Xilinx RocketIO User Manual page 81

Other Important Design Notes
UG024 (v1.5) October 16, 2002
RocketIO™ Transceiver User Guide
-- which are used only to
-- maintain output sync; this provides some idea
-- of when the output is properly
-- aligned, with the comma in aligned_data[31:24].
-- The 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.
PROCESS (usrclk2)
BEGIN
IF (usrclk2'EVENT AND usrclk2 = '1') THEN
IF (rxreset = '1') THEN
wait_to_sync <= "1111";
count <= '0';
ELSE
IF (rxrealign = '1') THEN
wait_to_sync <= "1111";
count <= '1';
ELSE
IF (count = '1') THEN
IF(wait_to_sync /= "0000") THEN
wait_to_sync <= wait_to_sync - "0001";
END IF;
END IF;
IF (rxcommadet = '1') THEN
count <= '1';
END IF;
END IF;
END IF;
END IF;
END PROCESS;
-- 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.
PROCESS (usrclk2)
BEGIN
IF (usrclk2'EVENT AND usrclk2 = '1') THEN
IF ((rxreset OR rxrealign) = '1') THEN
sync_hold <= '0';
ELSE
IF (wait_to_sync = "0000")THEN
IF ((rxchariscomma3 OR rxchariscomma1) = '1') THEN
sync_hold <= '1';
END IF;
END IF;
END IF;
END IF;
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