freechips/semicustom/semicustom.ipynb

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    "# Semicustom digital design crashbook\n",
    "\n",
    "## Sources\n",
    "\n",
    "### RTL description (Verilog)\n",
    "\n",
    "The OpenROAD workflow takes the circuit's RTL description as an input. For instance, it can be a three bits XOR gate.\n",
    "\n",
    "\n",
    "<blockquote><details>\n",
    "\n",
    "<summary>\n",
    "    \n",
    "#### ↕️ Types in Verilog\n",
    "\n",
    "</summary>\n",
    "    \n",
    "```verilog\n",
    "// Three scalar nets\n",
    "wire op_b, op_a, result;\n",
    "// One 16-bit net\n",
    "wire [15:0] word_bus;\n",
    "// 1K-array of 8-bit nets\n",
    "wire [7:0] byte_array [0:1023];\n",
    "```\n",
    "    \n",
    "</details></blockquote>\n",
    "\n",
    "<blockquote><details>\n",
    "\n",
    "<summary>\n",
    "    \n",
    "#### ↕️ Assignation (non-blocking) in Verilog\n",
    "\n",
    "</summary>\n",
    "    \n",
    "```verilog\n",
    "// 16-bit, hexadecimal constant\n",
    "assign address = 16'hCAFE;\n",
    "// Unsized, decimal constant\n",
    "assign counter = 'd42;\n",
    "// 1-bit, binary constant\n",
    "assign answer = 1'b1;\n",
    "\n",
    "// Ternary assignation\n",
    "assign muxed = which ? source_1 : source_2;\n",
    "\n",
    "// Concatenation\n",
    "assign padded_packet = {5'b00000,body,suffix};\n",
    "// Replication\n",
    "assign odd_mask = {10{2'b10}};\n",
    "\n",
    "// Indexing\n",
    "assign one_bit = bus[4];\n",
    "assign bits = bus[15:12];\n",
    "```\n",
    "    \n",
    "</details></blockquote>\n",
    "\n",
    "<blockquote><details>\n",
    "\n",
    "<summary>\n",
    "    \n",
    "#### ↕️ Operators in Verilog\n",
    "\n",
    "</summary>\n",
    "    \n",
    "```verilog\n",
    "// Addition, substraction, negation\n",
    "assign sum = op_a + op_b; assign sub = op_a + op_b; assign opp = -op_a\n",
    "// Multiplication, division, modulo\n",
    "assign prod = op_a * op_b; assign div = op_a / op_b; assign rem = op_a & op_b\n",
    "    \n",
    "// Bitwise not, or, and, xor\n",
    "assign n = ~m; assign a = b | c; assign d = e & f; assign x = y ^ z\n",
    "\n",
    "// Logical not, and, or\n",
    "assign ans = !v; assign ans = v || w; assign ans = v && w;\n",
    "// Logical equality, difference\n",
    "assign ans = v == w; assign ans = v != w;\n",
    "// Relations (strictly) greater, (strictly) lower than\n",
    "assign sg = a > b; assign gt = a >= b; assign sl = a < b; assign lt = a <= b;\n",
    "    \n",
    "// Left, right shift by n bits\n",
    "assign l << n; assign r >> n;\n",
    "// Left, right arithmetic shift by n bits\n",
    "assign l <<< n; assign r >>> n;\n",
    "```\n",
    "    \n",
    "</details></blockquote>"
   ]
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   "source": [
    "%%writefile v/xor3.v\n",
    "module xor3(\n",
    "    input wire a,\n",
    "    input wire b,\n",
    "    input wire c,\n",
    "    output wire out\n",
    ");\n",
    "    assign out = a ^ b ^ c;\n",
    "endmodule"
   ]
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   "source": [
    "### Configuration file (JSON)\n",
    "\n",
    "A configuration file should be provided. It describes constraints and strategies applied during synthesis and implementation of the circuit."
   ]
  },
  {
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   "source": [
    "%%writefile build/config.json\n",
    "{\n",
    "    \"DESIGN_NAME\": \"xor3\",\n",
    "    \"VERILOG_FILES\": \"dir::../v/xor3.v\",\n",
    "    \"RUN_CTS\": false,\n",
    "    \"CLOCK_PORT\": null,\n",
    "    \"FP_SIZING\": \"absolute\",\n",
    "    \"DIE_AREA\": \"0 0 35 45\",\n",
    "    \"FP_PDN_AUTO_ADJUST\": false,\n",
    "    \"FP_PDN_VOFFSET\": 0,\n",
    "    \"FP_PDN_HOFFSET\": 0,\n",
    "    \"DIODE_INSERTION_STRATEGY\": 3\n",
    "}"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Workflow\n",
    "The provided `flow.tcl` is a script describing the OpenROAD workflow. A `.gds` file will be generated using the RTL circuit description, the PDK and the configuration file."
