add forgotten start of register interface

hdl/ip/vhd/i2c/BUCK

@@ -1,4 +1,13 @@
 load("//tools:hdl.bzl", "vhdl_unit", "vunit_sim")
+load("//tools:hdl.bzl", "vhdl_unit", "vunit_sim")
+load("//tools:rdl.bzl", "rdl_file")
+
+rdl_file(
+    name = "i2c_core_regs_pkg",
+    src = "i2c_core_regs.rdl",
+    outputs = ["i2c_core_regs_pkg.vhd", "i2c_core_regs.html"],
+    visibility = ['PUBLIC']
+)
 
 vhdl_unit(
     name = "i2c_txn_layer",

hdl/ip/vhd/i2c/i2c_core_regs.rdl

@@ -0,0 +1,51 @@
+// Copyright 2024 Oxide Computer Company
+// This is a SystemRDL description of the SW-accessible registers for the I2C core.
+
+addrmap i2c_core_regs {
+    name = "I2C core registers";
+    desc = "Registers accessible on the AXI bus for interacting with the I2C core.";
+
+    default regwidth = 32;
+    default sw = rw;
+    default hw = r;
+
+    reg {
+        name = "Receive data";
+        default sw = r;
+        default hw = rw;
+
+        field {
+            desc = "Last 4 bytes recieved";
+        } DATA[31:0] = 0;
+    } RXD;
+
+    reg {
+        name = "Transmit data";
+        default sw = r;
+        default hw = rw;
+
+        field {
+            desc = "Next 4 bytes to send";
+        } DATA[31:0] = 0;
+    } TXD;
+
+    reg {
+        name = "Control bits for I2C communication.";
+
+        field {
+            desc = "Number of bytes to read/write in the I2C transaction. up to 128 bytes.";
+        } COUNT[22:16] = 1;
+
+        field {
+            desc = "I2C Address of target";
+        } ADDR[14:8] = 0;
+
+        field {
+            desc = "2'b00 to read, 2'b01 to write, 2'b10 to random-read.";
+        } OP[2:1] = 0;
+
+        field {
+            desc = "'1' to start next transaction.";
+        } START[0:0] = 0;
+    } CONTROL;
+};

hdl/ip/vhd/i2c/i2c_core_regs.vhd

@@ -0,0 +1,71 @@
+-- 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/.
+--
+-- Copyright 2024 Oxide Computer Company
+
+-- AXI-accessible registers for the I2C block
+
+library ieee;
+use ieee.std_logic_1164.all;
+use ieee.numeric_std.all;
+use ieee.numeric_std_unsigned.all;
+
+use work.axil8x32_pkg.all;
+
+use work.i2c_core_regs_pkg.all;
+
+entity i2c_core_regs is
+    port (
+        clk     : in    std_logic;
+        reset   : in    std_logic;
+        axi_if  : view  axil_target;
+    );
+end entity;
+
+architecture rtl of i2c_core_regs is
+    constant AXI_OKAY           : std_logic_vector(1 downto 0) := "00";
+    signal   axi_read_ready_int : std_logic;
+    signal   axi_awready        : std_logic;
+    signal   axi_wready         : std_logic;
+    signal   axi_bvalid         : std_logic;
+    signal   axi_bready         : std_logic;
+    signal   axi_arready        : std_logic;
+    signal   axi_rvalid         : std_logic;
+    signal   axi_rdata          : std_logic_vector(31 downto 0);
+begin
+
+    -- AXI wiring
+    axi_if.write_response.resp  <= AXI_OKAY;
+    axi_if.write_response.valid <= axi_bvalid;
+    axi_if.read_data.resp       <= AXI_OKAY;
+    axi_if.write_data.ready     <= axi_wready;
+    axi_if.write_address.ready  <= axi_awready;
+    axi_if.read_address.ready   <= axi_arready;
+    axi_if.read_data.data       <= axi_rdata;
+    axi_if.read_data.valid      <= axi_rvalid;
+
+    axi_bready          <= axi_if.write_response.ready;
+    axi_wready          <= awready;
+    axi_arready         <= not rvalid;
+    axi_read_ready_int  <= axi_if.read_address.valid and axi_arready;
+
+    axi: process(clk, reset)
+    begin
+        if reset then
+            axi_awready <= '0';
+            axi_bvalid  <= '0';
+            axi_rvalid  <= '0';
+        elsif rising_edge(clk) then
+
+            -- bvalid is set on every write and then cleared after bv
+            if axi_awready then
+                axi_bvalid  <= '1';
+            elsif axi_bready then
+                axi_bvalid  <= '0';
+            end if;
+
+        end if;
+    end process;
+
+end architecture;