Xilinx RocketIO User Manual page 78

R
78
// counter is set to a high value
// whenever the elastic buffer is reinitialized;
// that is, upon asserted RXRESET or
// RXREALIGN. Count-down is enabled whenever a
// comma is known to have
// come through the comma detection circuit,
// that is, upon an asserted RXREALIGN
// or RXCOMMADET.
always @ ( posedge usrclk2 )
begin
if ( rxreset )
begin
wait_to_sync <= 4'b1111;
count
end
else if ( rxrealign )
begin
wait_to_sync <= 4'b1111;
count
end
else
begin
if ( count && ( wait_to_sync != 4'b0000 ) )
wait_to_sync <= wait_to_sync - 4'b0001;
if ( rxcommadet )
count
end
end
// This process maintains output sync, which indicates
// when outgoing aligned_data
// should be properly aligned, with the comma in aligned_data[31:24].
// Output aligned_data is
// considered to be in sync when a comma is seen on
// rxdata (as indicated
// by rxchariscomma3 or 1) after the counter wait_to_sync
// has reached 0, indicating
// that commas seen by the comma detection circuit
// have had time to propagate to
// aligned_data after initialization of the elastic buffer.
always @ ( posedge usrclk2 )
begin
if ( rxreset | rxrealign )
sync <= 1'b0;
else if ( ( wait_to_sync == 4'b0000 ) &
( rxchariscomma3 | rxchariscomma1 ) )
sync <= 1'b1;
end
// This process generates aligned_data with commas aligned in [31:24],
// assuming that incoming commas are aligned to [31:24] or [15:8].
// Here, you could add code to use ENPCOMMAALIGN and
// ENMCOMMAALIGN to enable a move back into the byte_sync=0 state.
always @ ( posedge usrclk2 or posedge rxreset )
begin
if ( rxreset )
begin
rxdata_reg <= 16'h0000;
aligned_data
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Chapter 3: Digital Design Considerations
<= 1'b0;
<= 1'b1;
<= 1'b1;
<= 32'h0000_0000;
RocketIO™ Transceiver User Guide
UG024 (v1.5) October 16, 2002