Add reset controller to Sonata

The reset controller generates a PoR based upon the PLL locked signal as
well as generating a new reset when the reset button is pushed. With the
new reset controller user JTAG works reliably to program Ibex demo
system.

Co-authored-by: Marno van der Maas <mvdmaas+git@lowrisc.org>

ibex_demo_system.core

@@ -40,6 +40,7 @@ filesets:
     files:
       - rtl/fpga/top_sonata.sv
       - rtl/fpga/clkgen_sonata.sv
+      - rtl/fpga/rst_ctrl.sv
     file_type: systemVerilogSource
 
   files_verilator:

rtl/fpga/clkgen_sonata.sv

@@ -5,11 +5,9 @@
 module clkgen_sonata (
     input IO_CLK,
     output IO_CLK_BUF,
-    input IO_RST_N,
     output clk_sys,
-    output rst_sys_n
+    output locked
 );
-  logic locked_pll;
   logic io_clk_buf;
   logic clk_50_buf;
   logic clk_50_unbuf;
@@ -56,7 +54,7 @@ module clkgen_sonata (
     .DRDY                (),
     .DWE                 (1'b0),
     // Other control and status signals
-    .LOCKED              (locked_pll),
+    .LOCKED              (locked),
     .PWRDWN              (1'b0),
     // Do not reset PLL on external reset, otherwise ILA disconnects at a reset
     .RST                 (1'b0));
@@ -77,7 +75,4 @@ module clkgen_sonata (
   // outputs
   // clock
   assign clk_sys = clk_50_buf;
-
-  // reset
-  assign rst_sys_n = locked_pll & IO_RST_N;
 endmodule

rtl/fpga/rst_ctrl.sv

@@ -0,0 +1,102 @@
+// Copyright lowRISC contributors.
+// Licensed under the Apache License, Version 2.0, see LICENSE for details.
+// SPDX-License-Identifier: Apache-2.0
+
+// Produces an active-low power-on-reset (PoR) on output `rst_no`. A reset will only happen once
+// `pll_locked_i` is set to indicate the systems PLL(s) have locked and clock(s) are stable. Further
+// resets can be triggered by the 'rst_btn_i' input (active high). This input is debounced so it can
+// be easily connected to a physical button.
+//
+// The PoR has three phases:
+// * phase 0 - Reset is left deasserted
+// * phase 1 - Reset is asserted and held asserted (giving a negative edge on rst_no from phase 0 to
+//   phase 1)
+// * phase 2 - Reset is deasserted (giving a positive edge on rst_no from phase 1 to phase 2)
+//
+// The 'ResetPhase0Count' and 'ResetPhase1Count' specify the length of time for phase 0 and phase
+// 1 in clock cycles (phase 2 is an unbounded length)
+//
+// When 'rst_btn_i' is asserted (set to 1) the controller is held at the beginning of phase 1 (reset
+// is asserted, and a negative edge seen on `rst_no` when the debounced `rst_btn_i` is first
+// asserted). It proceds to phase 2 as normal when the debounced `rst_btn_i` is deasserted.
+//
+// Debouncing of `rst_btn_i` occurs internally to the module. The `rst_btn_i` input must be in
+// a single state for more than `DebounceCount` cycles for that state to take effect.
+//
+// If the `pll_locked_i` input goes low a new reset will be produced when it goes high again.
+//
+// The provided `clk_i` is assumed to always be stable and must be independent of the output reset
+// `rst_no` and of the PLL providing the `pll_locked_i` input.
+//
+// The `rst_no` signal is produced directly from a flop to prevent glitches. This flop is clocked
+// from `clk_i`.
+//
+// This module is designed for FPGA implementation as it relies on an 'initial' statement to set the
+// power-on contents of registers.
+
+module rst_ctrl #(
+  parameter int unsigned ResetPhase0Count = 5,
+  parameter int unsigned ResetPhase1Count = 200,
+  parameter int unsigned DebounceCount    = 500
+) (
+  input clk_i,
+  input pll_locked_i,
+  input rst_btn_i,
+
+  output rst_no
+);
+  localparam CounterWidth = $clog2(ResetPhase1Count + 1);
+
+  logic [CounterWidth-1:0] reset_counter_d, reset_counter_q;
+  logic rst_btn_debounce;
+  logic rst_n_d, debounce_rst_n_d, rst_n_q, debounce_rst_n_q;
+
+  initial begin
+    reset_counter_q = '0;
+    rst_n_q = 1'b1;
+    debounce_rst_n_q = 1'b1;
+  end
+
+  always_comb begin
+    reset_counter_d = reset_counter_q;
+
+    // The output is driven when the counter is between the value of phase
+    // 0 and phase 1. When a press on the reset button is detected, we can
+    // reset the counter value and make sure that the reset pulse is the same
+    // length as when we reset the system at startup.
+    if (rst_btn_debounce && (reset_counter_q >= ResetPhase0Count)) begin
+      reset_counter_d = ResetPhase0Count;
+    end else begin
+      if (pll_locked_i) begin
+        if (reset_counter_q < ResetPhase1Count) begin
+          reset_counter_d <= reset_counter_d + 1;
+        end
+      end else begin
+        reset_counter_d = '0;
+      end
+    end
+  end
+
+  always_ff @(posedge clk_i) begin
+    reset_counter_q  <= reset_counter_d;
+    rst_n_q          <= rst_n_d;
+    debounce_rst_n_q <= debounce_rst_n_d;
+  end
+
+  debounce #(.ClkCount(DebounceCount)) u_rst_btn_debounce (
+    .clk_i,
+    .rst_ni(debounce_rst_n_q),
+
+    .btn_i(rst_btn_i),
+    .btn_o(rst_btn_debounce)
+  );
+
+  assign rst_n_d = reset_counter_q < ResetPhase0Count ? 1'b1 :
+                   reset_counter_q < ResetPhase1Count ? 1'b0 :
+                                                        1'b1;
+
+  assign debounce_rst_n_d = reset_counter_q <= ResetPhase0Count ? 1'b1 :
+                            reset_counter_q <  ResetPhase1Count ? 1'b0 :
+                                                                  1'b1;
+  assign rst_no = rst_n_q;
+endmodule

