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AM3874 clock input

Michi Yama
Guru 10030 points
Other Parts Discussed in Thread: AM3874

Hi,

I have one question regarding clock input circuit of AM3874 device.

Now my customer uses an oscillator for supplying clock to AM3874.

The osc's output is connected to DEV_CLKIN(AH2) pin. And Both GND pin of the osc and VSSA_DEVOSC(AG3) pin are connected to each other, and also osc's gnd pin is connected to Gnd(0V).

Recently, our customer found one oscillator that is not connected to gnd(0V) by failure mounted. However, according to the measurement of clock wave, the waveform of failure mounted is not bad. It was almost same as normal mounted. There was no effect by power line noise.

Do you know the internal circuit of relation of VSSA_DEVOSC and VSS? Customer would like to know whether AM3874 CLOCK INPUT(DEV_OSC) is not influenced from VSSA_DEVOSC pin condition.

Please advise me.

Best regards,

Michi

  • 0
    TI__Guru**** 170625 points

    Hi Michi,

    Michi Yama said:

    Now my customer uses an oscillator for supplying clock to AM3874.

    The osc's output is connected to DEV_CLKIN(AH2) pin.

    The DEV clock can be sourced in two ways:

    1. Using an external crystal in conjunction with the internal oscillator - DEVOSC_MXI

    2. Using an external 1.8-V LVCMOS-compatible clock input - DEV_CLKIN

    From what I undersnatnd, you are in the 2nd case - DEV_CLKIN.

    Michi Yama said:
    And Both GND pin of the osc and VSSA_DEVOSC(AG3) pin are connected to each other, and also osc's gnd pin is connected to Gnd(0V).

    This is the configuration recommended in the AM3874 datasheet:

    7.4.1.2 Using a 1.8V LVCMOS-Compatible Clock Input
    A 1.8-V LVCMOS-Compatible Clock Input can be used instead of the internal oscillators as the DEV and
    AUX clock inputs to the system. The external connections to support this are shown in Figure 7-9 and
    Figure 7-10. The DEV_CLKIN and AUX_CLKIN pins are connected to the 1.8-V LVCMOS-Compatible
    clock sources. The DEV_MXO and AUX_MXO pins are left unconnected. The VSSA_DEVOSC and
    VSSA_AUXOSC pins are connected to board ground (VSS).

    Regards,
    Pavel

  • 0 in reply to Pavel Botev
    Guru 10030 points

    Dear Pavel-san,

    Thank you for your support.

    Please see the below picture.

    As you said, customer's schematic is 2nd case (Using an external 1.8-V LVCMOS-compatible clock input - DEV_CLKIN). And red color "x" shows connection was opened by failure amount. As you understanding,

    this condition is not recommended on data sheet. But customer's system had no error with this condition. Customer measured the influence (waveform, noise on the power line). But they never found any difference compared with normal system.

    So customer would like to know the reason why AM3874 does not influence with VSSA_DEVOSC contion.  I need

    the explanation for customer satisfaction.

    I appreciate your quick reply.

    Best regards,

    Michi

  • 0 in reply to Michi Yama
    TI__Guru**** 170625 points

    Michi,

    Michi Yama said:

    As you understanding,

    this condition is not recommended on data sheet. But customer's system had no error with this condition. Customer measured the influence (waveform, noise on the power line). But they never found any difference compared with normal system.

    This works for your specific use case, but there will be problems if you cover all possible conditions and use cases.

    Regards,
    Pavel