   ]
  },
  {
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   "source": [
    "%env PDK=sky130A\n",
    "!flow.tcl -design build"
   ]
  },
  {
   "cell_type": "markdown",
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    "id": "luguFgZ43AeL"
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   "source": [
    "## Output products\n",
    "\n",
    "### Display layout\n",
    "\n",
    "The implemented layout can be retrieved as follows:"
   ]
  },
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   "source": [
    "import glob\n",
    "import gdstk\n",
    "import IPython.display\n",
    "\n",
    "gdsii = sorted(glob.glob(\"./build/runs/*/results/final/gds/*.gds\"))[-1]\n",
    "top = gdstk.read_gds(gdsii).top_level()\n",
    "top[0].write_svg('svg/inverter.svg')\n",
    "IPython.display.SVG('svg/inverter.svg')"
   ]
  },
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   "cell_type": "markdown",
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   "source": [
    "### Reporting\n",
    "\n",
    "Extensive reports are available under:\n",
    "\n",
    "```\n",
    "freeechips/semicustom/runs/RUN_YYYY.MM.DD_HH.MM.SS/reports/.\n",
    "```\n",
    "\n",
    "An overview of the main figures can be retrieved as well:"
   ]
  },
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   "source": [
    "import glob\n",
    "import pandas as pd\n",
    "pd.options.display.max_rows = None\n",
    "\n",
    "pd.read_csv(sorted(glob.glob(\"./build/runs/*/reports/metrics.csv\"))[-1]).T"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# To have fun going further...\n",
    "\n",
    "## Sequential circuits in Verilog\n",
    "\n",
    "We focused on combinational circuits. Sequential circuits can obviously be described in Verilog as well. Sequential blocks feature an `always` block. Refer to the vendor's reference to get the sensitivity list's syntax (e.g. to determine if reset is synchronous or asynchronous).\n",
    "\n",
    "<blockquote><details>\n",
    "\n",
    "<summary>\n",
    "    \n",
    "#### ↕️ Sequential structures in Verilog\n",
    "\n",
    "</summary>\n",
    "\n",
    "Nets which store data are declared as `reg`s:\n",
    "    \n",
    "```verilog\n",
    "// Three scalar register\n",
    "reg op_b, op_a, result;\n",
    "// One 16-bit register\n",
    "reg [15:0] word_bus;\n",
    "// 1K-array of 8-bit registers\n",
    "reg [7:0] byte_array [0:1023];\n",
    "```\n",
    "\n",
    "The following structure updates the memory points:\n",
    "    \n",
    "```verilog\n",
    "always @(posedge clk or negedge rst) begin\n",
    "  if (!rst) begin \n",
    "    counter <= 0;\n",
    "  end else begin\n",
    "    counter <= counter + 1;\n",
    "  end\n",
    "end\n",
    "```\n",
    "    \n",
    "</details></blockquote>\n",
    "\n",
    "Advanced structures like the `case` structure can be used to describe finite state machines. FSM decoders can be described in an abstract way using `always` blocks in a fully combinational way:\n",
    "\n",
    "<blockquote><details>\n",
    "\n",
    "<summary>\n",
    "    \n",
    "#### ↕️ Advanced structures in Verilog\n",
    "\n",
    "</summary>\n",
    "\n",
    "```verilog\n",
    "    \n",
    "/*\n",
    " *   Case structure\n",
    " */\n",
    "    \n",
    "case (state)\n",
    "\n",
    "  3'b000: idle_led <= 1'b1;\n",
    "  \n",
    "  3'b001,\n",
    "  3'b010: work_led <= 1'b1;\n",
    "  \n",
    "  3'b011: begin\n",
    "    muxed <= spi_1;\n",
    "    work_led <= 1'b1;\n",
    "  end\n",
    "  \n",
    "  default: begin\n",
    "    muxed <= 0;\n",
    "    work_led <= 1'b0; \n",
    "    idle_led <= 1'b1;\n",
    "  end\n",
    "\n",
    "endcase\n",
    "\n",
    "/*\n",
    " *   Combinational always structure\n",
    " *     To describe priorities in a procedural fashion,\n",
