5.9. Loopback Modes
The PMA supports three serial loopback modes:
- Serial loopback
- Pre-CDR reverse serial loopback
- Post-CDR reverse serial loopback
The 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 serial loopback takes the data before the buffer. Note that this also skips the receiver CTLE, but passes through the VGA before going into the CDR. The VGA can be adjusted accordingly.
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