Xilinx Virtex UltraScale+ FPGAs - Page 19

Figure 9: Internal Dual Clocking Architecture
LCPLL
GTM_DUAL
REFCLK Distribution
GTM Channel 0 (CH0)
TX PMA
TX PCS
RX PMA
RX PCS
GTM Channel 1 (CH1)
TX PMA
TX PCS
RX PMA
RX PCS
X20900-061418
The LCPLL input clock selecon is described in Reference Clock Selecon and Distribuon. The
LCPLL outputs feed the TX and RX clock divider blocks, which control the generaon of serial
and parallel clocks used by the PMA and PCS blocks. The LCPLL is shared between the TX and
RX datapaths.
The gure below illustrates a conceptual view of the LCPLL architecture. The input clock can be
divided by a factor of M before it is fed into the phase frequency detector. The feedback divider
N determines the voltage-controlled oscillator (VCO) mulplicaon rao. For line rates below
28.1 Gb/s (NRZ) and 56.2 Gb/s (PAM4), a fraconal-N divider is supported where the eecve
rao is a combinaon of the N factor plus a fraconal part. The LCPLL output frequency depends
on the sengs of LCPLLCLKOUT_RATE. When LCPLLCLKOUT_RATE is set to HALF, the output
frequency is half of the VCO frequency. When it is set to FULL, the output frequency is the same
as the VCO frequency. A lock indicator block compares the frequencies of the reference clock
and VCO feedback clock to determine if a frequency lock has been achieved.
Chapter 2: Shared Features
UG581 (v1.0) January 4, 2019 www.xilinx.com
Virtex UltraScale+ GTM Transceivers 19
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