    " *     use blocking `<=` assignations instead of\n",
    " *     non-blocking `=` assignations.\n",
    " */\n",
    "                \n",
    "always @( * ) begin\n",
    "  flag <= 1'b0;\n",
    "  if (error) begin \n",
    "    flag <= 1'b1;\n",
    "  end\n",
    "end\n",
    "```\n",
    "\n",
    "</details></blockquote>\n",
    "    \n",
    "The counter example can be synthetized:"
   ]
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   "source": [
    "%%writefile v/cnt.v\n",
    "module cnt(\n",
    "    input wire clk,\n",
    "    input wire rst,\n",
    "    output wire [7:0] count\n",
    ");\n",
    "\n",
    "  reg [7:0] counter;\n",
    "  assign count = counter;\n",
    "\n",
    "  always @(posedge clk or negedge rst) begin\n",
    "    if (!rst) begin \n",
    "      counter <= 0;\n",
    "    end else begin\n",
    "      counter <= counter + 1;\n",
    "    end\n",
    "  end\n",
    "\n",
    "endmodule"
   ]
  },
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   "metadata": {},
   "source": [
    "For sequential circuits, one must specify to synthesize the clock tree as well:"
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "%%writefile build/config.json\n",
    "{\n",
    "    \"DESIGN_NAME\": \"cnt\",\n",
    "    \"VERILOG_FILES\": \"dir::../v/cnt.v\",\n",
    "    \"RUN_CTS\": true,\n",
    "    \"CLOCK_PORT\": \"clk\",\n",
    "    \"FP_SIZING\": \"absolute\",\n",
    "    \"DIE_AREA\": \"0 0 40 50\",\n",
    "    \"FP_PDN_AUTO_ADJUST\": false,\n",
    "    \"FP_PDN_VOFFSET\": 0,\n",
    "    \"FP_PDN_HOFFSET\": 0,\n",
    "    \"DIODE_INSERTION_STRATEGY\": 3\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The flow can be started:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true
   },
   "outputs": [],
   "source": [
    "%env PDK=sky130A\n",
    "!flow.tcl -design build"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The cell above can be reused to display the lyaout."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## High-Level Synthesis (HLS)\n",
    "\n",
    "RTL description of circuits does not follow an imperative programming paradigm. It is a description language that produces highly parallelized designs.\n",
    "\n",
    "High-Level Synthesis provides an imperative language and a compiler that synthesizes the imperative instructions into RTL. For instance, _XLS_ provides a _Rust_-like language:"
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {},
   "outputs": [],
   "source": [
    "%%writefile xls/x/find_max.x\n",
    "\n",
    "//\n",
    "// Input:  an array of 32-bit unsigned integers (u32) of parametrized length N\n",
    "// Output: the largest element of the array\n",
    "//\n",
    "pub fn find_max<N: u32>(array: u32[N]) -> u32 {\n",
    "  let max: u32 = u32:0;\n",
    "  for (i, max): (u32, u32) in range(u32:0,N) {\n",
    "    if (array[i] > max) {array[i]} else {max}\n",
    "  }(max)\n",
    "}\n",
    "\n",
    "//\n",
    "// Input:  an array of 32-bit unsigned integers (u32) of parametrized length 4\n",
    "// Output: the largest element of the array\n",
    "//\n",
    "pub fn find_max_impl(array: u32[4]) -> u32 {\n",
    "  find_max<u32:4>(array)\n",
    "}\n",
    "    \n",
    "#[test]\n",
    "fn find_max_impl_test() {\n",
    "  assert_eq(find_max_impl(u32[4]:[45,3,15,6]), u32:45);\n",
    "  assert_eq(find_max_impl(u32[4]:[3,45,15,6]), u32:45);\n",
    "  assert_eq(find_max_impl(u32[4]:[15,3,45,6]), u32:45);\n",
    "  assert_eq(find_max_impl(u32[4]:[6,3,15,45]), u32:45);\n",
    "}\n",
    "\n",
    "#[test]\n",
    "fn find_max_test() {\n",
    "  assert_eq(find_max<u32:1>(u32[1]:[39]), u32:39);\n",
    "  assert_eq(find_max<u32:2>(u32[2]:[4,90]), u32:90);\n",
    "  assert_eq(find_max<u32:3>(u32[3]:[7,21,15]), u32:21);\n",
