import spi v0.2

firmware/rtl/spi/spi_master.v

@@ -1,5 +1,12 @@
-/* SPI master.
- * Written by Peter McGoron, 2022.
+/* (c) Peter McGoron 2022 v0.2
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v.2.0. If a copy of the MPL was not distributed with this
+ * 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
@@ -12,11 +19,11 @@ 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
-	parameter CYCLE_HALF_WAIT = 1, // Half of the wait time of a cycle
+	parameter CYCLE_HALF_WAIT = 1, // Half of the wait time of a cycle minus 1.
+	                               // One SCK cycle is 2*(CYCLE_HALF_WAIT + 1) clock cycles.
 	parameter TIMER_LEN = 3, // Length in bits required to store CYCLE_HALF_WAIT
 	parameter POLARITY = 0, // 0 = sck idle low, 1 = sck idle high
 	parameter PHASE = 0 // 0 = rising-read falling-write, 1 = rising-write falling-read.
@@ -29,13 +36,30 @@ spi_master
 `endif
 `ifndef SPI_MASTER_NO_WRITE
 	input [WID-1:0] to_slave,
-	output mosi,
+	output reg mosi,
 `endif
-	output sck_wire,
-	output finished,
+	output reg sck_wire,
+	output reg finished,
 	input arm
 );
 
+`ifndef SPI_MASTER_NO_READ
+/* MISO is almost always an external wire, so buffer it.
+ * This might not be necessary, since the master and slave do not respond
+ * immediately to changes in the wires, but this is just to be safe.
+ * It is trivial to change, just do
+ *    wire read_miso = miso;
+ */
+
+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;
@@ -68,7 +92,7 @@ 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
 
@@ -100,6 +124,16 @@ task setup_bits();
 	end
 endtask
 
+task cycle_change();
+	// Stop transfer when the clock returns to its original polarity.
+	if (bit_counter == WID[WID_LEN-1:0] && sck == POLARITY[0]) begin
+		state <= WAIT_FINISHED;
+	end else begin
+		sck <= !sck;
+		state <= ON_CYCLE;
+	end
+endtask
+
 always @ (posedge clk) begin
 	case (state)
 	WAIT_ON_ARM: begin
@@ -132,19 +166,17 @@ always @ (posedge clk) begin
 				bit_counter <= bit_counter + 1;
 			end
 		end
-		state <= CYCLE_WAIT;
+
+		if (CYCLE_HALF_WAIT == 0) begin
+			cycle_change();
+		end else begin
+			state <= CYCLE_WAIT;
+		end
 	end
 	CYCLE_WAIT: begin
 		if (timer == CYCLE_HALF_WAIT) begin
-			timer <= 0;
-			// Stop transfer when the clock returns
-			// to its original polarity.
-			if (bit_counter == WID && sck == POLARITY) begin
-				state <= WAIT_FINISHED;
-			end else begin
-				state <= ON_CYCLE;
-				sck <= !sck;
-			end
+			timer <= 1;
+			cycle_change();
 		end else begin
 			timer <= timer + 1;
 		end

firmware/rtl/spi/spi_master_ss.v

@@ -0,0 +1,4 @@
+`define SPI_MASTER_SS_NAME spi_master_ss
+`define SPI_MASTER_NAME spi_master
+/* verilator lint_off DECLFILENAME */
+`include "spi_master_ss_template.v"

firmware/rtl/spi/spi_master_ss_no_read.v

@@ -0,0 +1,5 @@
+`define SPI_MASTER_SS_NAME spi_master_ss_no_read
+`define SPI_MASTER_NAME spi_master_no_read
+`define SPI_MASTER_NO_READ
+/* verilator lint_off DECLFILENAME */
+`include "spi_master_ss_template.v"

firmware/rtl/spi/spi_master_ss_no_write.v

@@ -0,0 +1,5 @@
+`define SPI_MASTER_SS_NAME spi_master_ss_no_write
+`define SPI_MASTER_NAME spi_master_no_write
+`define SPI_MASTER_NO_WRITE
+/* verilator lint_off DECLFILENAME */
+`include "spi_master_ss_template.v"

