From 0128cfe497224105f4e055692169c6a06c8bde18 Mon Sep 17 00:00:00 2001
From: Meinhard Kissich <privat@meinhard-kissich.at>
Date: Wed, 3 Apr 2024 14:30:59 +0200
Subject: [PATCH] Manually instantiate buffers

---
 rtl/fazyrv_pc.sv      | 15 ++++++++++++++-
 rtl/fazyrv_shftreg.sv | 14 +++++++++++++-
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/rtl/fazyrv_pc.sv b/rtl/fazyrv_pc.sv
index f2fe7a2..aff71a1 100644
--- a/rtl/fazyrv_pc.sv
+++ b/rtl/fazyrv_pc.sv
@@ -95,7 +95,20 @@ assign add_vec = {{(ADD_VEC_WIDTH-3){1'b0}}, inc_i, 2'b00};
 
 assign carry_vec[0] = carry_r[0];
 
-assign pc_n = shift_i ? {din_i, pc_r[31:CHUNKSIZE]} : pc_r;
+
+// Insert sky130 buffers manually to achieve a higher
+// placement density. Inspired by MichaelBell/tinyQV
+// www.github.com/MichaelBell/tinyQV/blob/69ce898bf1122e91a3114f3f0fe8e4bdf242f7f0/cpu/register.v#L58
+//
+
+logic [31-CHUNKSIZE:0] pc_dlyd;
+`ifdef SKY130
+  sky130_fd_sc_hd__dlygate4sd3_1 i_buf[31-CHUNKSIZE:0] ( .X(pc_dlyd), .A(pc_r[31:CHUNKSIZE]) );
+`else
+  buf #1 i_buf[31:CHUNKSIZE] (pc_dlyd, pc_r[31:CHUNKSIZE]);
+`endif
+
+assign pc_n = shift_i ? {din_i, pc_dlyd} : pc_r;
 
 always_ff @(posedge clk_i) begin
   if (~rst_in) begin
diff --git a/rtl/fazyrv_shftreg.sv b/rtl/fazyrv_shftreg.sv
index 7f61f89..2d97e06 100644
--- a/rtl/fazyrv_shftreg.sv
+++ b/rtl/fazyrv_shftreg.sv
@@ -35,11 +35,23 @@ logic [31:0] reg_r = 'b0;
 logic [31:0] reg_r;
 `endif
 
+// Insert sky130 buffers manually to achieve a higher
+// placement density. Inspired by MichaelBell/tinyQV
+// www.github.com/MichaelBell/tinyQV/blob/69ce898bf1122e91a3114f3f0fe8e4bdf242f7f0/cpu/register.v#L58
+//
+
+logic [31-CHUNKSIZE:0] reg_dlyd;
+`ifdef SKY130
+  sky130_fd_sc_hd__dlygate4sd3_1 i_buf[31-CHUNKSIZE:0] ( .X(reg_dlyd), .A(reg_r[31:CHUNKSIZE]) );
+`else
+  buf #1 i_buf[31:CHUNKSIZE] (reg_dlyd, reg_r[31:CHUNKSIZE]);
+`endif
+
 assign dat_o = reg_r[CHUNKSIZE-1:0];
 
 always_ff @(posedge clk_i) begin
   if (shft_i) begin
-    reg_r <= {dat_i, reg_r[31:CHUNKSIZE]};
+    reg_r <= {dat_i, reg_dlyd};
   end
 end