MVB Reordering

ENTITY MVB_REORDERING IS

The MVB_REORDERING component moves MVB items to other positions within the same MVB word. Each RX item has its own key (see REORDER_KEY) which says to which TX position that item must be moved. It is a pure scatter network (ITEMS multiplexers controlled by ITEMS*ITEMS comparators) without any storage of items: an item is always transmitted in the same word in which it was received, it is never moved to another word. The amount of logic grows quadratically with ITEMS.

The requested reordering does not have to be a complete permutation, gaps are allowed. A TX position which is not targeted by any valid RX item gets TX_VLD=’0’ (data on that position are undefined). Compaction (all valid items moved to the lowest positions) as well as scattering (valid items moved to arbitrary positions) are therefore both valid use cases.

RX_DST_RDY is directly connected to TX_DST_RDY, the component never generates backpressure of its own. Latency is one clock cycle when OUT_REG_EN=True, otherwise the whole datapath is combinational.

The reordering logic needs at least 2 items, because the key of a single item would be zero bits wide. ITEMS=1 is therefore allowed only together with the bypass mode (REORDERING_EN=False), any other use is refused by an assert during elaboration.

Warning

The keys are not checked in any way and there is no output reporting a discarded item. The user must guarantee these conditions for each valid RX item (an item with RX_VLD(j)=’1’):

Unique keys - two valid RX items must not target the same TX position. In case of such collision, the item on the higher RX position (higher index j) wins and the other item is silently discarded.
Key range - the key value must be lower than ITEMS. When ITEMS is not a power of two, the key is able to encode also higher values (for ITEMS=5 the key is 3 bits wide and encodes values 0 to 7) and an item with such key is silently discarded, because no TX position matches it.

Keys of invalid RX items (RX_VLD(j)=’0’) are don’t care.

Generics

Generic	Type	Default	Description
ITEMS	natural	5	Number of MVB items in word. The minimum value is 2, ITEMS=1 is allowed only with REORDERING_EN=False.
ITEM_WIDTH	natural	64	Width of one MVB item in bits.
OUT_REG_EN	boolean	True	Enable the output register on the TX MVB interface. It breaks the combinational path through the multiplexers at the cost of one clock cycle of latency.
REORDERING_EN	boolean	True	Enable the reordering logic. When False, the RX MVB word is passed to the TX MVB interface unchanged and REORDER_KEY is ignored. This bypass mode is the only mode supported with ITEMS=1.

Ports

Port	Type	Mode	Description
CLK	std_logic	in	Clock input
RESET	std_logic	in	Reset input synchronized with CLK
=====	RX MVB INTERFACE (original item positions)	=====	=====
RX_DATA	std_logic_vector(ITEMS*ITEM_WIDTH-1 downto 0)	in
RX_VLD	std_logic_vector(ITEMS-1 downto 0)	in
RX_SRC_RDY	std_logic	in
RX_DST_RDY	std_logic	out
=====	TX MVB INTERFACE (new item positions)	=====	=====
TX_DATA	std_logic_vector(ITEMS*ITEM_WIDTH-1 downto 0)	out
TX_VLD	std_logic_vector(ITEMS-1 downto 0)	out
TX_SRC_RDY	std_logic	out
TX_DST_RDY	std_logic	in
REORDER_KEY	std_logic_vector(ITEMS*log2(ITEMS)-1 downto 0)	in	Target TX position of each RX item, log2(ITEMS) bits per item. The key of RX item j is REORDER_KEY((j+1)log2(ITEMS)-1 downto jlog2(ITEMS)). Valid with RX_VLD and RX_SRC_RDY. The keys must meet the conditions described in the warning above.