rtl/fpga/top_sonata.sv

@@ -34,29 +34,19 @@ module top_sonata (
 );
   parameter SRAMInitFile = "";
 
-  logic top_rst_n;
   logic mainclk_buf;
-  logic clk_sys, rst_sys_n;
+  logic clk_sys;
+  logic rst_sys_n;
   logic [7:0] reset_counter;
 
   logic [4:0] nav_sw_n;
   logic [7:0] user_sw_n;
 
-  initial begin
-    reset_counter = 0;
-  end
+  logic pll_locked;
+  logic rst_btn;
 
-  always_ff @(posedge mainclk_buf) begin
-    if (reset_counter != 8'hff) begin
-      reset_counter <= reset_counter + 8'd1;
-    end
-  end
 
-  assign top_rst_n = reset_counter < 8'd5   ? 1'b1 :
-                     reset_counter < 8'd200 ? 1'b0 :
-                                              nrst_btn;
-
-  assign led_bootok = 1'b1;
+  assign led_bootok = rst_sys_n;
 
   // Switch inputs have pull-ups and switches pull to ground when on. Invert here so CPU sees 1 for
   // on and 0 for off.
@@ -95,9 +85,16 @@ module top_sonata (
   clkgen_sonata clkgen(
     .IO_CLK(main_clk),
     .IO_CLK_BUF(mainclk_buf),
-    .IO_RST_N(top_rst_n),
     .clk_sys,
-    .rst_sys_n
+    .locked(pll_locked)
   );
 
+  assign rst_btn = ~nrst_btn;
+
+  rst_ctrl u_rst_ctrl (
+    .clk_i       (mainclk_buf),
+    .pll_locked_i(pll_locked),
+    .rst_btn_i   (rst_btn),
+    .rst_no      (rst_sys_n)
+  );
 endmodule