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AM35xx LVCMOS I/O capacitance (3pF)

paddu
Intellectual 915 points
Other Parts Discussed in Thread: AM3505

Hi,

 

 

Regarding AM3505's "DC Electrical Characteristics"

the datasheet Table 3-4 mentions that

Input/Outuput capacitance for "dual-voltage LVCMOS I/O" as "3pF".

 

Can anyone please let me know what is this value(3pF) about

and do we use this capacitance in the application?

 

Best Regards.

Paddu.

 

  • 0
    TI__Guru 62855 points

    3pf is the typical load capacitance of the package and input buffer of the I/O.  This capacitive load will be seen looking into the pin of the device.  The output buffer of the device would also see this capacitive load looking out of the pin when the pin is open circuit.

     

    You may need to use this load capacitance when doing finite signal integrity and timing analysis.  In most cases this internal capacitance is much smaller that external capacitance of PCB traces and external devices, so you may be able to ignore this capacitive load.

     

     

    Regards,

    Paul

  • 0 in reply to peaves
    TI__Expert 5725 points

    Hi Peaves,

     

    What type of LVCMOS type of signal is required for this processor? 1.8 V LVCMOS or 3.3 V LVCMOS clock input?

    Best regards,

    MJ

  • 0 in reply to mapefer3
    TI__Guru 62855 points

    Are you asking if the input voltage of the SYS_XTALIN input is 1.8 V or 3.3 V?

    If so, the absolute maximum for this input is VDDSOSC + 0.3 volts and the absolute maximum for VDDSOSC is 2.1 volts as defined in the Absolute Maximum Ratings Table of the data manual.  VIH minimum is defined as 0.8 x VDDSOSC and VIL maximum is defined as 0.2 x VDDSOSC in the DC Electrical Characteristics Table of the data manual.  I was able to find the recommended operating voltage for VDDSOSC defined as 1.8 volts in section 4.1 of the data manual.  

    4.1 Oscillator

    The sys_xtalin (26 MHz) oscillator provides the primary reference clock for the device. The on-chip oscillator requires an external crystal connected across the sys_xtalin and sys_xtalout pins, along with two load capacitors, as shown in Figure 4-3. The external crystal load capacitors must be connected only to the oscillator ground pin (VSSOSC). Do not connect to board ground (VSS).

     

    Note: If an external oscillator is to be used, the external oscillator clock signal should be connected to the

    sys_xtalin pin with a 1.8V amplitude. The sys_xtalout should be left unconnected and the VSSOSC signal should be connected to board ground (VSS).

     

    Regards,

    Paul