diff --git a/XSPICE_CODE_MODELS.md b/XSPICE_CODE_MODELS.md
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+# Writing XSPICE Code Models for pyTesla
+
+## What This Is
+
+XSPICE code models let you write circuit components in C. During simulation,
+ngspice calls your C function at each timestep with port voltages/states and
+you compute the outputs. This is how we'll add custom parts (gate drivers,
+controllers, digital logic) to the pyTesla part library.
+
+## How a Code Model Looks in a Netlist
+
+```spice
+* Analog component using a code model
+A1 in out mymodel
+.model mymodel custom_model(gain=2.0 offset=0.1)
+
+* Digital component with bridge to analog world
+A2 [digital_in] [digital_out] my_gate
+.model my_gate d_and(rise_delay=1n fall_delay=1n)
+
+* Analog-to-digital bridge (connects analog node to digital code model)
+A3 [analog_node] [digital_node] adc1
+.model adc1 adc_bridge(in_low=0.8 in_high=2.0)
+```
+
+The `A` device letter tells ngspice this is an XSPICE code model instance.
+
+## File Structure for a New Code Model
+
+Each code model needs two files in `src/xspice/icm/<category>/<model_name>/`:
+
+### 1. `ifspec.ifs` — Interface Specification
+
+Declares the model's name, ports, and parameters. Not C — it's a simple
+declarative format that the `cmpp` preprocessor converts to C.
+
+```
+NAME_TABLE:
+C_Function_Name:    cm_my_driver
+Spice_Model_Name:   my_driver
+Description:        "Half-bridge gate driver model"
+
+PORT_TABLE:
+Port_Name:      hin         lin         ho          lo
+Description:    "high in"   "low in"    "high out"  "low out"
+Direction:      in          in          out         out
+Default_Type:   d           d           v           v
+Allowed_Types:  [d]         [d]         [v]         [v]
+Vector:         no          no          no          no
+Vector_Bounds:  -           -           -           -
+Null_Allowed:   no          no          no          no
+
+PARAMETER_TABLE:
+Parameter_Name:     vcc             dead_time
+Description:        "supply volts"  "dead time seconds"
+Data_Type:          real            real
+Default_Value:      12.0            100e-9
+Limits:             [0.1 1000]      [0 -]
+Vector:             no              no
+Vector_Bounds:      -               -
+Null_Allowed:       yes             yes
+```
+
+### 2. `cfunc.mod` — C Implementation
+
+Plain C with XSPICE macros for accessing ports/parameters. This is where
+your component's behavior goes.
+
+```c
+#include <math.h>
+
+void cm_my_driver(ARGS)
+{
+    /* Read parameters */
+    double vcc       = PARAM(vcc);
+    double dead_time = PARAM(dead_time);
+
+    /* Read digital input states */
+    Digital_State_t hin = INPUT_STATE(hin);
+    Digital_State_t lin = INPUT_STATE(lin);
+
+    /* Compute outputs */
+    if (INIT) {
+        /* First call — initialize state */
+        OUTPUT(ho) = 0.0;
+        OUTPUT(lo) = 0.0;
+    } else {
+        /* Normal operation */
+        if (hin == ONE)
+            OUTPUT(ho) = vcc;
+        else
+            OUTPUT(ho) = 0.0;
+
+        if (lin == ONE)
+            OUTPUT(lo) = vcc;
+        else
+            OUTPUT(lo) = 0.0;
+    }
+
+    /* Set output delay */
+    OUTPUT_DELAY(ho) = dead_time;
+    OUTPUT_DELAY(lo) = dead_time;
+}
+```
+
+## Key XSPICE Macros
+
+| Macro | Purpose |
+|-------|---------|
+| `ARGS` | Function signature placeholder (always use in the function declaration) |
+| `INIT` | True on first call — use for initialization |
+| `PARAM(name)` | Read a model parameter value |
+| `INPUT(name)` | Read analog input voltage/current |
+| `INPUT_STATE(name)` | Read digital input (ONE, ZERO, UNKNOWN) |
+| `OUTPUT(name)` | Set analog output value |
+| `OUTPUT_STATE(name)` | Set digital output state |
+| `OUTPUT_DELAY(name)` | Set propagation delay for output |
