Altera Stratix GX Transceiver - Manual (Page 201)

Altera Corporation 7–1
January 2005
7. Loopback Modes
Introduction You can apply several loopback modes to the Stratix ® GX block. The main
forms of loopback are as follows:
■ Serial loopback
■ Parallel loopback
■ Reverse serial loopback
Loopback refers to feeding the data from the transmitter directly to the
receiver. Reverse loopback refers to feeding the data from the receiver
directly to the transmitter. Serial loopback and parallel loopback feed
data from the transmitter block to the receiver. Reverse serial loopback
feeds the data from the receiver to the transmitter.
Serial Loopback Figure 7–1 shows the data path for serial loopback. A data stream is fed
to the transmitter from the FPGA logic array and has the option of using
all the blocks in the transmitter. The data then traverses from the
transmitter in serial form to the receiver. The serial data is the data that is
transmitted from the Stratix GX device. Once the data enters the receiver
in serial form, it can use any of the receiver blocks and is then fed into the
FPGA logic array. The PRBS block generates data when using serial
loopback.
Serial loopback is dynamically enabled on a channel-by-channel basis
using the rx_slpbk port. When rx_slpbk is high, all blocks that are
active when the signal is low are still active. The serial loopback is
enabled but output is still seeing data on the tx_out[] port.
Serial loopback is often used to check the analog portion of the
transceiver. The data is retimed through different clock domains and an
alignment pattern is still necessary for the word aligner.