Minimal Skidbuffer with AXI-like Handshaking

Sat, 19 Apr 2025 19:14:03 +0000

Created in less than an hour:
A compact skidbuffer that only decouples the ready signal – with zero added latency.

The background here is the use of AXI-like handshaking and the need to decouple the ready signal in order to optimize data processing. In the desisngs I use, the ready signal is often the bottleneck, as it usually has to be propagated through several pipelines. A skid buffer can help here to reduce latency and optimize data processing. In contrast, the ‘valid’ signal is not decoupled as it is decoupled by each pipeline itself.

Translated with DeepL.com (free version)

The design follows a simple principle:

If ready = '1', the signal is passed through directly (MUX = 0)
If ready = '0', a buffer is activated (MUX = 1)
valid is either forwarded directly or taken from the buffer

The system implements complete AXI-like handshaking
and was successfully tested with randomly delayed upstream and downstream.

Resource usage (after synthesis, Xilinx Spartan-3):

1 flip-flop
4 LUTs
0 added latency

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--@ Set mux to buffered mode if data is available in the buffer.
C_MUX <= R_IsBuffered;
--@ Enable the buffer register if not buffered and chip enable is high.
C_Enable <= I_CE and not R_IsBuffered;
--@ Set the ready signal to high if not buffered.
O_Ready <= not R_IsBuffered;
--@ Set the valid signal to high if data is available in the buffer or if data is valid.
O_Valid <= R_IsBuffered or I_Valid;

process (I_CLK)
begin
 if rising_edge(I_CLK) then
 if I_RST = G_ResetActiveAt then
 R_IsBuffered <= '0';
 elsif I_CE = '1' then
 if R_IsBuffered = '0' and I_Valid = '1' then
 R_IsBuffered <= '1';
 elsif I_Ready = '1' and (R_IsBuffered or I_Valid) = '1' then
 R_IsBuffered <= '0';
 end if;
 end if;
 end if;
end process;

This architecture is especially suited for deep pipeline systems with timing bottlenecks on ready.

No overhead – just data flow.

AXI on 0xMax42 - Flatfile purist. Autodidact. Systems thinker.

Minimal Skidbuffer with AXI-like Handshaking