Correct two howling errors in the capture logic – how did I not notice these before?

Fixed small problem when serial_number is not specified.

README.md

@@ -8,3 +8,58 @@
 - Also requires **NumPy** and **SciPy** if you want to do analog range calibration
 - Having **Pandas** is useful for wrapping capture results for further processing
 
+## Longer Notes
+
+### Why have I written this?
+
+BitScope helpfully provide applications and libraries for talking to their USB
+capture devices, so one can just use those. I wrote this code because I want
+to be able to grab the raw data and further process it in various ways. I'm
+accustomed to working at a Python prompt with various toolkits like SciPy and
+Matplotlib, so I wanted a simple way to just grab a trace as an array.
+
+The BitScope library is pretty simple to use, but also requires you to
+understand a fair amount about how the scope works and make a bunch of decisions
+about what capture mode and rate to use. I want to just specify a time period,
+voltage range and a rough number of samples and have all of that worked out for
+me, same way as I'd use an actual oscilloscope: twiddle the knobs and look at
+the trace.
+
+Of course, I could have wrapped the BitScope library in something that would do
+this, but after reading a bit about how the scope works I was fascinated with
+understanding it further and so I decided to go back to first principles and
+start with just talking to it with a serial library. This code thus serves as
+(sort of) documentation for the VM registers, capture modes and how to use them.
+It also has the advantage of being pure Python.
+
+The code prefers the highest capture resolution possible and will do the mapping
+from high/low/trigger voltages to the mysterious magic numbers that the device
+needs. It can also do logic and mixed-signal capture.
+
+In addition to capturing, the code can also generate waveforms at arbitrary
+frequencies – something that is tricky to do as the device operates at specific
+frequencies and so one has to massage the width of the waveform buffer to get a
+frequency outside of these. It can also control the clock generator.
+
+I've gone for an underlying async design as it makes it easy to integrate the
+code into UI programs or network servers – both of which interest me as the end
+purpose for this code. However, for shell use there are synchronous wrapper
+functions. Of particular note is that the synchronous wrapper understands
+keyboard interrupt and will cancel a capture returning the trace around the
+cancel point. This is useful if your trigger doesn't fire and you want to
+understand why.
+
+### Where's the documentation, mate?
+
+Yeah, yeah. I know.
+
+### Also, I see no unit tests...
+
+It's pretty hard to do unit tests for a physical device. That's my excuse and
+I'm sticking to it.
+
+### Long lines and ignoring E221, eh?
+
+"A foolish consistency is the hobgoblin of little minds"
+
+Also, I haven't used an 80 character wide terminal in this century.

analysis.py

@@ -1,3 +1,11 @@
+"""
+analysis
+========
+
+Library code for analysing captures returned by `Scope.capture()`.
+"""
+
+# pylama:ignore=C0103,R1716
 
 import numpy as np
 
@@ -62,7 +70,7 @@ def extract_waveform(series, period):
     p = int(round(series.sample_rate * period))
     n = len(series.samples) // p
     if n <= 2:
-        return None, None
+        return None, None, None, None
     samples = np.array(series.samples)[:p*n]
     cumsum = samples.cumsum()
     underlying = (cumsum[p:] - cumsum[:-p]) / p
@@ -94,7 +102,7 @@ def normalize_waveform(samples, smooth=7):
                 crossings.append((i - last_rising, last_rising))
     if first_falling is not None:
         crossings.append((n + first_falling - last_rising, last_rising))
-    width, first = min(crossings)
+    first = min(crossings)[1]
     wave = (np.hstack([samples[first:], samples[:first]]) - offset) / scale
     return wave, offset, scale, first, sorted((i - first % n, w) for (w, i) in crossings)
 
@@ -104,7 +112,7 @@ def characterize_waveform(samples, crossings):
     possibles = []
     if len(crossings) == 1:
         duty_cycle = crossings[0][1] / n
-        if duty_cycle > 0.45 and duty_cycle < 0.55:
+        if 0.45 < duty_cycle < 0.55:
             possibles.append((rms(samples - sine_wave(n)), 'sine', None))
             possibles.append((rms(samples - triangle_wave(n)), 'triangle', None))
             possibles.append((rms(samples - sawtooth_wave(n)), 'sawtooth', None))

scope.py

@@ -1,4 +1,13 @@
-#!/usr/bin/env python3
+"""
+scope
+=====
+
+Code for talking to the BitScope series of USB digital mixed-signal scopes.
+Only supports the BS000501 at the moment, but that's only because it's never
+been tested on any other model.
+"""
+
+# pylama:ignore=E0611,E1101,W0201,W1203,W0631,C0103,R0902,R0912,R0913,R0914,R0915,C0415,W0601,W0102
 
