TXUSRCLK2 is the main synchronization clock for all signals into the TX side of the GTM
transceiver. Most signals into the TX side of the GTM transceiver are sampled on the positive
edge of TXUSRCLK2. TXUSRCLK2 and TXUSRCLK have a fixed-rate relationship based on the
TX_DATA_WIDTH and TX_INT_DATA_WIDTH settings. The following table shows the
relationship between TXUSRCLK2 and TXUSRCLK per TX_DATA_WIDTH and
TX_INT_DATA_WIDTH values.
Table 29: Relationship between TXUSRCLK2 and TXUSRCLK
Encoding
TX Data Width
TX Internal Datapath
TXUSRCLK2 Frequency
NRZ
64
64
F
TXUSRCLK2
= F
TXUSRCLK
NRZ
128
64
F
TXUSRCLK2
= F
TXUSRCLK
/2
PAM4
80
80
F
TXUSRCLK2
= F
TXUSRCLK
PAM4
160
80
F
TXUSRCLK2
= F
TXUSRCLK
/2
PAM4
128
128
F
TXUSRCLK2
= F
TXUSRCLK
PAM4
256
128
F
TXUSRCLK2
= F
TXUSRCLK
/2
These rules about the relationships between clocks must be observed for TXUSRCLK and
TXUSRCLK2:
• TXUSRCLK and TXUSRCLK2 must be positive-edge aligned, with as little skew as possible
between them. As a result, low-skew clock resources (BUFG_GTs) must be used to drive
TXUSRCLK and TXUSRCLK2.
• Even though they might run at different frequencies, TXUSRCLK, TXUSRCLK2, and the
transmitter reference clock must have the same oscillator as their source. Thus TXUSRCLK
and TXUSRCLK2 must be multiplied or divided versions of the transmitter reference clock.
Ports and Attributes
The following table defines the TX interface ports.
Table 30: TX Interface Ports
Port
Dir
Clock Domain
Description
TXDATA[255:0]
In
TXUSRCLK2
The bus for transmitting data. The width of this port
is equal to the TX data width selection.
64: TXDATA[63:0].
80: TXDATA[79:0].
128: TXDATA[127:0].
160: TXDATA[159:0].
256: TXDATA[255:0].
TXUSRCLK
In
Clock
This port is used to provide a clock for the internal
TX PCS datapath.
Chapter 3: Transmitter
UG581 (v1.0) January 4, 2019
Virtex Ult GTM Transceivers
57