18.104.22.168. FHT PMA Loopback Modes
Loopback modes are design-for-test features that verify different blocks of the PMA. FHT PMAs have loopback modes to debug different blocks of the PMA.
- A PMA-transmitter-to-receiver internal serial loopback
- B PMA-transmitter-to-receiver digital parallel loopback
- C PMA-receiver-to-transmitter reverse parallel loopback
PMA-Transmitter-to-Receiver Internal Serial Loopback
The internal serial loopback path sets the CDR to recover data from the serializer instead of the receiver serial input pin. The transmitter buffer sends data normally, but internal serial loopback takes the data before the buffer. It is implemented completely on the PMA and does not require any connector on the serial path.
The FHT lane also supports external connectivity to connect the transmitter differential outputs to the receiver differential inputs.
- Mission mode is an external connection where the data source is something other than the FHT transmitter.
- External connectivity supports, for example, a bit-error rate tester (BERT) or another device's transmitter providing data to the FHT receiver.
PMA-Transmitter-to-Receiver Digital Parallel Loopback
In a digital parallel loopback path, the parallel data stream of the transmitter is looped back as the parallel data input stream for the receiver.
PMA-Receiver-to-Transmitter Reverse Parallel Loopback
The reverse parallel loopback path sets the transmitter buffer to transmit data fed directly from the CDR recovered data. From an external instrument, data is fed to the receiver buffer, and the deserialized parallel data stream of the receiver is looped back as the parallel data input stream for the transmitter.