 import argparse
 import array
@@ -43,8 +52,8 @@ class Scope(vm.VirtualMachine):
                 break
             else:
                 raise RuntimeError("No matching serial device found")
-        Log.info(f"Connecting to scope at {url}")
         self.close()
+        Log.info(f"Connecting to scope at {url}")
         parts = urlparse(url, scheme='file')
         if parts.scheme == 'file':
             self._reader = self._writer = streams.SerialStream(device=parts.path)
@@ -63,8 +72,8 @@ class Scope(vm.VirtualMachine):
         await self.issue_get_revision()
         revision = ((await self.read_replies(2))[1]).decode('ascii')
         if revision == 'BS000501':
-            self.master_clock_rate = 40000000
-            self.master_clock_period = 1/self.master_clock_rate
+            self.primary_clock_rate = 40000000
+            self.primary_clock_period = 1/self.primary_clock_rate
             self.capture_buffer_size = 12 << 10
             self.awg_wavetable_size = 1024
             self.awg_sample_buffer_size = 1024
@@ -74,8 +83,8 @@ class Scope(vm.VirtualMachine):
             self.analog_params = {'x1':  self.AnalogParams(1.1, -.05, 0, 1.1, -.05, -.05, 18.333, -7.517, -5.5, 8, 0)}
             self.analog_lo_min = 0.07
             self.analog_hi_max = 0.88
-            self.timeout_clock_period = (1 << 8) * self.master_clock_period
-            self.timestamp_rollover = (1 << 32) * self.master_clock_period
+            self.timeout_clock_period = (1 << 8) * self.primary_clock_period
+            self.timestamp_rollover = (1 << 32) * self.primary_clock_period
         else:
             raise RuntimeError(f"Unsupported scope, revision: {revision}")
         self._awg_running = False
@@ -115,8 +124,8 @@ class Scope(vm.VirtualMachine):
         self.close()
 
     def close(self):
-        super().close()
-        Log.info("Closed scope")
+        if super().close():
+            Log.info("Closed scope")
 
     def calculate_lo_hi(self, low, high, params):
         if not isinstance(params, self.AnalogParams):
@@ -160,14 +169,16 @@ class Scope(vm.VirtualMachine):
         logic_enable = sum(1 << channel for channel in logic_channels)
 
         for capture_mode in vm.CaptureModes:
-            ticks = int(round(period / self.master_clock_period / nsamples))
+            ticks = int(round(period / self.primary_clock_period / nsamples))
             clock_scale = 1
             if capture_mode.analog_channels == len(analog_channels) and capture_mode.logic_channels == bool(logic_channels):
                 Log.debug(f"Considering trace mode {capture_mode.trace_mode.name}...")
                 if ticks > capture_mode.clock_high and capture_mode.clock_divide > 1:
-                    clock_scale = int(math.ceil(period / self.master_clock_period / nsamples / capture_mode.clock_high))
-                    ticks = int(round(period / self.master_clock_period / nsamples / clock_scale))
-                    if ticks in range(capture_mode.clock_low, capture_mode.clock_high+1):
+                    clock_scale = min(capture_mode.clock_divide, int(math.ceil(period / self.primary_clock_period / nsamples / capture_mode.clock_high)))
+                    ticks = int(round(period / self.primary_clock_period / nsamples / clock_scale))
+                    if ticks > capture_mode.clock_low:
+                        if ticks > capture_mode.clock_high:
+                            ticks = capture_mode.clock_high
                         Log.debug(f"- try with tick count {ticks} x {clock_scale}")
                     else:
                         continue
@@ -178,23 +189,23 @@ class Scope(vm.VirtualMachine):
                 else:
                     Log.debug("- mode too slow")
                     continue
-                n = int(round(period / self.master_clock_period / ticks / clock_scale))
+                actual_nsamples = int(round(period / self.primary_clock_period / ticks / clock_scale))
                 if len(analog_channels) == 2:
-                    n -= n % 2
+                    actual_nsamples -= actual_nsamples % 2
                 buffer_width = self.capture_buffer_size // capture_mode.sample_width
                 if logic_channels and analog_channels:
                     buffer_width //= 2
-                if n <= buffer_width:
-                    Log.debug(f"- OK; period is {n} samples")
-                    nsamples = n
+                if actual_nsamples <= buffer_width:
+                    Log.debug(f"- OK; period is {actual_nsamples} samples")
+                    nsamples = actual_nsamples
                     break
-                Log.debug(f"- insufficient buffer space for necessary {n} samples")
+                Log.debug(f"- insufficient buffer space for necessary {actual_nsamples} samples")
         else:
             raise ConfigurationError("Unable to find appropriate capture mode")
-        sample_period = ticks*clock_scale*self.master_clock_period
+        sample_period = ticks*clock_scale*self.primary_clock_period
         sample_rate = 1/sample_period
         if trigger_position and sample_rate > 5e6:
-            Log.warn("Pre-trigger capture not supported above 5M samples/s; forcing trigger_position=0")
+            Log.warning("Pre-trigger capture not supported above 5M samples/s; forcing trigger_position=0")
             trigger_position = 0
 