+| `OUTPUT_STRENGTH(name)` | Set digital output drive strength |
+| `PORT_SIZE(name)` | Number of elements in a vector port |
+| `PARTIAL(out, in)` | Set partial derivative (for analog convergence) |
+| `STATIC_VAR(name)` | Access persistent state between timesteps |
+| `T(n)` | Current time at call (n=0) or previous calls (n=1,2) |
+| `cm_event_alloc(tag, size)` | Allocate event-driven state memory |
+| `cm_event_get_ptr(tag, n)` | Get state pointer (n=0 current, n=1 previous) |
+
+## Port Types
+
+- **v** — analog voltage
+- **i** — analog current
+- **vd** — analog voltage differential
+- **id** — analog current differential
+- **d** — digital (ONE, ZERO, UNKNOWN)
+
+## Analog-Digital Bridges
+
+To connect analog and digital ports, use bridge models in the netlist:
+
+```spice
+* Analog voltage -> Digital state
+A_bridge1 [v_node] [d_node] adc1
+.model adc1 adc_bridge(in_low=0.8 in_high=2.0)
+
+* Digital state -> Analog voltage
+A_bridge2 [d_node] [v_node] dac1
+.model dac1 dac_bridge(out_low=0.0 out_high=3.3)
+```
+
+## Verilog Co-Simulation (Future)
+
+Once code models work, Verilog support follows:
+
+1. Write your FPGA design in Verilog
+2. Compile with Verilator: `verilator --cc design.v --exe`
+3. Build the output into a shared library (.dll)
+4. Use `d_cosim` in the netlist:
+
+```spice
+A1 [clk din] [dout pwm] cosim1
+.model cosim1 d_cosim(simulation="my_fpga_design.dll")
+```
+
+ngspice loads the DLL and runs the compiled Verilog alongside the analog sim.
+
+## Directory Layout for pyTesla Parts
+
+```
+src/xspice/icm/
+├── analog/          # Built-in analog models (gain, limit, etc.)
+├── digital/         # Built-in digital models (d_and, d_cosim, etc.)
+├── pytesla/         # Our custom part library (new category)
+│   ├── modpath.lst  # List of model directories
+│   ├── gate_driver/
+│   │   ├── cfunc.mod
+│   │   └── ifspec.ifs
+│   ├── spark_gap/
+│   │   ├── cfunc.mod
+│   │   └── ifspec.ifs
+│   └── ...
+```
+
+## Build Process
+
+After adding a new code model:
+
+1. Place `cfunc.mod` and `ifspec.ifs` in the appropriate directory
+2. Add the model name to `modpath.lst`
+3. Rebuild: `python build_mingw.py`
+
+The CMake build will run `cmpp` to preprocess the files, compile
+the generated C, and register the model with ngspice automatically.
+
+(Note: the ICM build step in CMakeLists.txt is not yet implemented —
+this document describes the target workflow once it is.)
diff --git a/build_cmpp.py b/build_cmpp.py
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+"""Build the cmpp preprocessor and compile XSPICE .cm code model libraries."""
+import subprocess
+import os
+import sys
+import shutil
+
+MINGW_BIN = r"C:\mingw64\bin"
+NGSPICE_ROOT = r"C:\git\ngspice"
+CMPP_SRC = os.path.join(NGSPICE_ROOT, "src", "xspice", "cmpp")
+ICM_DIR = os.path.join(NGSPICE_ROOT, "src", "xspice", "icm")
+INCLUDE_DIR = os.path.join(NGSPICE_ROOT, "src", "include")
+CONFIG_H_DIR = os.path.join(NGSPICE_ROOT, "visualc", "src", "include")
+BUILD_DIR = os.path.join(NGSPICE_ROOT, "build", "cmpp_build")
+CM_OUTPUT_DIR = os.path.join(NGSPICE_ROOT, "pyngspice", "codemodels")
+CMPP_EXE = os.path.join(BUILD_DIR, "cmpp.exe")
+
+def get_env():
+    env = os.environ.copy()
+    env["PATH"] = MINGW_BIN + ";" + env["PATH"]
+    return env
+
+def run(cmd, cwd=None):
+    print(f"  > {' '.join(cmd)}")
+    r = subprocess.run(cmd, capture_output=True, text=True, cwd=cwd, env=get_env())
+    if r.stdout.strip():
+        for line in r.stdout.strip().split('\n')[:20]:
+            print(f"    {line}")
+    if r.stderr.strip():
+        lines = r.stderr.strip().split('\n')
+        # Show first 10 and last 30 lines of errors
+        if len(lines) > 40:
+            for line in lines[:5]:
+                print(f"    {line}")
+            print(f"    ... ({len(lines) - 35} lines omitted) ...")
