add metastability workaround

1 files changed, 13 insertions, 2 deletions

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