110 www.xilinx.com Virtex-4 RocketIO MGT User GuideUG076 (v4.1) November 2, 2008Chapter 3: PCS Digital Design Considerations RTXCHARDISPVAL and TXCHARDISPMODETXCHARDISPVAL and TXCHARDISPMODE are dual-purpose ports for the transmitterdepending on whether 8B/10B encoding is done. Table 3-4 shows this dual functionality.When encoding is enabled, these ports function as byte-mapped control ports controllingthe running disparity of the transmitted serial data (Table 3-5).In the encoding configuration, the disparity of the serial transmission can be controlledwith the TXCHARDISPVAL and TXCHARDISPMODE ports. When TXCHARDISPMODEis set to a logic 1, the running disparity is set before encoding the specific byte.TXCHARDISPVAL determines if the disparity is negative (set to a logic 0) or positive (setto a logic 1).When TXCHARDISPMODE is set to a logic 0, the running disparity is maintained ifTXCHARDISPVAL is also set to a logic 0. However, the disparity is inverted beforeencoding the byte when the TXCHARDISPVAL is set to a logic 1. Most applications use theTable 3-4: 8B/10B Signal DefinitionsSignal FunctionTXENC8B10BBUSE 1 8B/10B encoding is enabled.0 8B/10B encoding is disabled.8B/10B Encoding Block Enabled 8B/10B Encoding Block DisabledTXBYPASS8B10B1 8B/10B encoding is bypassed on a clock-by-clock basis.Defined by no encoding.0 8B/10B encoding block is enabled on a clock-by-clock basis.TXCHARDISPMODE,TXCHARDISPVAL00 Maintain running disparity normally. Part of 10-bit encoded byte(see Figure 3-10):TXCHARDISPMODE[0],(or: [1] /[2] /[3] /[4]/[5]/[6]/[7])TXCHARDISPVAL[0],(or: [1] /[2] /[3] /[4]/[5]/[6]/[7])TXDATA[7:0](or: [15:8]/[23:16]/[31:24]/[39:32]/[47:40]/[55:48]/[63:56})01 Invert the normally generated runningdisparity before encoding this byte.10 Set negative running disparity beforeencoding this byte.11 Set positive running disparity beforeencoding this byte.TXCHARISK 1 Byte to transmit is a K-character. Undefined. These bits should be set to alogic 0.0 Byte to transmit is a data character.Notes:1. TXCHARDISPVAL and TXCHARDISPMODE become part of the 10-, 20-, 40-, or 80-bit data only if the internal data width is 40.Table 3-5: Running Disparity Control{TXCHARDISPMODE,TXCHARDISPVAL} Function00 Maintain running disparity normally.01 Invert normally generated running disparity beforeencoding this byte.10 Set negative running disparity before encoding this byte.11 Set positive running disparity before encoding this byte.
