Core with Transceiver - 16.2 English

The loopback path is implemented in the core as shown in the following figure. When placed into loopback, the data is routed from the transmitter path to the receiver path at the last possible point in the core. This point is immediately before the device-specific transceiver (or LVDS transceiver) interface. When placed in loopback, the core creates a constant stream of Idle code groups that are transmitted through the serial or GTP transceiver in accordance with the IEEE 802.3-2008 specification.

Earlier versions (before v5.0) of the core implemented loopback differently. The serial loopback feature of the device-specific transceiver was used by driving the loopback[1:0] port of the device-specific (serial or GTP) transceiver. This is no longer the case, and the loopback[1:0] output port of the core is now permanently set to logic “00.” However, for debugging purposes, the loopback[1:0] input port of the device-specific transceiver can be directly driven by the user logic to place it in either parallel or serial loopback mode.