1.10 AC and DC Coupling

Each transmit channel of a PCIe lane must be AC-coupled to allow the 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 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 FPGA Datasheet electrical specifications.

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