Part Number: AM5729
Hello,
I have a custom board with the AM5729 Processor using the PCIe interface. The LJCB_CLK differential pins on the board are being used as inputs for a 100 MHz LVDS reference clock for the PCIe. As suggested on page 402 of the datasheet, there are AC coupling capacitors between the oscillator and the processor pins. The reference clock common mode voltage is ~1.2V as defined by the LVDS spec, but I'm measuring the common mode input voltage of the pins on the processor at ~150 mV. This means the clock oscillates between about -25 mV and 325 mV at the processor pins referenced to vss. The real problem is when the board is powered off, the AC coupling capacitors discharge the energy stored by the DC voltage across them, causing a transient negative voltage as low as -640 mV on the processor pins referenced to vss. The voltage takes about 20 msec to rise above -300 mV.
My question is two part:
1. Can the pins on the processor handle this transient behavior without being overstressed?
2. If the answer to question 1 is no, how can I compensate for this behavior? Is there a parameter to change the input common mode voltage to be closer to that of the oscillator? Do I need to adjust the common mode voltage of my oscillator?
Thank you,
Ryan
