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OMAP L138 CLKOUT: drive multiple ICs?

Anonymous
Anonymous


Hi,

I would like to ask a question OMAP L138 [pin T18, CLKOUT/PRU1_R30[13]/GP6[14]/UHPI_HDS2].

Besides debugging usage, this pin can also function as an output pin to drive other ICs on board. This pin is in power group C. In the discussion follow let's assume the voltage for group C (hence CLKOUT) is 1.8V.

Shape

Is CLKOUT an analog sinusoid signal or digital squire wave signal?

Driving capability

How many external ICs can it drive? DACs, ADCs, CODECs and a variety of ICs on the chip needs external clock, and it goes without saying that at least one can be driven be CLKOUT provided that CLKOUT frequency and the required clock frequency of the IC match. But if CLKOUT is used to drive two, three, four or even more ICs, would it get overloaded to the extent that it cannot successfully drive some of the ICs it connects to?

Does trace impedance matter here? If there are multiple ICs to be driven by CLKOUT, should I make traces as shorter as possible?

Boosting

Additionally, is there any ICs that could "boost" the CLKOUT signal, so that even if there are multiple ICs be driven by it, the boosted CLKOUT signal still have enough energy (voltage and current) to drive them?

 

Zheng

  • 0
    TI__Mastermind 27585 points

    Shape: It is a digital square wave

    Drive capability: The IBIS model includes data for this buffer, which you can simulate to determine what the signal looks like under various loads. In general, 2-3 loads could be driven without external buffers.

    Boosting: Yes an external buffer that can switch fast enough for your frequency would be necessary if you plan to connect many ICs. Again, simulation will help determine this.

    Jeff

  • Anonymous
    0 Anonymous in reply to jc-ti

    jc-ti,

    I got it, thanks.

     

    Zheng

  • Anonymous
    0 Anonymous in reply to jc-ti

    Jeff,

    Amplitude:

    What are

    1. VMIN
    2. VMAX
    3. VPP

    of the CLKOUT?

    Since it belongs to power group C, is there any relationship like

    1. VMIN=0V
    2. VMAX=IOVDD
    3. VPP=IOVDD

    For example, if voltage supply of power group C is 1.8V, does it mean for CLKOUT:

    1. VMIN=0V
    2. VMAX=1.8V
    3. VPP=1.8V

    ?

     

    Could you also have a look at DM6437: CLKOUT and VCLK ? For DM6437, although the group voltage (unlike OMAP L138, DM6437 has only a single power group) is 3.3V, I observed around 5V VPP value. I don't know why. Do you know the reason?

    And would this also happen on L138's CLKOUT?

     

    Zheng

  • 0 in reply to Anonymous
    TI__Mastermind 27585 points

    Yes your example is correct. For questions on DM6437, you should post in the appropriate forum. The CLKOUT on L138 have a VPP of 1.8 or 3.3 depending on how power group C is sourced.

    Jeff