When many Virtex-6 GTX transceivers are powered up or down, variationswill be seen on theserial transceiver analog voltage supplies. If this happens during operation, adjacent operating transceivers will have a reduced margin because of this variation.
The power supply voltage regulator must maintain a constant output voltage even with variations in the load current.When a Virtex-6 GTX serial transceiver is powered up or down, a step change in the power supply load current occurs. As a result of this current change, the voltage regulator must adjust to the load change and the resulting variation in the IR drop in the power distribution network.The response time of a typical voltage regulator depends on the amount of output capacitance on the voltage regulator and the operating frequency of the power supply regulator. Typically, the response time is on the order of 10's or 100's of microseconds.It is during the time between the load current change caused by the powering up or down of the GTX transceiver that the power supply voltage will vary and during this time the adjacent operating GTX transceivers will have a reduced margin.
The workaround for this is to not power up or down all the transceivers simultaneously, but staggered.
When transceivers are in operation, assert/deassert the following signals only for one transceiver at a time with intervals of 100 us:
* TXPOWERDOWN
* RXPOWERDOWN
* TXPLLPOWERDOWN
* RXPLLPOWERDOWN
The following signals must be asserted/deasserted in groups of four transceivers at a time, also with intervals of 100 us:
* GTXTXRESET
* GTXRXRESET
When PROG_B signal is pulsed, a drop in the current request is expected and this causes a positive peak in the voltage level. Depending on the power regulators and the number of simultaneously running GTX, the peak could exceed the Absolute Maximum Ratings. When a non-supported voltage peak in the power supply is possible, a staggered power down is recommended before the PROG_B assertion.