1.12 AC and DC Coupling

Each transmit channel of a PCIe lane must be AC-coupled to allow link detection. Capacitors used for AC coupling must be external to the device and large enough to avoid excessive low-frequency drops when the data signal contains a long string of consecutive identical bits. For non-PCIe applications, Microchip recommends that a PolarFire SoC device receives inputs that are AC-coupled to prevent common-mode mismatches between devices. Suitable values (for example, 0.1 μF) for AC-coupling capacitors must be used to maximize link signal quality and must conform to PolarFire SoC Datasheet electrical specifications.

For lower data rates as per the datasheet, DC coupling is supported by PolarFire SoC Transceiver Tx and Rx interfaces through a configuration option. If a PolarFire SoC transmitter is used to drive a PolarFire SoC receiver in DC-coupled mode, select the lowest common mode settings for the transmitter.