Loopback modes are specialized configurations of the transceiver datapath where the traffic stream is folded back to the source. Typically, a specific pattern is transmitted then compared to check for errors. The following figure illustrates a loopback test configuration with four different loopback modes.
Figure 1. Loopback Testing Overview
Loopback test modes fall into two broad categories:
- Near-end loopback modes loop transmitted data back in the transceiver closest to the traffic generator.
- Far-end loopback modes loop received data back in the transceiver at the far end of the link.
Loopback testing can be used either during development or in deployed equipment for fault isolation. The traffic patterns used can either be application traffic patterns or specialized pseudo-random bit sequences. Each GTM transceiver has a built-in PRBS generator and checker.
Each GTM transceiver features several loopback modes to facilitate testing:
- Near-end PCS loopback (path 1 in the above figure). To set the GTM
transceiver in near-end PCS loopback, set the following:
- Set CH[0/1/2/3]_RXCDRHOLD = 1'b1
- Near-end PMA loopback (path 2 in the above figure).
- Far-end PMA loopback (path 3 in the above figure). The transceiver in far-end PMA loopback must use the same reference clock used by the transceiver that is the source of the loopback data because synchronous operation is required.
- Far-end PCS loopback (path 4 in the above figure). The transceiver in far-end PCS loopback must use the same reference clock used by the transceiver that is the source of the loopback data because synchronous operation is required.