From 568f59017164ccd8c70e09e676af6d05249d7f91 Mon Sep 17 00:00:00 2001
From: Eugene Lozovoy <uzix.ls@gmail.com>
Date: Tue, 9 Aug 2022 12:49:27 +0300
Subject: [PATCH] fix random garbage on screen (again)

I hope I've found and fixed root cause of issue in this commit.
---
 fpga/rtl/screen.sv     | 16 +++++++++-------
 fpga/rtl/turbosound.sv | 24 +++++++++++++++++-------
 fpga/syn/clocks.sdc    |  7 +++++++
 3 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/fpga/rtl/screen.sv b/fpga/rtl/screen.sv
index 175f7fc..7f4b508 100755
--- a/fpga/rtl/screen.sv
+++ b/fpga/rtl/screen.sv
@@ -17,7 +17,7 @@ module screen(
     input fetch_allow,
     output reg fetch,
     output fetch_next,
-    output [14:0] addr,
+    output reg [14:0] addr,
     input [7:0] fetch_data,
 
     output contention,
@@ -184,7 +184,7 @@ wire screen_show = (vc < V_AREA) && (hc0 >= (SCREEN_DELAY<<2) - 1) && (hc0 < ((H
 wire screen_update = hc0[4:0] == 5'b10011;
 wire border_update = (hc0[4:0] == 5'b10011) || (machine == MACHINE_PENT && ck7);
 wire bitmap_shift = hc0[1:0] == 2'b11;
-wire next_addr = hc0[4:0] == 5'b10001;
+wire next_addr = hc0[4:0] == 5'b10000;
 
 reg [7:0] vaddr;
 reg [7:3] haddr;
@@ -214,18 +214,16 @@ reg [7:0] bitmap, attr, bitmap_next, attr_next;
 reg [7:0] up_ink0, up_paper0;
 
 reg fetch_step;
-wire fetch_bitmap = fetch && fetch_step == 2'd0;
-wire fetch_attr   = fetch && fetch_step == 2'd1;
+wire fetch_bitmap = fetch && fetch_step == 1'd0;
+wire fetch_attr   = fetch && fetch_step == 1'd1;
 assign fetch_next = loading && fetch_allow;
 
-assign addr = fetch_bitmap?
-    { 2'b10, vaddr[7:6], vaddr[2:0], vaddr[5:3], haddr[7:3] } :
-    { 5'b10110, vaddr[7:3], haddr[7:3] };
 assign up_ink_addr = { attr_next[7:6], 1'b0, attr_next[2:0] };
 assign up_paper_addr = { attr_next[7:6], 1'b1, attr_next[5:3] };
 
 always @(posedge clk28 or negedge rst_n) begin
     if (!rst_n) begin
+        addr <= 0;
         fetch <= 0;
         fetch_step <= 0;
         attr <= 0;
@@ -237,6 +235,10 @@ always @(posedge clk28 or negedge rst_n) begin
     end
     else begin
         if (ck14) begin
+            addr <= ((fetch && fetch_step == 1'd1) || (!fetch && fetch_step == 1'b0))?
+                { 2'b10, vaddr[7:6], vaddr[2:0], vaddr[5:3], haddr[7:3] } :
+                { 5'b10110, vaddr[7:3], haddr[7:3] };
+
             if (fetch)
                 fetch_step <= fetch_step + 1'b1;
             fetch <= fetch_next;
diff --git a/fpga/rtl/turbosound.sv b/fpga/rtl/turbosound.sv
index fbad16f..6dfb49f 100644
--- a/fpga/rtl/turbosound.sv
+++ b/fpga/rtl/turbosound.sv
@@ -7,8 +7,8 @@ module turbosound(
     input en_ts,
 
     cpu_bus bus,
-    output [7:0] d_out,
-    output d_out_active,
+    output reg [7:0] d_out,
+    output reg d_out_active,
 
     output [7:0] ay_a0,
     output [7:0] ay_b0,
@@ -18,12 +18,15 @@ module turbosound(
     output [7:0] ay_c1
 );
 
+//              bdir bc1 description
+// bffd read  |   0   0  inactive
+// bffd write |   1   0  write to psg
+// fffd read  |   0   1  read from psg
+// fffd write |   1   1  latch address
 
 reg ay_bdir;
 reg ay_bc1;
 reg ay_sel;
-wire ay_rd0 = bus.rd && ay_bc1 == 1'b1 && ay_bdir == 1'b0 && ay_sel == 1'b0;
-wire ay_rd1 = bus.rd && ay_bc1 == 1'b1 && ay_bdir == 1'b0 && ay_sel == 1'b1;
 wire port_bffd = en && bus.ioreq && bus.a_reg[15] == 1'b1 && bus.a_reg[1] == 0;
 wire port_fffd = en && bus.ioreq && bus.a_reg[15] == 1'b1 && bus.a_reg[14] == 1'b1 && bus.a_reg[1] == 0;
 always @(posedge clk28 or negedge rst_n) begin
@@ -87,8 +90,15 @@ YM2149 ym2149_1(
     );
 
 
-assign d_out_active = ay_rd0 | ay_rd1;
-assign d_out = ay_rd1? ay_dout1 : ay_dout0;
-
+always @(posedge clk28 or negedge rst_n) begin
+    if (!rst_n) begin
+        d_out_active <= 0;
+        d_out <= 0;
+    end
+    else begin
+        d_out_active <= bus.rd && port_fffd;
+        d_out <= ay_sel? ay_dout1 : ay_dout0;
+    end
+end
 
 endmodule
diff --git a/fpga/syn/clocks.sdc b/fpga/syn/clocks.sdc
index 1a5036d..60ab9bf 100755
--- a/fpga/syn/clocks.sdc
+++ b/fpga/syn/clocks.sdc
@@ -8,3 +8,10 @@ derive_clocks -period 14MHz
 
 set_multicycle_path -from {vencode:*|*} -to {vencode:*|*} -setup 4
 set_multicycle_path -from {vencode:*|*} -to {vencode:*|*} -hold 3
+
+# One screen read cycle = ~71ns. SRAM speed = 55ns
+# So we have about 16ns to setup control signals (n_vrd, n_vwr, va - 10ns) and read back data (vd - 6ns)
+set_max_delay -from [get_pins -compatibility_mode screen0|*] -to [get_ports n_vrd] 10ns
+set_max_delay -from [get_pins -compatibility_mode screen0|*] -to [get_ports n_vwr] 10ns
+set_max_delay -from [get_pins -compatibility_mode screen0|*] -to [get_ports va[*]] 10ns
+set_max_delay -from [get_ports vd[*]] -to [get_pins -compatibility_mode screen0|*] 6ns