¿Qué es ERROR: Paquete: 1107 en la asignación en Xilinx ISE?

Aparece un mensaje de error al procesar el mapeo ISE de Xilinx. y estoy tratando de resolver este problema ERROR: Pack: 1107. Pero no puedo entender esto, ¿qué significa?

mi tablero objetivo es Spartan6lx16 2c. y el reloj externo se asigna como 50Mhz.

ERROR:Pack:1107 - Pack was unable to combine the symbols listed below into a
   single IOBS component because the site type selected is not compatible. 

   Further explanation:
   The component type is determined by the types of logic and the properties and
   configuration of the logic it contains. In this case an IO component of type
   IOBS was chosen because the IO contains symbols and/or properties consistent
   with differential slave usage. Please double check that the types of logic
   elements and all of their relevant properties and configuration options are
   compatible with the physical site type of the constraint.

   Summary:
   Symbols involved:
    PAD symbol "TMDSn_clock" (Pad Signal = TMDSn_clock)
    SlaveBuffer symbol "OBUFDS_clock/SLAVEBUF.DIFFOUT" (Output Signal =
   TMDSn_clock)
   Component type involved: IOBS
   Site Location involved: B14
   Site Type involved: IOBM

¿Qué se supone que debo hacer para resolver este error de asignación?

Este es el código completo.

// (c) fpga4fun.com & KNJN LLC 2013

////////////////////////////////////////////////////////////////////////
module HDMI_test(
    input wire pixclk,  // 25MHz
    output wire [2:0] TMDSp, 
    output wire [2:0] TMDSn,
    output wire TMDSp_clock,
    output wire TMDSn_clock
);

////////////////////////////////////////////////////////////////////////
reg [9:0] CounterX, CounterY;
reg hSync, vSync, DrawArea;
always @(posedge pixclk) DrawArea <= (CounterX<640) && (CounterY<480);

always @(posedge pixclk) CounterX <= (CounterX==799) ? 0 : CounterX+1;
always @(posedge pixclk) if(CounterX==799) CounterY <= (CounterY==524) ? 0 : CounterY+1;

always @(posedge pixclk) hSync <= (CounterX>=656) && (CounterX<752);
always @(posedge pixclk) vSync <= (CounterY>=490) && (CounterY<492);

////////////////
wire [7:0] W = {8{CounterX[7:0]==CounterY[7:0]}};
wire [7:0] A = {8{CounterX[7:5]==3'h2 && CounterY[7:5]==3'h2}};
reg [7:0] red, green, blue;
always @(posedge pixclk) red <= ({CounterX[5:0] & {6{CounterY[4:3]==~CounterX[4:3]}}, 2'b00} | W) & ~A;
always @(posedge pixclk) green <= (CounterX[7:0] & {8{CounterY[6]}} | W) & ~A;
always @(posedge pixclk) blue <= CounterY[7:0] | W | A;

////////////////////////////////////////////////////////////////////////
wire [9:0] TMDS_red, TMDS_green, TMDS_blue;
TMDS_encoder encode_R(.clk(pixclk), .VD(red  ), .CD(2'b00)        , .VDE(DrawArea), .TMDS(TMDS_red));
TMDS_encoder encode_G(.clk(pixclk), .VD(green), .CD(2'b00)        , .VDE(DrawArea), .TMDS(TMDS_green));
TMDS_encoder encode_B(.clk(pixclk), .VD(blue ), .CD({vSync,hSync}), .VDE(DrawArea), .TMDS(TMDS_blue));

////////////////////////////////////////////////////////////////////////
wire clk_TMDS, DCM_TMDS_CLKFX;  // 25MHz x 10 = 250MHz
DCM_SP #(.CLKFX_MULTIPLY(5)) DCM_TMDS_inst(.CLKIN(pixclk), .CLKFX(DCM_TMDS_CLKFX), .RST(1'b0));
BUFG BUFG_TMDSp(.I(DCM_TMDS_CLKFX), .O(clk_TMDS));

