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TMS320C6748 power supply "bleed through"

Rafael Salcedo
Prodigy 155 points
Other Parts Discussed in Thread: TMS320C6748

I have an application using the TMS320C6748 where:

-I/O banks A and C are powered by 3.3VDC 

-I/O banks B, and the 1.8 VCC input are powerewd by 1.8VDC

-Core is powered by 1.2VDC

Individual load switches are used on each of the three supplies.  Outputs of the load switches are connected only to the processor and, in the case of the 1.8V supply, to the LPDDR memory that connects to the memory controller in the processor.

The power up sequence turns on the 1.2V first, the 1.8VDC 2nd and the 3.3VDC last.

When the 1.2VDC is turned on, the 1.2V bus at the processor goes to 1.2V correctly

When the 1.8VDC is turned on, the 1.8V bus at the processor goes to 1.8V correctly,

However, when the 3.3VDC is turned on, the 3.3VDC bus at the processor goes to 3.3V correctly, BUT the 1.8V bus at the processor now goes UP to 2.2VDC.  Turning off the 3.3V brings the 1.8V bus back to 1.8V.

I have confirmed that none of the external pull-up resistors, on the I/O pins, are connected to the wrong voltage.  I have tested two, newly bult (First Article) boards and both have the same problem.

Need your help to find out the cause of this problem.

 

 

  • 0
    Prodigy 155 points

    I found a solution to this problem:

    In my application, the RESET input to the processor was not asserted (was at a logic high) when I executed the power-up sequnce describe above.

    Re-reading the the section of the data sheet that describes the power up sequnce, I noticed this statement:

     RESET must be maintained active until all power supplies have reached their nominal values

    Asserting the RESET signal (setting it to a logic low) throughout the power-up sequence allowed all power supplies to come up correctly and they remained so after the RESET was de-asseted.