+            for line in lines[-30:]:
+                print(f"    {line}")
+        else:
+            for line in lines:
+                print(f"    {line}")
+    if r.returncode != 0:
+        print(f"  FAILED (rc={r.returncode})")
+        return False
+    return True
+
+def build_cmpp():
+    """Build the cmpp preprocessor tool."""
+    print("=== Step 1: Building cmpp preprocessor ===")
+    os.makedirs(BUILD_DIR, exist_ok=True)
+
+    if os.path.exists(CMPP_EXE):
+        print(f"  cmpp.exe already exists, skipping")
+        return True
+
+    cmpp_sources = [
+        "main.c", "file_buffer.c", "pp_ifs.c", "pp_lst.c", "pp_mod.c",
+        "read_ifs.c", "util.c", "writ_ifs.c", "ifs_yacc.c", "mod_yacc.c",
+        "ifs_lex.c", "mod_lex.c"
+    ]
+
+    cmd = [
+        "gcc", "-o", CMPP_EXE,
+    ] + [os.path.join(CMPP_SRC, f) for f in cmpp_sources] + [
+        f"-I{CMPP_SRC}", f"-I{INCLUDE_DIR}",
+        "-lshlwapi"
+    ]
+
+    if not run(cmd):
+        return False
+
+    print(f"  OK: {CMPP_EXE}")
+    return True
+
+def preprocess_category(category):
+    """Run cmpp on a code model category (analog, digital, etc.)."""
+    print(f"\n=== Step 2: Preprocessing '{category}' code models ===")
+    src_dir = os.path.join(ICM_DIR, category)
+    out_dir = os.path.join(BUILD_DIR, category)
+
+    # Copy the category directory to build dir for cmpp to work in
+    if os.path.exists(out_dir):
+        shutil.rmtree(out_dir)
+    shutil.copytree(src_dir, out_dir)
+
+    # Step 2a: Generate registration headers from modpath.lst
+    print(f"  Generating registration headers...")
+    env = get_env()
+    env["CMPP_IDIR"] = src_dir
+    env["CMPP_ODIR"] = out_dir
+    r = subprocess.run([CMPP_EXE, "-lst"], capture_output=True, text=True,
+                       cwd=out_dir, env=env)
+    if r.returncode != 0:
+        print(f"  cmpp -lst FAILED: {r.stderr}")
+        return None
+    print(f"  OK: cminfo.h, cmextrn.h generated")
+
+    # Step 2b: Read modpath.lst to get model names
+    modpath = os.path.join(src_dir, "modpath.lst")
+    with open(modpath) as f:
+        models = [line.strip() for line in f if line.strip()]
+
+    # Step 2c: Preprocess each model's .ifs and .mod files
+    for model in models:
+        model_src = os.path.join(src_dir, model)
+        model_out = os.path.join(out_dir, model)
+
+        if not os.path.isdir(model_src):
+            print(f"  SKIP: {model} (directory not found)")
+            continue
+
+        # Preprocess ifspec.ifs -> ifspec.c
+        env_mod = get_env()
+        env_mod["CMPP_IDIR"] = model_src
+        env_mod["CMPP_ODIR"] = model_out
+        r = subprocess.run([CMPP_EXE, "-ifs"], capture_output=True, text=True,
+                           cwd=model_out, env=env_mod)
+        if r.returncode != 0:
+            print(f"  FAIL: {model}/ifspec.ifs: {r.stderr}")
+            return None
+
+        # Preprocess cfunc.mod -> cfunc.c
+        r = subprocess.run([CMPP_EXE, "-mod"], capture_output=True, text=True,
+                           cwd=model_out, env=env_mod)
+        if r.returncode != 0:
+            print(f"  FAIL: {model}/cfunc.mod: {r.stderr}")
+            return None
+
+    print(f"  OK: {len(models)} models preprocessed")
+    return models
+
+# Category-specific extra source files and include paths
+CATEGORY_EXTRAS = {
+    "tlines": {
+        "extra_sources": [
+            os.path.join(NGSPICE_ROOT, "src", "xspice", "tlines", "tline_common.c"),
+            os.path.join(NGSPICE_ROOT, "src", "xspice", "tlines", "msline_common.c"),
+        ],
+        "extra_includes": [
+            os.path.join(NGSPICE_ROOT, "src", "xspice", "tlines"),
+        ],
+    },
+}
+
+def compile_cm(category, models):
+    """Compile preprocessed code models into a .cm DLL."""