////////////////////////////////////////////////////////////////////////
reg [3:0] TMDS_mod10=0;  // modulus 10 counter
reg [9:0] TMDS_shift_red=0, TMDS_shift_green=0, TMDS_shift_blue=0;
reg TMDS_shift_load=0;
always @(posedge clk_TMDS) TMDS_shift_load <= (TMDS_mod10==4'd9);

always @(posedge clk_TMDS)
begin
    TMDS_shift_red   <= TMDS_shift_load ? TMDS_red   : TMDS_shift_red  [9:1];
    TMDS_shift_green <= TMDS_shift_load ? TMDS_green : TMDS_shift_green[9:1];
    TMDS_shift_blue  <= TMDS_shift_load ? TMDS_blue  : TMDS_shift_blue [9:1];   
    TMDS_mod10 <= (TMDS_mod10==4'd9) ? 4'd0 : TMDS_mod10+4'd1;
end

OBUFDS OBUFDS_red  (.I(TMDS_shift_red  [0]), .O(TMDSp[2]), .OB(TMDSn[2]));
OBUFDS OBUFDS_green(.I(TMDS_shift_green[0]), .O(TMDSp[1]), .OB(TMDSn[1]));
OBUFDS OBUFDS_blue (.I(TMDS_shift_blue [0]), .O(TMDSp[0]), .OB(TMDSn[0]));
OBUFDS OBUFDS_clock(.I(pixclk), .O(TMDSp_clock), .OB(TMDSn_clock));
endmodule


////////////////////////////////////////////////////////////////////////
module TMDS_encoder(
    input clk,
    input [7:0] VD,  // video data (red, green or blue)
    input [1:0] CD,  // control data
    input VDE,  // video data enable, to choose between CD (when VDE=0) and VD (when VDE=1)
    output reg [9:0] TMDS = 0
);

wire [3:0] Nb1s = VD[0] + VD[1] + VD[2] + VD[3] + VD[4] + VD[5] + VD[6] + VD[7];
wire XNOR = (Nb1s>4'd4) || (Nb1s==4'd4 && VD[0]==1'b0);
wire [8:0] q_m = {~XNOR, q_m[6:0] ^ VD[7:1] ^ {7{XNOR}}, VD[0]};

reg [3:0] balance_acc = 0;
wire [3:0] balance = q_m[0] + q_m[1] + q_m[2] + q_m[3] + q_m[4] + q_m[5] + q_m[6] + q_m[7] - 4'd4;
wire balance_sign_eq = (balance[3] == balance_acc[3]);
wire invert_q_m = (balance==0 || balance_acc==0) ? ~q_m[8] : balance_sign_eq;
wire [3:0] balance_acc_inc = balance - ({q_m[8] ^ ~balance_sign_eq} & ~(balance==0 || balance_acc==0));
wire [3:0] balance_acc_new = invert_q_m ? balance_acc-balance_acc_inc : balance_acc+balance_acc_inc;
wire [9:0] TMDS_data = {invert_q_m, q_m[8], q_m[7:0] ^ {8{invert_q_m}}};
wire [9:0] TMDS_code = CD[1] ? (CD[0] ? 10'b1010101011 : 10'b0101010100) : (CD[0] ? 10'b0010101011 : 10'b1101010100);

always @(posedge clk) TMDS <= VDE ? TMDS_data : TMDS_code;
always @(posedge clk) balance_acc <= VDE ? balance_acc_new : 4'h0;
endmodule


////////////////////////////////////////////////////////////////////////

y ucf

//global system clock
NET "pixclk"        LOC = A10   | IOSTANDARD = LVTTL;

//# HDMI Out (J3)
NET "TMDSp_clock" LOC = "C13" | IOSTANDARD = TMDS_33 ; # Clock
NET "TMDSn_clock" LOC = "B14" | IOSTANDARD = TMDS_33 ;
NET "TMDSp[0]"      LOC = "B10" | IOSTANDARD = TMDS_33 ; # Blue
NET "TMDSn[0]"      LOC = "A11" | IOSTANDARD = TMDS_33 ;
NET "TMDSp[1]"      LOC = "C11" | IOSTANDARD = TMDS_33 ; # Red
NET "TMDSn[1]"      LOC = "A12" | IOSTANDARD = TMDS_33 ;
NET "TMDSp[2]"      LOC = "B12" | IOSTANDARD = TMDS_33 ; # Green
NET "TMDSn[2]"      LOC = "A13" | IOSTANDARD = TMDS_33 ;