From 9bbe1407bae7b4aeca46dbba30814a9962cce670 Mon Sep 17 00:00:00 2001
From: Peter McGoron <code@mcgoron.com>
Date: Sun, 23 Oct 2022 14:03:19 -0400
Subject: [PATCH] add metastability workaround

---
 spi_master.v | 19 +++++++++++++++++--
 spi_slave.v  | 15 +++++++++++++--
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/spi_master.v b/spi_master.v
index d6018c0..82e5ee8 100644
--- a/spi_master.v
+++ b/spi_master.v
@@ -4,6 +4,11 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
+/* CYCLE_HALF_WAIT should take into account the setup time of the slave
+ * device, and also master buffering (MISO is one cycle off to stabilize
+ * the input).
+ */
+
 module
 `ifdef SPI_MASTER_NO_READ
 spi_master_no_read
@@ -14,7 +19,6 @@ spi_master_no_write
 spi_master
 `endif
 `endif
-
 #(
 	parameter WID = 24, // Width of bits per transaction.
 	parameter WID_LEN = 5, // Length in bits required to store WID
@@ -39,6 +43,17 @@ spi_master
 	input arm
 );
 
+`ifndef SPI_MASTER_NO_READ
+/* MISO is almost always an external wire, so buffer it. */
+reg miso_hot = 0;
+reg read_miso = 0;
+
+always @ (posedge clk) begin
+	read_miso <= miso_hot;
+	miso_hot <= miso;
+end
+`endif
+
 parameter WAIT_ON_ARM = 0;
 parameter ON_CYCLE = 1;
 parameter CYCLE_WAIT = 2;
@@ -71,7 +86,7 @@ endtask
 task read_data();
 `ifndef SPI_MASTER_NO_READ
 	from_slave <= from_slave << 1;
-	from_slave[0] <= miso;
+	from_slave[0] <= read_miso;
 `endif
 endtask
 
diff --git a/spi_slave.v b/spi_slave.v
index 83ab335..c4f7a7c 100644
--- a/spi_slave.v
+++ b/spi_slave.v
@@ -24,7 +24,7 @@ spi_slave
 	input ss_L,
 `ifndef SPI_SLAVE_NO_READ
 	output reg [WID-1:0] from_master,
-	input reg mosi,
+	input mosi,
 `endif
 `ifndef SPI_SLAVE_NO_WRITE
 	input [WID-1:0] to_master,
@@ -35,6 +35,17 @@ spi_slave
 	output reg err
 );
 
+`ifndef SPI_SLAVE_NO_READ
+/* MOSI is almost always an external wire, so buffer it. */
+reg mosi_hot = 0;
+reg read_mosi = 0;
+
+always @ (posedge clk) begin
+	read_mosi <= mosi_hot;
+	mosi_hot <= mosi;
+end
+`endif
+
 wire ss = !ss_L;
 reg sck_delay = 0;
 reg [WID_LEN-1:0] bit_counter = 0;
@@ -48,7 +59,7 @@ reg [WID-1:0] send_buf = 0;
 task read_data();
 `ifndef SPI_SLAVE_NO_READ
 	from_master <= from_master << 1;
-	from_master[0] <= mosi;
+	from_master[0] <= read_mosi;
 `endif
 endtask