+    print(f"\n=== Step 3: Compiling '{category}.cm' DLL ===")
+    src_dir = os.path.join(ICM_DIR, category)
+    out_dir = os.path.join(BUILD_DIR, category)
+    os.makedirs(CM_OUTPUT_DIR, exist_ok=True)
+
+    # Collect all .c files to compile
+    c_files = []
+
+    # dlmain.c (the DLL entry point with exports)
+    dlmain = os.path.join(ICM_DIR, "dlmain.c")
+    c_files.append(dlmain)
+
+    # Each model's cfunc.c and ifspec.c
+    for model in models:
+        model_dir = os.path.join(out_dir, model)
+        cfunc_c = os.path.join(model_dir, "cfunc.c")
+        ifspec_c = os.path.join(model_dir, "ifspec.c")
+        if os.path.exists(cfunc_c) and os.path.exists(ifspec_c):
+            c_files.append(cfunc_c)
+            c_files.append(ifspec_c)
+        else:
+            print(f"  SKIP: {model} (generated .c files missing)")
+
+    # UDN (user-defined node) types — udnfunc.c files from udnpath.lst
+    udnpath = os.path.join(src_dir, "udnpath.lst")
+    if os.path.exists(udnpath):
+        with open(udnpath) as f:
+            udns = [line.strip() for line in f if line.strip()]
+        for udn in udns:
+            udnfunc = os.path.join(src_dir, udn, "udnfunc.c")
+            if os.path.exists(udnfunc):
+                c_files.append(udnfunc)
+                print(f"  UDN: {udn}/udnfunc.c")
+
+    # Also need dstring.c for some models that use DS_CREATE
+    dstring_c = os.path.join(NGSPICE_ROOT, "src", "misc", "dstring.c")
+    if os.path.exists(dstring_c):
+        c_files.append(dstring_c)
+
+    # Category-specific extra sources
+    extras = CATEGORY_EXTRAS.get(category, {})
+    for src in extras.get("extra_sources", []):
+        if os.path.exists(src):
+            c_files.append(src)
+            print(f"  EXTRA: {os.path.basename(src)}")
+
+    cm_file = os.path.join(CM_OUTPUT_DIR, f"{category}.cm")
+
+    cmd = [
+        "gcc", "-shared", "-o", cm_file,
+    ] + c_files + [
+        f"-I{INCLUDE_DIR}",
+        f"-I{CONFIG_H_DIR}",
+        f"-I{out_dir}",           # For cminfo.h, cmextrn.h
+        f"-I{ICM_DIR}",           # For dlmain.c includes
+    ]
+
+    # Category-specific extra include paths
+    for inc in extras.get("extra_includes", []):
+        cmd.append(f"-I{inc}")
+
+    cmd += [
+        "-DHAS_PROGREP",
+        "-DXSPICE",
+        "-DSIMULATOR",
+        "-DNG_SHARED_BUILD",
+        "-DNGSPICEDLL",
+        "-Wall", "-Wno-unused-variable", "-Wno-unused-function",
+        "-Wno-sign-compare", "-Wno-maybe-uninitialized",
+        "-Wno-implicit-function-declaration",  # GCC 15 makes this an error; some models missing stdlib.h
+    ]
+
+    if not run(cmd):
+        return False
+
+    size_kb = os.path.getsize(cm_file) // 1024
+    print(f"  OK: {cm_file} ({size_kb} KB)")
+    return True
+
+def build_category(category):
+    """Full pipeline: preprocess + compile a code model category."""
+    models = preprocess_category(category)
+    if models is None:
+        return False
+    return compile_cm(category, models)
+
+if __name__ == "__main__":
+    categories = sys.argv[1:] if len(sys.argv) > 1 else ["analog"]
+
+    if not build_cmpp():
+        sys.exit(1)
+
+    for cat in categories:
+        if not build_category(cat):
+            print(f"\nFailed to build {cat}.cm")
+            sys.exit(1)
+
+    print(f"\n=== Done! Code models in: {CM_OUTPUT_DIR} ===")
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