FIFO interface (ready/valid)
The FIFO interface
Data is the wire that actually passes data from the transmitter to the Receiver.
Valid and Ready are known as handshaking signals which allow the Transmitter and the Receiver to communicate with regards to when it is time to pass the data.
The Valid signal (output from the Transmitter and input to the Receiver) indicates that the Transmitter has put valid data on the Data line this cycle.
The Ready signal (output from the Receiver and input to the Transmitter) indicates that the Receiver is ready to receive new data. Ready can be asserted as soon as the Receiver is ready to receive new data.
State machine
This state machine indicates once the transmitter assert the vld or the receiver assert the rdy signal, these signals must hold until their counterpart assert
The handshake and data transfer
The FIFO Interface handshake ensures that data passes from the source to the sink only when the source has valid data to pass and when the sink is ready to receive that data.
Example waveform explanation
The positive clock edge marked with arrow indicating the transfer of the data is made (i.e. the handshake occurs) at the beginning of clock cycle 3, 5, 6 and 7 as demonstrated in Figure 1
Sample code snippet and sample waveform
Transmitter
module transmitter(
input clk,
input rst_n,
output vld,
output [7:0] data,
input rdy
);
reg next_vld;
reg [7:0] next_data;
reg [7:0] hold_data;
reg reg_vld;
reg [7:0] reg_data;
logic [7:0] sent_data[$];
typedef enum {IDLE, WAIT_RDY, WAIT_VLD, TRANSFER} ff_state;
ff_state current_state = IDLE;
ff_state next_state = IDLE;
always @(*) begin
case (current_state)
IDLE: begin
if (reg_vld == 1 && rdy == 0) begin
next_state = WAIT_RDY;
end else if (reg_vld == 0 && rdy == 1) begin
next_state = WAIT_VLD;
end else if (reg_vld == 1 && rdy == 1) begin
next_state = TRANSFER;
end else begin
next_state = current_state;
end
end
WAIT_RDY: begin
if (reg_vld == 1 && rdy == 1) begin
next_state = TRANSFER;
end else begin
next_state = current_state;
end
end
WAIT_VLD: begin
if (reg_vld == 1 && rdy == 1) begin
next_state = TRANSFER;
end else begin
next_state = current_state;
end
end
TRANSFER: begin
if (reg_vld == 1 && rdy == 0) begin
next_state = WAIT_RDY;
end else if (reg_vld == 0 && rdy == 1) begin
next_state = WAIT_VLD;
end else if (reg_vld == 0 && rdy == 0) begin
next_state = IDLE;
end else begin
next_state = current_state;
end
end
endcase
end
always @(posedge clk or negedge rst_n) begin
// Next state here is the previous state from the previous cycle
$display("[DEBUG %d] next_vld: %d, next_state: %d, current_state: %d", $time, next_vld, next_state, current_state);
if (!rst_n) begin
current_state <= IDLE;
reg_vld <= 0;
reg_data <= 0;
end else begin
current_state <= next_state;
next_vld = $urandom;
if (next_vld == 1 && next_state != WAIT_RDY) begin
// Send next_data on the next cycle
next_data = $urandom_range(1,10);
hold_data = next_data;
sent_data.push_back(next_data);
end else if (next_state == WAIT_RDY) begin
next_data = hold_data;
end else begin
next_data = 0;
end
reg_vld <= #0.1 (next_state == WAIT_RDY) ? 1 : next_vld;
reg_data <= #0.1 next_data;
end
end
assign vld = reg_vld;
assign data = reg_data;
endmodule
Receiver
module receiver(
input clk,
input rst_n,
input vld,
input [7:0] data,
output rdy
);
reg next_rdy;
reg reg_rdy;
logic [7:0] received_data[$];
reg no_random = 0;
typedef enum {IDLE, WAIT_RDY, WAIT_VLD, TRANSFER} ff_state;
ff_state current_state = IDLE;
ff_state next_state = IDLE;
always @(*) begin
case (current_state)
IDLE: begin
no_random = 0;
if (reg_rdy == 1 && vld == 0) begin
next_state = WAIT_VLD;
end else if (reg_rdy == 0 && vld == 1) begin
next_state = WAIT_RDY;
end else if (reg_rdy == 1 && vld == 1) begin
next_state = TRANSFER;
end else begin
next_state = current_state;
end
end
WAIT_VLD: begin
no_random = 1;
if (reg_rdy == 1 && vld == 1) begin
next_state = TRANSFER;
end else begin
next_state = current_state;
end
end
WAIT_RDY: begin
no_random = 0;
if (reg_rdy == 1 && vld == 1) begin
next_state = TRANSFER;
end else begin
next_state = current_state;
end
end
TRANSFER: begin
if (reg_rdy == 1 && vld == 0) begin
next_state = WAIT_VLD;
end else if (reg_rdy == 0 && vld == 1) begin
next_state = WAIT_RDY;
end else if (reg_rdy == 0 && vld == 0) begin
next_state = IDLE;
end else begin
next_state = current_state;
end
end
endcase
end
always @(posedge clk or negedge rst_n) begin
$display("[DEBUG %d] current data: %d, current_state: %d, next_state: %d", $time, data, current_state, next_state);
if (next_state == TRANSFER) begin
// Sample the data
$display("[DEBUG %0t] pushing %d data value", $time, data);
received_data.push_back(data);
end
if (!rst_n) begin
reg_rdy <= 0;
current_state <= #0.1 IDLE;
end else begin
reg_rdy <= #0.1 (next_state == WAIT_VLD) ? 1 : $urandom;
current_state <= #0.1 next_state;
end
end
assign rdy = reg_rdy;
endmodule
Testbench
module test_top;
reg _clk;
reg _rst_n;
wire [7:0] _data;
wire _vld;
wire _rdy;
transmitter tx(_clk, _rst_n, _vld, _data, _rdy);
receiver rx(_clk, _rst_n, _vld, _data, _rdy);
initial begin
$dumpfile("dump.vcd");
$dumpvars(0);
_clk = 0;
_rst_n = 1;
#10ns;
_rst_n = 0;
#10ns;
_rst_n = 1;
#1000ns;
foreach (rx.received_data[i]) begin
$display("[DEBUG] RX RECEIVED %d: %d", i, rx.received_data[i]);
end
foreach (tx.sent_data[i]) begin
$display("[DEBUG] TX SENT %d: %d", i, tx.sent_data[i]);
end
$finish;
end
always #10ns _clk = ~_clk;
endmodule
Sample waveform
References
- Drake, C. (2020, August 3). Ready/Valid Protocol Primer. Drake Enterprises. http://www.cjdrake.com/readyvalid-protocol-primer.html
- W. Fletcher, C. (2009). EECS150: Interfaces: “FIFO” (a.k.a. Ready/Valid) (B). https://inst.eecs.berkeley.edu/~cs150/Documents/Interfaces.pdf