From 4aba165166b711dd4a6c967993b854e60f35aedf Mon Sep 17 00:00:00 2001
From: Luke Wren <wren6991@gmail.com>
Date: Fri, 20 Aug 2021 21:49:05 +0100
Subject: [PATCH] First pass at a 64-bit system timer

---
 hdl/peri/hazard3_riscv_timer.f |   2 +
 hdl/peri/hazard3_riscv_timer.v | 120 +++++++++++++++++++++++++++++++++
 2 files changed, 122 insertions(+)
 create mode 100644 hdl/peri/hazard3_riscv_timer.f
 create mode 100644 hdl/peri/hazard3_riscv_timer.v

diff --git a/hdl/peri/hazard3_riscv_timer.f b/hdl/peri/hazard3_riscv_timer.f
new file mode 100644
index 0000000..ae9ce66
--- /dev/null
+++ b/hdl/peri/hazard3_riscv_timer.f
@@ -0,0 +1,2 @@
+file hazard3_riscv_timer.v
+file ../debug/cdc/hazard3_sync_1bit.v
diff --git a/hdl/peri/hazard3_riscv_timer.v b/hdl/peri/hazard3_riscv_timer.v
new file mode 100644
index 0000000..99df9dd
--- /dev/null
+++ b/hdl/peri/hazard3_riscv_timer.v
@@ -0,0 +1,120 @@
+/**********************************************************************
+ * DO WHAT THE FUCK YOU WANT TO AND DON'T BLAME US PUBLIC LICENSE     *
+ *                    Version 3, April 2008                           *
+ *                                                                    *
+ * Copyright (C) 2021 Luke Wren                                       *
+ *                                                                    *
+ * Everyone is permitted to copy and distribute verbatim or modified  *
+ * copies of this license document and accompanying software, and     *
+ * changing either is allowed.                                        *
+ *                                                                    *
+ *   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION  *
+ *                                                                    *
+ * 0. You just DO WHAT THE FUCK YOU WANT TO.                          *
+ * 1. We're NOT RESPONSIBLE WHEN IT DOESN'T FUCKING WORK.             *
+ *                                                                    *
+ *********************************************************************/
+
+// Implementation of standard RISC-V timer (mtime/mtimeh mtimecmp/mtimecmph)
+// accessed over 32-bit data bus. Ticks on both edges of tick_nrz, which is
+// provided by some external timebase, and is assumed to be asynchronous to
+// clk. Nothing fancy, just a simple implementation of the spec.
+
+module hazard3_riscv_timer (
+	input  wire               clk,
+	input  wire               rst_n,
+
+	input  wire               psel,
+	input  wire               penable,
+	input  wire               pwrite,
+	input  wire [7:0]         paddr,
+	input  wire [31:0]        pwdata,
+	output reg  [31:0]        prdata,
+	output wire               pready,
+	output wire               pslverr,
+
+	input  wire               dbg_halt,
+	input  wire               tick_nrz,
+
+	output reg                timer_irq
+);
+
+wire bus_write = pwrite && psel && penable;
+wire bus_read = !pwrite && psel && penable;
+
+localparam W_ADDR = 8;
+localparam W_DATA = 32;
+
+localparam ADDR_CTRL      = 8'h00;
+localparam ADDR_MTIME     = 8'h08;
+localparam ADDR_MTIMEH    = 8'h0c;
+localparam ADDR_MTIMECMP  = 8'h10;
+localparam ADDR_MTIMECMPH = 8'h14;
+
+wire tick_nrz_sync;
+
+hazard3_sync_1bit tick_sync_u (
+	.clk    (clk),
+	.rst_n  (rst_n),
+	.i      (tick_nrz),
+	.o      (tick_nrz_sync)
+);
+
+reg tick_nrz_prev;
+always @ (posedge clk or negedge rst_n)
+	if (!rst_n)
+		tick_nrz_prev <= 1'b0;
+	else
+		tick_nrz_prev <= tick_nrz_sync;
+
+reg ctrl_en;
+always @ (posedge clk or negedge rst_n)
+	if (!rst_n)
+		ctrl_en <= 1'b1;
+	else if (bus_write && paddr == ADDR_CTRL)
+		ctrl_en <= pwdata[0];
+
+wire tick = ctrl_en && !dbg_halt && tick_nrz_prev != tick_nrz_sync;
+
+reg [63:0] mtime;
+
+always @ (posedge clk or negedge rst_n) begin
+	if (!rst_n) begin
+		mtime <= 64'h0;
+	end else begin
+		if (tick)
+			mtime <= mtime + 1'b1;
+		if (bus_write && paddr == ADDR_MTIME)
+			mtime[31:0] <= pwdata;
+		if (bus_write && paddr == ADDR_MTIMEH)
+			mtime[63:32] <= pwdata;
+	end
+end
+
+reg [63:0] mtimecmp;
+
+always @ (posedge clk or negedge rst_n) begin
+	if (!rst_n) begin
+		mtimecmp <= 64'h0;
+		timer_irq <= 1'b0;
+	end else begin
+		if (bus_write && paddr == ADDR_MTIMECMP)
+			mtimecmp[31:0] <= pwdata;
+		if (bus_write && paddr == ADDR_MTIMECMPH)
+			mtimecmp[63:32] <= pwdata;
+		timer_irq <= mtime >= mtimecmp; // oof
+	end
+end
+
+always @ (*) begin
+	case (paddr)
+	ADDR_CTRL:      prdata = {{W_DATA-1{1'b0}}, ctrl_en};
+	ADDR_MTIME:     prdata = mtime[31:0];
+	ADDR_MTIMEH:    prdata = mtime[63:32];
+	ADDR_MTIMECMP:  prdata = mtimecmp[31:0];
+	ADDR_MTIMECMPH: prdata = mtimecmp[63:32];
+	default:        prdata = {W_DATA{1'b0}};
+	endcase
+end
+
+endmodule