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authorJosh Blum <josh@joshknows.com>2010-01-22 16:00:45 -0800
committerJosh Blum <josh@joshknows.com>2010-01-22 16:00:45 -0800
commit8b377a9d6d0ad281474a8dbff49ea3b093178b28 (patch)
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parente92d36dcfe02afaedec348f2d8fc4523fb4e633b (diff)
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diff --git a/control_lib/ss_rcvr.v b/control_lib/ss_rcvr.v
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-
-
-// Source-synchronous receiver
-// Assumes both clocks are at the same rate
-// Relative clock phase is
-// unknown
-// variable
-// bounded
-// The output will come several cycles later than the input
-
-// This should synthesize efficiently in Xilinx distributed ram cells,
-// which is why we use a buffer depth of 16
-
-// FIXME Async reset on rxclk side?
-
-module ss_rcvr
- #(parameter WIDTH=16)
- (input rxclk,
- input sysclk,
- input rst,
-
- input [WIDTH-1:0] data_in,
- output [WIDTH-1:0] data_out,
- output reg clock_present);
-
- wire [3:0] rd_addr, wr_addr;
-
- // Distributed RAM
- reg [WIDTH-1:0] buffer [0:15];
- always @(posedge rxclk)
- buffer[wr_addr] <= data_in;
-
- assign data_out = buffer[rd_addr];
-
- // Write address generation
- reg [3:0] wr_counter;
- always @(posedge rxclk or posedge rst)
- if (rst)
- wr_counter <= 0;
- else
- wr_counter <= wr_counter + 1;
-
- assign wr_addr = {wr_counter[3], ^wr_counter[3:2], ^wr_counter[2:1], ^wr_counter[1:0]};
-
- // Read Address generation
- wire [3:0] wr_ctr_sys, diff, abs_diff;
- reg [3:0] wr_addr_sys_d1, wr_addr_sys_d2;
- reg [3:0] rd_counter;
-
- assign rd_addr = {rd_counter[3], ^rd_counter[3:2], ^rd_counter[2:1], ^rd_counter[1:0]};
-
- always @(posedge sysclk)
- wr_addr_sys_d1 <= wr_addr;
-
- always @(posedge sysclk)
- wr_addr_sys_d2 <= wr_addr_sys_d1;
-
- assign wr_ctr_sys = {wr_addr_sys_d2[3],^wr_addr_sys_d2[3:2],^wr_addr_sys_d2[3:1],^wr_addr_sys_d2[3:0]};
-
- assign diff = wr_ctr_sys - rd_counter;
- assign abs_diff = diff[3] ? (~diff+1) : diff;
-
- always @(posedge sysclk)
- if(rst)
- begin
- clock_present <= 0;
- rd_counter <= 0;
- end
- else
- if(~clock_present)
- if(abs_diff > 5)
- clock_present <= 1;
- else
- ;
- else
- if(abs_diff<3)
- clock_present <= 0;
- else
- rd_counter <= rd_counter + 1;
-
-endmodule // ss_rcvr