Embedded Tri-mode Ethernet MAC Wrapper v4.1 - How can I switch to 250 MHz reference clock to avoid jitter problems
Description
Keywords: gigabit, TBI, GMAC, GEMAC, patch, installation, instruction, v7.0, IP Update 1, ip1_1, clk, patch, CES4, GT11, V4, FX
Virtex-4 FX GT11 jitter errata exceeds the IEEE Std 802.3-2002 specification when using a reference clock of 125 MHz. To improve the jitter performance of the transmitted serial line, Xilinx recommends a 250 MHz reference clock. For more information, see the Virtex-4 FX errata at:
http://xwebpub/xlnx/xweb/xil_publications_display.jsp?category=Errata&iLanguageID=1
Solution
Use the following information to switch between reference clock frequencies of 250 MHz and 125 MHz.250 MHz
GT11 Attributes:
TXOUTDIV2SEL = 4
TXPLLNDIVSEL = 10
RXOUTDIV2SEL = 1
RXPLLNDIVSEL = 20
DCM attribute to divide down reference clock to 125 MHz:
CLKIN_DIVIDE_BY_2 = TRUE
125 MHz
TXOUTDIV2SEL = 4
TXPLLNDIVSEL = 20
RXOUTDIV2SEL = 1
RXPLLNDIVSEL = 40
The clocking will be updated to use a 250 MHz reference clock for the GT11 and use the internal clock dividers in Embedded Tri-mode Ethernet MAC Wrapper v4.2 Core available in 8.2i IP Update 1, scheduled for release in July 2006.