Improve ToD timestep generation in PTP clock and PTP CDC module

Signed-off-by: Alex Forencich <[email protected]>
alexforencich · Nov 7, 2023 · 957f98e · 957f98e
1 parent 9fb450b
commit 957f98e
Show file tree

Hide file tree

Showing 2 changed files with 6 additions and 20 deletions.
diff --git a/rtl/ptp_clock.v b/rtl/ptp_clock.v
@@ -112,10 +112,6 @@ reg [15:0] drift_rate_reg = DRIFT_RATE;
 
 reg [INC_NS_WIDTH-1:0] ts_inc_ns_reg = 0;
 reg [FNS_WIDTH-1:0] ts_inc_fns_reg = 0;
-reg [INC_NS_WIDTH-1:0] ts_inc_ns_delay_reg = 0;
-reg [FNS_WIDTH-1:0] ts_inc_fns_delay_reg = 0;
-reg [30:0] ts_inc_ns_ovf_reg = 0;
-reg [FNS_WIDTH-1:0] ts_inc_fns_ovf_reg = 0;
 
 reg [47:0] ts_96_s_reg = 0;
 reg [29:0] ts_96_ns_reg = 0;
@@ -254,17 +250,14 @@ always @(posedge clk) begin
     end
 
     // 96 bit timestamp
-    {ts_inc_ns_delay_reg, ts_inc_fns_delay_reg} <= {ts_inc_ns_reg, ts_inc_fns_reg};
-    {ts_inc_ns_ovf_reg, ts_inc_fns_ovf_reg} <= {NS_PER_S, {FNS_WIDTH{1'b0}}} - {ts_inc_ns_reg, ts_inc_fns_reg};
-
-    {ts_96_ns_inc_reg, ts_96_fns_inc_reg} <= {ts_96_ns_inc_reg, ts_96_fns_inc_reg} + {ts_inc_ns_delay_reg, ts_inc_fns_delay_reg};
-    {ts_96_ns_ovf_reg, ts_96_fns_ovf_reg} <= {ts_96_ns_inc_reg, ts_96_fns_inc_reg} - {ts_inc_ns_ovf_reg, ts_inc_fns_ovf_reg};
+    {ts_96_ns_inc_reg, ts_96_fns_inc_reg} <= {ts_96_ns_inc_reg, ts_96_fns_inc_reg} + {ts_inc_ns_reg, ts_inc_fns_reg};
+    {ts_96_ns_ovf_reg, ts_96_fns_ovf_reg} <= {ts_96_ns_inc_reg, ts_96_fns_inc_reg} + (-{NS_PER_S, {FNS_WIDTH{1'b0}}} | {ts_inc_ns_reg, ts_inc_fns_reg});
     {ts_96_ns_reg, ts_96_fns_reg} <= {ts_96_ns_inc_reg, ts_96_fns_inc_reg};
 
     if (!ts_96_ns_ovf_reg[30]) begin
         // if the overflow lookahead did not borrow, one second has elapsed
         // increment seconds field, pre-compute normal increment, force overflow lookahead borrow bit set
-        {ts_96_ns_inc_reg, ts_96_fns_inc_reg} <= {ts_96_ns_ovf_reg, ts_96_fns_ovf_reg} + {ts_inc_ns_delay_reg, ts_inc_fns_delay_reg};
+        {ts_96_ns_inc_reg, ts_96_fns_inc_reg} <= {ts_96_ns_ovf_reg, ts_96_fns_ovf_reg} + {ts_inc_ns_reg, ts_inc_fns_reg};
         ts_96_ns_ovf_reg[30] <= 1'b1;
         {ts_96_ns_reg, ts_96_fns_reg} <= {ts_96_ns_ovf_reg, ts_96_fns_ovf_reg};
         ts_96_s_reg <= ts_96_s_reg + 1;

diff --git a/rtl/ptp_clock_cdc.v b/rtl/ptp_clock_cdc.v
@@ -101,8 +101,6 @@ localparam TIME_ERR_INT_WIDTH = NS_WIDTH+FNS_WIDTH;
 localparam [30:0] NS_PER_S = 31'd1_000_000_000;
 
 reg [NS_WIDTH+FNS_WIDTH-1:0] period_ns_reg = 0, period_ns_next = 0;
-reg [NS_WIDTH+FNS_WIDTH-1:0] period_ns_delay_reg = 0, period_ns_delay_next = 0;
-reg [31+FNS_WIDTH-1:0] period_ns_ovf_reg = 0, period_ns_ovf_next = 0;
 
 reg [47:0] src_ts_s_capt_reg = 0;
 reg [TS_NS_WIDTH+CMP_FNS_WIDTH-1:0] src_ts_ns_capt_reg = 0;
@@ -595,19 +593,16 @@ always @* begin
     gain_sel_next = gain_sel_reg;
 
     // PTP clock
-    period_ns_delay_next = period_ns_reg;
-    period_ns_ovf_next = {NS_PER_S, {FNS_WIDTH{1'b0}}} - period_ns_reg;
-
     if (TS_WIDTH == 96) begin
         // 96 bit timestamp
-        ts_ns_inc_next = ts_ns_inc_reg + period_ns_delay_reg;
-        ts_ns_ovf_next = ts_ns_inc_reg - period_ns_ovf_reg;
+        ts_ns_inc_next = ts_ns_inc_reg + period_ns_reg;
+        ts_ns_ovf_next = ts_ns_inc_reg + (-{NS_PER_S, {FNS_WIDTH{1'b0}}} | period_ns_reg);
         ts_ns_next = ts_ns_inc_reg;
 
         if (!ts_ns_ovf_reg[30+FNS_WIDTH]) begin
             // if the overflow lookahead did not borrow, one second has elapsed
             // increment seconds field, pre-compute normal increment, force overflow lookahead borrow bit set
-            ts_ns_inc_next = ts_ns_ovf_reg + period_ns_delay_reg;
+            ts_ns_inc_next = ts_ns_ovf_reg + period_ns_reg;
             ts_ns_ovf_next[30+FNS_WIDTH] = 1'b1;
             ts_ns_next = ts_ns_ovf_reg;
             ts_s_next = ts_s_reg + 1;
@@ -767,8 +762,6 @@ end
 
 always @(posedge output_clk) begin
     period_ns_reg <= period_ns_next;
-    period_ns_delay_reg <= period_ns_delay_next;
-    period_ns_ovf_reg <= period_ns_ovf_next;
 
     ts_s_reg <= ts_s_next;
     ts_ns_reg <= ts_ns_next;