usb15: make USB support optional

setup.py

@@ -29,6 +29,9 @@ setup(
     version = stcgal.__version__,
     packages = find_packages(exclude=["doc"]),
     install_requires = ["pyserial"],
+    extras_require = {
+        "usb": ["pyusb>=1.0.0"]
+    },
     entry_points = {
         "console_scripts": [
             "stcgal = stcgal.frontend:cli",

stcgal/protocols.py

@@ -26,9 +26,15 @@ import argparse
 import collections
 from stcgal.models import MCUModelDatabase
 from stcgal.utils import Utils
-import usb, usb.core, usb.util
 import functools
 
+try:
+    import usb.core, usb.util
+    _usb_available = True
+except ImportError:
+    _usb_available = False
+
+
 class StcFramingException(Exception):
     """Something wrong with packet framing or checksum"""
     pass
@@ -2048,6 +2054,11 @@ class StcUsb15Protocol(Stc15Protocol):
     def connect(self, autoreset=False):
         """Connect to USB device and read info packet"""
 
+        # USB support is optional. Provide an error if pyusb is not available.
+        if _usb_available == False:
+            raise StcProtocolException("USB support not available. "
+                + "pyusb is not installed or not working correctly.")
+
         print("Waiting for MCU, please cycle power: ", end="")
         sys.stdout.flush()