[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
  • 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 as long as the push/pull network is designed carefully to avoid conflicts.

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