foil 1 meter WS2812b strips, using about 8 inches of a Cat5 pair, that I need more like a 470 to 1K resistor to get the lights not to flicker. Any thoughts on this? Anything less doesn't seem to work (this was with a TXB0104).


    Thanks in advance.

    --Jim

  2. #2
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    1) If I want OE low and DIR high, can I just connect them directly to ground and vcc, respectively? Or, do I need resistors in there?
    Yes you an connect them directly; no resistors needed

    2) I have used a TXB0104 and you had to connect your Vlow to the chip as well as Vhigh. For the 74HC245, does it simply take the IO and raise it to Vcc? That is, am I right in that you don't need to connect the input voltage anywhere, just the IO pins you are trying to shift and your resultant voltage is shifted to Vcc?
    Correct, no input voltage needed.

    The TXB0104 uses both voltages because it can switch between a large range; e.g. 1.2V input to 5V output. The 74hc245 just triggers when it sees input > 1/2 of Vcc; in this case the 3.3V signal from the Teensy is > 2.5V from the 5V line so it works.

    3) In testing with other components and cutting up one of those foil 1 meter WS2812b strips, using about 8 inches of a Cat5 pair, that I need more like a 470 to 1K resistor to get the lights not to flicker. Any thoughts on this? Anything less doesn't seem to work (this was with a TXB0104).
    You should be safe using a 470 Ohm resistor between the 74HC245 and the first WS2812B.

  3. #3
    Senior Member PaulStoffregen's Avatar
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    I agree with everything Potatotron said.

    In addition, I'd like to add....


    Quote Originally Posted by jrowe88 View Post
    For the 74HC245, does it simply take the IO and raise it to Vcc?
    You should use 74HCT245. The extra "T" in the part number means it recognizes any voltage over 2.0 as logic high, and any under 0.8 as logic low.

    Maybe you're wondering how such magic can be accomplished, without another power supply? Internally, they change the ratio of transistor sizes inside the chip, so the N-channel input transistor has higher transconductance than the P-channel transistor. The logic threshold switching point, where the N and P transistor input voltages cause equal source-drain current, is a lower voltage because the N-channel transistor conducts more current at lower voltage. The chip "knows" where to switch based on the power supply voltage and the ratio of those transistor sizes.

    Without the T, the threshold is higher and might not properly receive the 3.3V signals, when powered by 5V. Get the one with a "T" in the part number!


    Any thoughts on this? Anything less doesn't seem to work (this was with a TXB0104).
    Those bidirectional chips are absolutely terrible at driving long wires.

    You'll find the 74HCT245 works much better. That's why we use it on the Octo28 board. It's truly the right chip for this job.
    Last edited by PaulStoffregen; 01-19-2016 at 01:17 PM.

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