[FAQ] DS320PR810: How should PCIe redriver PD pins be configured?

Powerdown pins have a simple working principle:

• When the input to the pin is logical HIGH, the device channels are shut down into a reset state.
  • There is no signal output from the TX side, and the corresponding RX inputs will be in a high-impedance state instead of presenting a 50 Ohm termination.
  • RX detection will only start again once this reset state is lifted.
• When the input to the pin is logical LOW, the device is in a normal operational or active state.

There are several ways the powerdown pins could be configured depending on the application:

• Connect powerdown pins to inverted system PERST# signal (TI suggested)
  
  • Inversion is necessary to match the polarity of the PD pins, important to remember
  • At powerup, the device will wait to begin RX detection until PERST# is asserted, meaning that the detection will be synchronized with the endpoint instead of starting RX detection as soon as it is ready
  • If the system performs a warm reset by cycling PERST# from high --> low --> high, the redriver will reset its RX detection to help ensure a proper reinitialization sequence
• Connect powerdown pins to system PRSNT# signal (TI suggested if system will have hot plug functionality)
  • As with PERST#, check the polarity of the PRSNT# signal - if it is an active high signal, it should be inverted before reaching PD, but most often it is active-low and does not need inversion
  • Device will be held in reset if endpoint is removed, and will restart RX detection properly when endpoint is inserted
• Strap powerdown pins to GND (not quite recommended)
  • The simplest option. At powerup, the device will start as soon as possible and will stay in the active state for as long as the power is supplied.
  • Due to lack of startup synchronization, some endpoints may have problems with incorrect link widths
  • Device will not restart RX detection during a warm reset - reinitialization will therefore be somewhat "off-script"
• Connect arbitrary external control signal for any other application-specific use case where the redriver needs to be held in reset

Powerdown pins can be connected to multiple control signals at the same time, in this case it's recommended to use a logic OR gate to prevent possible conflicts between the input signals (one may be logical high while the other may be logical low at the same time, better to avoid the possibility of a current short or unexpected voltage level).

The connection to inverted PERST# is the most common and applicable recommendation. It increases system flexibility and can prevent odd problems later on, so having at least a provision for an inverter and a PERST# trace is a good idea during schematic and layout design.

Best,

Evan Su