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OPT3101: Source of external clock

TSUTOMU HATTORI
Prodigy 90 points
Part Number: OPT3101

Dear team,

There are some examples in that External Reference CLK is provided from MCU,

like Typical Application Block Diagram, or some circuit schematics of reference designs.

Is there any specific reason to provide External Reference CLK from MCU to GP2 port?

How about using VCXOs as a source of External Reference CLK?

I wonder that a source of External Reference CLK is required a certain level of quality?

like accuracy, or level of jitter, etc.

If so, how much of quality?

Thank you in advance for your advice.

Best regards,

HATTORI, Tsutomu

  • 0
    TI__Mastermind 25895 points

    Hi,

    Please refer to the OPT3101EVM. This uses the external clock. It is used for frequency calibration. Please see below from datasheet

    7.3.4 Oscillator

    The system clock is generated using an on-chip oscillator with high stability across temperature. This oscillator is trimmed to a nominal frequency of 80 MHz within ±3%. For accurate distance conversion, this frequency is trimmed digitally to 10-bit accuracy. Additionally, the device can accept an external reference clock and correct for the on-chip oscillator variations for continuous background frequency calibration.

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 90 points

    Hi Alex,

    Thank you for your message.

    I understand the purpose of External Reference Clock.

    Would it be possible to confirm following things.

      1) In the circuit of OPT3101 EVM and the circuit of TIDA 010021 Reference Design, 

          why External Reference Clock is provided from GPIOs of MCU?

          Is  there any specific reason to use GPIOs of MCU?

      2) Is it a good idea using a VCXO as a source of External Reference Clock?

      3) How much do we need to take care of the quality of the source of External Reference Clock?

           for example, jitter or rise time and fall time, or noise, frequency drift, something like that.

    Best regards,

    HATTORI, Tsutomu

  • 0 in reply to TSUTOMU HATTORI
    TI__Mastermind 25895 points

    Hi,

    1) In MSP430 the clock can be output from certain GPIO pins that support GPIO or clock output. In this case GPIO is used as clock output.

    2) I don't see an issue with this as long as voltage level is matched to IC requirements and clock signal integrity is good. However, generally an MCU can be used to do this and is already available in the system. What is the reason for wanting to use VCXO?

    3) A stable clock needs to be supplied to the OPT3101 or you will not get benefit over the internal performance and may also get worse performance. For example, without external clock 3101 frequency drift will be per datasheet spec. With external clock it will correct to the clock reference.

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 90 points

    Hi Alex,

    Thank you for your quick response and detailed explanation.

    I just wanted to confirm if there is any specific reason to use GPIO of MCU,

    not oscillators as a clock source.

    I didn't mean I wanted to use VCXOs.

    Best regards,

    HATTORI, Tsutomu