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Original question:

SN74LVC157A Signal Rate

SN74LVC157A: SN74LVC157A Signal Rate

SYLVAIN BOURIOT
Intellectual 370 points
Part Number: SN74LVC157A

Hello, I don't understand Luke Sun's answer (Dec 5, 2016 3:18 PM) about 10MHZ SPI with SN74LVC157A.

Dennis said that we can use propagation delay to estimate maximal frequency:

With VCC=3.3V:

2x5.2=10.ns

fmax=1/10.4ns~96MHz

With VCC=1.8V:

2x14=28ns

fmax=1/28ns~35.7MHz

In all cases, this frequency is much greater than the frequency of 10MHZ of SPI.

So why Luke wrote "Then SN74LVC157A seems couldn't support 10MHz SPI".

Have you got a specification of maximal speed of 74lvc157A gate?

Regards.

Sylvain

  • 0
    TI__Mastermind 44520 points
    Hi Sylvain,

    This device will be able to handle 10 MHz. I'm not going to dive into Dennis's response, but I wouldn't use Tpd to estimate the max operating frequency. For future references, I always recommend finding a device in the same family with a clock input (for example a flip flop). Look at that devices Fclock spec and that value will a better estimate of what your device can handle.
  • 0 in reply to Dylan Hubbard
    Intellectual 370 points

    Thanks for your reply and your recommandation.

    But, in my design I need to multiplex signal in order to save I/O  on  microcontroler.;  that's why I need to know maximal input frequency of 74LVC157A

    Is there a general document with maximum input frequency of SN74LVCxx logic IC? .

    Fo example, in little logic guide (page 4), maximal speed of each IC family is specified.  

    Regards.

    Sylvain

     

  • 0 in reply to SYLVAIN BOURIOT
    TI__Mastermind 44520 points
    Sylvain,

    I thought you were just wanting to know if it could handle 10 MHz i'm sorry. This device should be able to handle 100 MHz (assuming good layout techniques reducing RC components on output).