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AM335X Schematic Checklist Question

Ralph Johnson86
Prodigy 30 points

Other Parts Discussed in Thread: AM3358

The following is from the checklist:

If ADC is not used...
  • Connect all TSC_ADC terminals (VREFP, VREFN, AIN[7:0], VDDA_ADC, and VSSA_ADC) to same ground as all VSS terminals.

I thought that VDDA_ADC is an output pin driven by the internal ADC voltage reference, and is always active.  Is there a method that places this pin in a high impedance state permiting it to be tied it to ground?

  • 0
    TI__Guru 63595 points

    VDDA_ADC is the power input terminal for the Touch Screen Controller/ADC. What made you think this terminal was an output from an internal reference?

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 30 points

    The external ADC reference is accomplished using VREFP/VREFN.  Likewise, I assumed that an internal ADC reference exists with a voltage source internal to the AM335X referred to as VDDA_ADC/VSSA_ADC.  There is a spec in the data sheet listing the maximum current rating at 10ma. This led my to think that this was an output current rating.  I'm not 100% clear on how the ADC reference voltage is selected.  How are the VDDA_ADC/VSSA_ADC pins to be utilized?  What is routed to the ADC reference mux when VDDA_ADC/VSSA_ADC is selected?  Both the TI AM3358 EVM and Beaglebone(s) utilize a PMIC LDO tied to BOTH the VREFP/VREFN and VDDA_ADC/VSSA_ADC.  In this configuration it makes sense that VDDA_ADC/VSSA_ADC are inputs. 

    i

  • 0 in reply to Ralph Johnson86
    Prodigy 30 points

    I'll take a stab at answering my own question.  Please correct if I am wrong.  The voltage present on the VDDA_ADC/VSSA_ADC pins feed the internal reference voltage circuit.  The output of said circuit is one of the reference options for the ADC mux.  The voltage present on VREFP/VREFN goes directly to input mux as a reference option.  Because the same voltage source is routed to both the internal and external reference AM3358 I/O pins on both the TI EVM and BeagleBone, the ADC reference can be set to either external or internal.  The unused input pins are not grounded but connected to this LDO voltage source from the PMIC.

  • 0 in reply to Ralph Johnson86
    TI__Guru 63595 points

    The VDDA_ADC/VSSA_ADC terminals power the entire ADC and may also be configured as the voltage references. As you mentioned the VDDA_ADC terminal is also connected to the 5:1 INT_VREFP analog mux and the VSSA_ADC terminal is also connected to the 4:1 VREFM analog mux. Please refer to the Functional Block Diagram of the Analog Front-In shown in the AM335x TRM and the appropriate register descriptions for configuring each analog mux.

    I also recommend reading Advisory 1.0.32 in the AM335x Silicon Errata for connecting unused AIN terminals.

    Regards,
    Paul