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SN54LS674: Cascading Parallel to Serial Conversion Shift Registers

Viraj Patel
Prodigy 130 points
Part Number: SN54LS674
Other Parts Discussed in Thread: SN74HC166, SN74HCS166, SN54HC165, CD54HC165

Hi, I am looking to use two (2) SN54LS674 shift registers to process 32 bits of information for my application. The SN54LS674 seems the ideal candidate to perform Parallel to Serial conversion.  Shown below in Figure 1 is the basic idea I have for my application. How should I implement two cascading shift registers to count from LSB to MSB? Also, the datasheet does not provide much information about the control inputs and only provides a truth table. Can you please explain how inputs _CS (Pin 2), SHCLK (Pin 3), R_W (Pin 4), and MODE (Pin 6) work? Can these inputs be shared by Micro GPIO? Please let me know as soon as possible. Thank you.

  • 0
    Guru 277220 points

    Why have you chosen this particular device? Do you really need a bipolar, military-grade device?

    As far as I can see, the '674 is not really designed for cascading. You can share all inputs and the SER/Q15 output, but use /CS to address them separately.

    If you have selected it because it has 16 bits, then that is probably the wrong choice. You have to cascade anyway, so use four 8-bit shift registers (e.g., SN74HCS166/5, SN74HC166/5, SN74LV166/5A).

  • 0 in reply to Clemens Ladisch
    Prodigy 130 points

    I chosen this device because I needed a military grade device in my application. I understand I can use four (4) 8-bit shift registers but I want to reduce the number of cascading registers required. 

  • 0 in reply to Viraj Patel
    Guru 277220 points

    LS is a bipolar family, i.e., it uses lots of power, and the output TTL levels do not go up to 5 V. And the '674 is not easy to cascade because SER/Q15 is a shared input/output; you need quite a lot GPIOs.

    The SN54HC165/6 or CD54HC165/6 can be cascaded easily.

  • 0 in reply to Clemens Ladisch
    Prodigy 130 points

    Thank you for the information. Yes I am restricted on board space. I will proceed with four 8-bit shift registers since they are surface mount.