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Question about SN74LS628

Guru 19555 points
Other Parts Discussed in Thread: SN74LS628

Please let me know about below information for SN74LS628.

(Datasheet have no described information)


①About calculation method of output frequency

 Datasheet is not described equation, graph only.

 Customer want to set 500kHz~1MHz finely, but they can't set without an equation.

②RNG and FC pin

 Are these pin mean to set output frequency? (Input to fixed voltage, OK?)

 If low frequency application (500kHz~1MHz), is RNG=5V and FC=0V correct?

③About CX1 and CX2, I think that these pin connect capacitor, is there recommended value?

④SN74LS628 have two RX pins, which is correct below?

 ・Both RX pins use common (connect)

 ・RX use one side pin, the other side pin is GND or Open

⑤Is there reference design?


Best regards,


  • Hello Satoshi-san,

    We are currently looking into your questions and will get back to you as soon as we can.
  • Satoshi-san,

    Having charts alone was common 30 years ago.

    Cext of 820pF is a good start for needed frequency. Capacitor connects to both CX pins.

    Timing resistor also connects to both RX pins.

    The timing cap and resistor do not connect to GND, VCC

    RNG and FC pins seems to have the same power per figure 3 in data sheet.

    Only the polarity is different. Increasing FC raises output frequency, RND does the opposite.

    Start with both FC and RNG at 1/2 VCC. You only have to tune one of the other to tune in desired frequency.