Current Mode Logic (CML) buffers are used as the common interface to the SERDES/PCS. Internal
input and output terminations are provided to simplify board level
interfacing and AC coupling capacitors are available within the CML
receiver. These allow the receiver input termination (50, 75, 2k ohm)
and transmitter output termination (50, 75, 5k ohm) to be biased at
different levels. These terminations can be updated by the GUI of
IPexpress or by changing attributes of CHx_RTERM_TX, CHx_RTERM_RX and
PLL_TERM in the source code.
In most applications, you should use AC coupling rather than DC coupling, except when external AC coupling capacitors are used. The usage of TX PLL termination resistors is related to whether 50 ohm termination resistors are or are not used. Termination resistors and their usage are detailed below.
RX I/O termination - for ECP2, ECP3 and ECP5:
\u00B7
50: All non-SMPTE protocols use a 50 ohm
termination resistor.
\u00B7
60: provided for flexibility.
\u00B7
75: SMTPE uses a 75 ohm termination resistor.
\u00B7
HIGH: such as PCIe RX detection. User does not
set this termination.
TX I/O termination - for ECP2, ECP3 and ECP5:
\u00B7
50: All non-SMTPE protocols use a 50 ohm
termination resistor.
\u00B7
75: SMTPE uses a 75 ohm termination resistor.
\u00B7
5k: such as PCIe electric idle and RX detection.
User does not set this termination value for RX detection.
TX PLL termination - for ECP2 and ECP3:
\u00B7
50: If there is no 50 ohm termination resistor
on the PCB.
\u00B7
2k: If there is a 50 ohm termination resistor on
the PCB.