Xilinx 7 Series User Manual

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Xilinx 7 Series User Manual Manual pdf 190 page image

190 www.xilinx.com 7 Series FPGAs GTP Transceivers User GuideUG482 (v1.9) December 19, 2016Chapter 4: ReceiverThe content of the RX elastic buffer becomes invalid if an RX elastic buffer overflow or underflowcondition occurs. When any of these conditions occur, the RX elastic buffer should be reset andreinitialized by using GTRXRESET, RXPCSRESET, RXBUFRESET, or the GTP transceiverinternally generated RX elastic buffer reset. The internally generated RX elastic buffer reset canoccur on channel bonding change, comma realignment, electrical idle, or rate change conditions.The RX elastic buffer is also used for clock correction (see RX Clock Correction) and channelbonding (see RX Channel Bonding, page 198). Clock correction is used in cases where XCLK andRXUSRCLK are not frequency matched. Table 4-35 lists common clock configurations and showswhether they require clock correction.When the RX elastic buffer is used, the setting of CLK_COR_MIN_LAT affects the latency throughthe buffer, regardless of whether clock correction is used.RX Clock CorrectionFunctional DescriptionThe RX elastic buffer is designed to bridge between two different clock domains, RXUSRCLK andXCLK, which is the recovered clock from CDR. Even if RXUSRCLK and XCLK are running atsame clock frequency, there is always a small frequency difference. Because XCLK andRXUSRCLK are not exactly the same, the difference can be accumulated to cause the RX elasticbuffer to eventually overflow or underflow unless it is corrected. To allow correction, each GTPtransceiver TX periodically transmits one or more special characters that the GTP transceiver RX isallowed to remove or replicate in the RX elastic buffer as necessary. By removing characters whenthe RX elastic buffer is too full and replicating characters when the RX elastic buffer is too empty,the receiver can prevent overflow or underflow.Table 4-35: Common Clock ConfigurationsTypes of Clocking Require Clock Correction?Synchronous system where both sides uses the reference clockfrom the same physical oscillator. NoAsynchronous system when separate reference clocks are used andthe GTP transceiver receiver uses an RX recovered clock. NoAsynchronous system when separate reference clocks are used andthe GTP transceiver receiver uses a local clock. YesSend Feedback