Windows time accuracy (#936)

* fixes #732: add support for VN8900 xlGetChannelTime function
* improve time stamp accuracy of the CAN messages for Kvaser and Vector buses on the Windows platform. (see #934)
On Windows the value returned by time.time is refreshed at best every 0.5ms, and on some PCs the default refresh rate is every 10ms. The idea is to get the value of time.perf_counter at the moment when time.time is refreshed and use this as a reference point.
* check the time.time resolution before using the new time correlation function

Author: danielhrisca

can/interfaces/kvaser/canlib.py

@@ -13,6 +13,7 @@
 
 from can import CanError, BusABC
 from can import Message
+from can.util import time_perfcounter_correlation
 from . import constants as canstat
 from . import structures
 
@@ -492,11 +493,20 @@ def __init__(self, channel, can_filters=None, **kwargs):
 
         timer = ctypes.c_uint(0)
         try:
-            kvReadTimer(self._read_handle, ctypes.byref(timer))
+            if time.get_clock_info("time").resolution > 1e-5:
+                ts, perfcounter = time_perfcounter_correlation()
+                kvReadTimer(self._read_handle, ctypes.byref(timer))
+                current_perfcounter = time.perf_counter()
+                now = ts + (current_perfcounter - perfcounter)
+                self._timestamp_offset = now - (timer.value * TIMESTAMP_FACTOR)
+            else:
+                kvReadTimer(self._read_handle, ctypes.byref(timer))
+                self._timestamp_offset = time.time() - (timer.value * TIMESTAMP_FACTOR)
+
         except Exception as exc:
             # timer is usually close to 0
             log.info(str(exc))
-        self._timestamp_offset = time.time() - (timer.value * TIMESTAMP_FACTOR)
+            self._timestamp_offset = time.time() - (timer.value * TIMESTAMP_FACTOR)
 
         self._is_filtered = False
         super().__init__(channel=channel, can_filters=can_filters, **kwargs)

can/interfaces/vector/canlib.py

@@ -32,7 +32,12 @@
 # Import Modules
 # ==============
 from can import BusABC, Message
-from can.util import len2dlc, dlc2len, deprecated_args_alias
+from can.util import (
+    len2dlc,
+    dlc2len,
+    deprecated_args_alias,
+    time_perfcounter_correlation,
+)
 from .exceptions import VectorError
 
 # Define Module Logger
@@ -295,11 +300,22 @@ def __init__(
         # Calculate time offset for absolute timestamps
         offset = xlclass.XLuint64()
         try:
-            try:
-                xldriver.xlGetSyncTime(self.port_handle, offset)
-            except VectorError:
-                xldriver.xlGetChannelTime(self.port_handle, self.mask, offset)
-            self._time_offset = time.time() - offset.value * 1e-9
+            if time.get_clock_info("time").resolution > 1e-5:
+                ts, perfcounter = time_perfcounter_correlation()
+                try:
+                    xldriver.xlGetSyncTime(self.port_handle, offset)
+                except VectorError:
+                    xldriver.xlGetChannelTime(self.port_handle, self.mask, offset)
+                current_perfcounter = time.perf_counter()
+                now = ts + (current_perfcounter - perfcounter)
+                self._time_offset = now - offset.value * 1e-9
+            else:
+                try:
+                    xldriver.xlGetSyncTime(self.port_handle, offset)
+                except VectorError:
+                    xldriver.xlGetChannelTime(self.port_handle, self.mask, offset)
+                self._time_offset = time.time() - offset.value * 1e-9
+
         except VectorError:
             self._time_offset = 0.0
 

can/util.py

@@ -4,6 +4,7 @@
 import functools
 import warnings
 from typing import Dict, Optional, Union
+from time import time, perf_counter, get_clock_info
 
 from can import typechecking
 
@@ -326,6 +327,35 @@ def rename_kwargs(func_name, kwargs, aliases):
                 warnings.warn("{} is deprecated".format(alias), DeprecationWarning)
 
 
+def time_perfcounter_correlation():
+    """Get the `perf_counter` value nearest to when time.time() is updated
+
+    Computed if the default timer used by `time.time` on this platform has a resolution
+    higher than 10μs, otherwise the current time and perf_counter is directly returned.
+    This was chosen as typical timer resolution on Linux/macOS is ~1μs, and the Windows
+    platform can vary from ~500μs to 10ms.
+
+    Note this value is based on when `time.time()` is observed to update from Python,
+    it is not directly returned by the operating system.
+
+    :returns:
+        (t, performance_counter) time.time value and time.perf_counter value when the time.time
+        is updated
+
+    """
+
+    # use this if the resolution is higher than 10us
+    if get_clock_info("time").resolution > 1e-5:
+        t0 = time()
+        while True:
+            t1, performance_counter = time(), perf_counter()
+            if t1 != t0:
+                break
+    else:
+        return time(), perf_counter()
+    return t1, performance_counter
+
+
 if __name__ == "__main__":
     print("Searching for configuration named:")
     print("\n".join(CONFIG_FILES))