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SN74HC164 CLOCK Rising Time

Patrick Zeng
Intellectual 1090 points
Other Parts Discussed in Thread: SN74HC164, SN54HC195

Hi team,

  My customer Midea runs into a major problem with SN74HC164DR.

  SN74HC164 is used for LED display board. There is a long cable (2 meters) between CLOCK signal source and SN74HC164(pin 8). Such long cable equals to large RC which slows down the rising slew rate of CLOCK signal. It causes abnormal behavior in the LED display. I noticed the datasheet shows when VCC=5V, the rising transition time is required to be within 500ns. But with this long cable, it's hard to guarantee the rising time is within 500ns.

  We tried to convince customer to modify external components to reduce RC but they wouldn't. And they claimed that other vendors like Toshiba works pretty good with the same board.

  Customer Midea is asking different vendors for the maximum value of "input transition rise and fall time". According to their words, some vendors can tolerate up to several us. So what I'm asking is can you please confirm what's maximum value that we can do according to the bench test? Is it just 500ns as it's said in the datasheet or it has bigger tolerance? If not, what's your suggestion to solve this issue?  Thanks.

Regards,

Patrick

  • 0
    Guru 317080 points

    The datasheet has no reason to lie, and the problems indicate that slower edges indeed do not work with the SN74HC164.

    Toshiba's 74HC164D datasheet specifies input rise and fall times of up to 50 µs. In this respect, these two devices are not perfect clones of each other.

    The easiest solution is to tell them to use the Toshiba device use a Schmitt-trigger buffer for the clock input (and for the other inputs, if needed).

    (And TI's web site wrongly claims the SN54HC195 has Schmitt trigger inputs.)