74HC595 Strobe and Latch Clock

Discussion in 'General Electronics Chat' started by Llamarama, Feb 9, 2014.

  1. Llamarama

    Thread Starter Active Member

    Feb 1, 2010
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    Hello everyone, I'm using a load of cascaded 74HC595s to drive a few bargraph LEDs, however instead of using a microcontroller to use them, i'm using pure logic.

    I have the data inputs tied high so a continuous stream of logic high is fed in, shift register clock, MR and /OE are used as usual.

    I'm not quite sure what to do with strobe and latch clock though. Going by datasheets it looks like the latch clock is simply the shift register clock inverted, so that shouldn't be a problem (I hope?). The main problem is strobe, the datasheet shows either high or doesn't matter for all functions, so if this is held high will it prevent the shift register functioning?

    Any help would be greatly appreciated, almost all the resources I've found online just mention the datasheet and are almost entirely geared towards Arduino use. Thanks everyone
     
  2. MrChips

    Moderator

    Oct 2, 2009
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  3. Llamarama

    Thread Starter Active Member

    Feb 1, 2010
    62
    2
    Sorry, I'm using the NXP (Phillips) datasheet that has

    SHCP - Shift register clock
    STCP - Latch clock
    /OE - /Output enable
    /MR - /Master reset
    DS - Data strobe (Marked as input)

    http://www.nxp.com/documents/data_sheet/74HC_HCT595.pdf

    Going by the pin numbers;

    SRCLR' = /MR
    SRCLK = SHCP
    RCLK = STCP
    OE' = /OE

    I usually stick with NXP datasheets because they're mainly digital ones, not scanned printed ones like a lot of the older TI ones, but their non-standard naming of pins annoys me. (Especially CP0 and CP1 on the 4017, that had me stumped for a while)
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    DS (14) is not Data Strobe. This Serial Data Input.

    STCP = RCLK (12) is a clock input used to transfer the data from the shift register to the output latch. Ideally you want this to appear after the SHCP = SRCLK (11), the shift register clock.

    You can connect STCP to SHCP. However the output will be one clock cycle behind the shift register, i.e. the correct data will appear on the outputs after one clock cycle later.
     
  5. Llamarama

    Thread Starter Active Member

    Feb 1, 2010
    62
    2
    Thanks for the heads up, been messing around with the circuit on breadboard today and it seems to work. Thanks for the help :)
     
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