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Why the SYSBOOT pin can't be pulled up?

yi li2
Intellectual 615 points


Hi,

     We are using 3359 in our custom board, the SYSBOOT pin is set like:SYSBOOT[0~4]=00100, and the SYSBOOT pin is connected to the RVDS chip F383B and the VGA chip 7123

     Phenomenon is 

     1、 If the 3359 power up before the VGA chip power up, it can't go into boot normally, and the SYSBOOT_3 pin is 1.3V, not 3.3V

     2、If the 3359 power up after the VGA chip power up, it can go into boot normally, and the SYSBOOT_3 pin is 3.3V

We test another board which use 3358, it can go into boot normally in both two condition

     

  • 0
    TI__Guru 63155 points

    Most CMOS devices have an internal parasitic diode between the input terminals and respective IO power supply.  This parasitic diode will be forward biased when the input voltage to a CMOS device exceeds the power supply voltage by more than about 0.3 volts.  If this is the case, the inputs will be clamped to about 0.3 volts when the VGA chip power supply is 0 volts unless enough current flows through the input(s) to power up the VGA power supply.

    In the case where you have AM335x powered up before the VGA chip, the parasitic diode is allowing current to flow from the SYSBOOT pull-up resistor into the VGA chip input to the power supply that is not turned on.  This current is enough to hold the pull-up low which causes the SYSBOOT inputs to be the wrong value when they are sampled on the rising edge of PWRONRSTn.

    You should power all IO signals connected together from the same power supply or if multiple power supplies are needed they should be designed to track each other during power-up and power-down so current does not flow through these parasitic diodes.  This may cause a long term reliabilty issue since these diodes were not design to conduct this current.

    The AM335x data sheet parameter "Steady State Max. Voltage at all IO pins" in Table 3-1 states the AM335x input voltage should always be in the range of "-0.5V to IO supply voltage + 0.3 V".  So if the respective VDDSHVx power supply is 0 volts the maximum input voltage should be 0.3 volts.  The VGA chip data sheet may have a similar parameter.

    I assume the voltage on the AM335x input is near the high/low threshold of the input buffer and in some cases AM335x sees the correct SYSBOOT value. This is the only explanation I have for some boards working while others do not.

    Regards,
    Paul