    "  assert_eq(find_max<u32:8>(u32[8]:[1,3,45,1,5,56,0,34]), u32:56);\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Testing, parsing and linting can be performed prior to RTL synhesis:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true
   },
   "outputs": [],
   "source": [
    "!interpreter_main xls/x/find_max.x"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now that the imperative instructions are tested, the RTL design can be synthesized by _XLS_:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "XLS_DESIGN_NAME = 'find_max_impl'\n",
    "XLS_DESIGN_FILE = 'find_max'\n",
    "!ir_converter_main --top={XLS_DESIGN_NAME} xls/x/{XLS_DESIGN_FILE}.x > xls/ir/{XLS_DESIGN_FILE}.ir\n",
    "!opt_main xls/ir/{XLS_DESIGN_FILE}.ir > xls/ir/{XLS_DESIGN_FILE}_opt.ir\n",
    "!codegen_main --generator=combinational xls/ir/{XLS_DESIGN_FILE}_opt.ir > v/{XLS_DESIGN_FILE}.v\n",
    "!cat v/{XLS_DESIGN_FILE}.v"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Verilog simulation\n",
    "\n",
    "A test bench can be written to simulate the `xor3` circuit described above:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
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   "source": [
    "%%writefile tb/xor3_tb.v\n",
    "module xor3_tb;\n",
    "\n",
    "  wire value;\n",
    "  reg w1, w2, w3;\n",
    "  initial begin\n",
    "    $dumpfile(\"tb/xor3_tb.vcd\");\n",
    "    $dumpvars(0,xor3_tb);\n",
    "    # 0 w1 = 0; w2 = 0; w3 = 0;\n",
    "    # 5 w1 = 0; w2 = 0; w3 = 1;\n",
    "    # 5 w1 = 0; w2 = 1; w3 = 0;\n",
    "    # 5 w1 = 0; w2 = 1; w3 = 1;\n",
    "    # 5 w1 = 1; w2 = 0; w3 = 0;\n",
    "    # 5 w1 = 1; w2 = 0; w3 = 1;\n",
    "    # 5 w1 = 1; w2 = 1; w3 = 0;\n",
    "    # 5 w1 = 1; w2 = 1; w3 = 1;\n",
    "    # 5 $finish;\n",
    "  end\n",
    "\n",
    "  xor3 xor3_i (w1, w2, w3, value);\n",
    "\n",
    "  initial\n",
    "     $monitor(\"At time %t, value = %h (%0d)\",\n",
    "              $time, value, value);\n",
    "endmodule"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The simulation can then be performed using _Icarus Verilog_:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true
   },
   "outputs": [],
   "source": [
    "!iverilog -o tb/xor3_tb tb/xor3_tb.v v/xor3.v\n",
    "!vvp tb/xor3_tb"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The waveforms are dumped in a `.vcd` file, located under:\n",
    "\n",
    "```\n",
    "tb/xor3_tb.vcd\n",
    "```\n",
    "\n",
    "It can be opened with https://vc.drom.io/ for instance."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Some ideas...\n",
    "\n",
    "Here are a couple ideas to spend a good time exploring those beautiful pieces of software:\n",
    "\n",
    " * Play with RTL2GDS configuration parameters and observe the impact on the layout.\n",
    " * Simulate the `cnt` sequential circuit.\n",
    " * Simulate the `find_max_impl` circuit synthesized using HLS.\n",
    " * Try to make _Klayout_ GUI work (no warranty).\n",
    "\n",
    " **Do not start from scratch! Use the provided examples, examples you can find on the internet and the documentation to adapt from them!**\n",
    " \n",
    " > Good luck and read the docs. 😉\n",
    " \n",
    "## More food for the brain\n",
    "\n",
    " * GDS2RTL flow configuration parameters: https://armleo-openlane.readthedocs.io/en/latest/docs/source/configuration.html\n",
    " * DSLX language reference: https://google.github.io/xls/dslx_reference/"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# References\n",
    "Inspired from:\n",
    "“Silicon Notebooks.” CHIPS Alliance, Apr. 08, 2023. Accessed: Apr. 10, 2023. [Online]. Available: https://github.com/chipsalliance/silicon-notebooks/blob/b65134a43b01ae31423f7ee87110740b2257ac42/digital-inverter-openlane.ipynb (Apache License 2.0)"
   ]
  }
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