firmware/rtl/spi/spi_master_ss_template.v

@@ -0,0 +1,108 @@
+/* spi master with integrated ability to wait a certain amount of cycles
+ * after activating SS.
+ */
+
+module `SPI_MASTER_SS_NAME
+#(
+	parameter WID = 24,
+	parameter WID_LEN = 5,
+	parameter CYCLE_HALF_WAIT = 1,
+	parameter TIMER_LEN = 3,
+
+	parameter SS_WAIT = 1,
+	parameter SS_WAIT_TIMER_LEN = 2,
+
+	parameter POLARITY = 0,
+	parameter PHASE = 0
+)
+(
+	input clk,
+`ifndef SPI_MASTER_NO_READ
+	output [WID-1:0] from_slave,
+	input miso,
+`endif
+`ifndef SPI_MASTER_NO_WRITE
+	input [WID-1:0] to_slave,
+	output reg mosi,
+`endif
+	output sck_wire,
+	output finished,
+	output ss_L,
+	input arm
+);
+
+reg ss = 0;
+reg arm_master = 0;
+assign ss_L = !ss;
+
+`SPI_MASTER_NAME #(
+	.WID(WID),
+	.WID_LEN(WID_LEN),
+	.CYCLE_HALF_WAIT(CYCLE_HALF_WAIT),
+	.TIMER_LEN(TIMER_LEN),
+	.POLARITY(POLARITY),
+	.PHASE(PHASE)
+) master (
+	.clk(clk),
+`ifndef SPI_MASTER_NO_READ
+	.from_slave(from_slave),
+	.miso(miso),
+`endif
+`ifndef SPI_MASTER_NO_WRITE
+	.to_slave(to_slave),
+	.mosi(mosi),
+`endif
+	.sck_wire(sck_wire),
+	.finished(finished),
+	.arm(arm_master)
+);
+
+localparam WAIT_ON_ARM = 0;
+localparam WAIT_ON_SS = 1;
+localparam WAIT_ON_MASTER = 2;
+localparam WAIT_ON_ARM_DEASSERT = 3;
+reg [2:0] state = WAIT_ON_ARM;
+reg [SS_WAIT_TIMER_LEN-1:0] timer = 0;
+
+task master_arm();
+	arm_master <= 1;
+	state <= WAIT_ON_MASTER;
+endtask
+
+always @ (posedge clk) begin
+	case (state)
+	WAIT_ON_ARM: begin
+		if (arm) begin
+			timer <= 1;
+			if (SS_WAIT == 0) begin
+				master_arm();
+			end else begin
+				timer <= 1;
+				state <= WAIT_ON_SS;
+			end
+			ss <= 1;
+		end
+	end
+	WAIT_ON_SS: begin
+		if (timer == SS_WAIT) begin
+			master_arm();
+		end else begin
+			timer <= timer + 1;
+		end
+	end
+	WAIT_ON_MASTER: begin
+		if (finished) begin
+			state <= WAIT_ON_ARM_DEASSERT;
+			ss <= 0;
+		end
+	end
+	WAIT_ON_ARM_DEASSERT: begin
+		if (!arm) begin
+			state <= WAIT_ON_ARM;
+			arm_master <= 0;
+		end
+	end
+	endcase
+end
+
+endmodule

firmware/rtl/spi/spi_slave.v

@@ -1,4 +1,4 @@
-/* (c) Peter McGoron 2022 v0.1
+/* (c) Peter McGoron 2022 v0.2
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v.2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
@@ -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
 
@@ -77,7 +88,7 @@ task setup_bits();
 endtask
 
 task check_counter();
-	if (bit_counter == WID) begin
+	if (bit_counter == WID[WID_LEN-1:0]) begin
 		err <= ready_at_start;
 	end else begin
 		bit_counter <= bit_counter + 1;

firmware/rtl/spi/spi_slave_no_read.v

@@ -0,0 +1,3 @@
+`define SPI_SLAVE_NO_READ
+/* verilator lint_off DECLFILENAME */
+`include "spi_slave.v"