Transactional Handshaking in RTL (Synchronous Example)

Another example is the LocalLink protocol from Xilinx.

This is a synchronous packet proctocol (c.f.\ compare with the asynchronous four-phase handshake just described).

Like the four-phase handshake, LocalLink has contra-flowing request and acknowledge signals. But data is not qualified by a request transition: instead it is qualified as valid on any positive clock edge where both request and acknowledge are asserted.

The nets for an eight-bit transmitting interface are

  input clk;             
  output [7:0] xxx_data; // The data itself
  output xxx_sof_n;      // Start of frame
  output xxx_eof_n;      // End of frame
  output xxx_src_rdy_n;  // Req
  input  xxx_dst_rdy_n;  // Ack

Start and end of frame signals delimit the packets. All control signals are active low (denoted with the underscore n suffix).