         if raw:
@@ -225,21 +236,13 @@ class Scope(vm.VirtualMachine):
         if trigger_level is None:
             trigger_level = (high + low) / 2
         analog_trigger_level = (trigger_level - analog_params.offset) / analog_params.scale if not raw else trigger_level
-        if trigger == 'A' or trigger == 'B':
-            if trigger == 'A':
-                spock_option |= vm.SpockOption.TriggerSourceA
-                trigger_logic = 0x80
-            elif trigger == 'B':
-                spock_option |= vm.SpockOption.TriggerSourceB
-                trigger_logic = 0x40
-            trigger_mask = 0xff ^ trigger_logic
-        elif isinstance(trigger, dict):
+        if isinstance(trigger, dict):
             trigger_logic = 0
             trigger_mask = 0xff
             for channel, value in trigger.items():
                 if isinstance(channel, str):
                     if channel.startswith('L'):
-                        channel = int(channel[1:])
+                        channel = int(channel[1:])  # noqa
                     else:
                         raise ValueError("Unrecognised trigger value")
                 if channel < 0 or channel > 7:
@@ -248,6 +251,14 @@ class Scope(vm.VirtualMachine):
                 trigger_mask &= ~mask
                 if value:
                     trigger_logic |= mask
+        elif trigger in {'A', 'B'}:
+            if trigger == 'A':
+                spock_option |= vm.SpockOption.TriggerSourceA
+                trigger_logic = 0x80
+            elif trigger == 'B':
+                spock_option |= vm.SpockOption.TriggerSourceB
+                trigger_logic = 0x40
+            trigger_mask = 0xff ^ trigger_logic
         else:
             raise ValueError("Unrecognised trigger value")
         trigger_type = trigger_type.lower()
@@ -263,13 +274,12 @@ class Scope(vm.VirtualMachine):
         if timeout is None:
             trigger_timeout = 0
         else:
-            trigger_timeout = int(math.ceil(((trigger_intro+trigger_outro+trace_outro+2)*ticks*clock_scale*self.master_clock_period
+            trigger_timeout = int(math.ceil(((trigger_intro+trigger_outro+trace_outro+2)*ticks*clock_scale*self.primary_clock_period
                                              + timeout)/self.timeout_clock_period))
             if trigger_timeout > vm.Registers.Timeout.maximum_value:
                 if timeout > 0:
                     raise ConfigurationError("Required trigger timeout too long")
-                else:
-                    raise ConfigurationError("Required trigger timeout too long, use a later trigger position")
+                raise ConfigurationError("Required trigger timeout too long, use a later trigger position")
 
         Log.info(f"Begin {('mixed' if logic_channels else 'analogue') if analog_channels else 'logic'} signal capture "
                  f"at {sample_rate:,.0f} samples per second (trace mode {capture_mode.trace_mode.name})")
@@ -315,7 +325,7 @@ class Scope(vm.VirtualMachine):
             value_multiplier, value_offset = (1, 0) if raw else (high-low, low-analog_params.ab_offset/2*(1 if channel == 'A' else -1))
             data = await self.read_analog_samples(asamples, capture_mode.sample_width)
             series = DotDict({'channel': channel,
-                              'capture_start': start_timestamp * self.master_clock_period,
+                              'capture_start': start_timestamp * self.primary_clock_period,
                               'timestamps': timestamps[dump_channel::len(analog_channels)] if len(analog_channels) > 1 else timestamps,
                               'samples': array.array('f', (value*value_multiplier+value_offset for value in data)),
                               'sample_period': sample_period*len(analog_channels),
@@ -337,7 +347,7 @@ class Scope(vm.VirtualMachine):
                 mask = 1 << i
                 channel = f'L{i}'
                 series = DotDict({'channel': channel,
-                                  'capture_start': start_timestamp * self.master_clock_period,
+                                  'capture_start': start_timestamp * self.primary_clock_period,
                                   'timestamps': timestamps,
                                   'samples': array.array('B', (1 if value & mask else 0 for value in data)),
                                   'sample_period': sample_period,
@@ -347,7 +357,7 @@ class Scope(vm.VirtualMachine):
                     series.trigger_timestamp = series.timestamps[trigger_samples]
                     series.trigger_level = trigger[i]
                     series.trigger_type = trigger_type
-            traces[channel] = series
+                traces[channel] = series
         Log.info(f"{nsamples} samples captured on {cause}, traces: {', '.join(traces)}")
         return traces
 
@@ -360,19 +370,19 @@ class Scope(vm.VirtualMachine):
             raise ValueError(f"high out of range (0-{self.awg_maximum_voltage})")
         if low < 0 or low > high:
             raise ValueError("low out of range (0-high)")
-        max_clock = min(vm.Registers.Clock.maximum_value, int(math.floor(self.master_clock_rate / frequency / min_samples)))
-        min_clock = max(self.awg_minimum_clock, int(math.ceil(self.master_clock_rate / frequency / self.awg_sample_buffer_size)))
+        max_clock = min(vm.Registers.Clock.maximum_value, int(math.floor(self.primary_clock_rate / frequency / min_samples)))
+        min_clock = max(self.awg_minimum_clock, int(math.ceil(self.primary_clock_rate / frequency / self.awg_sample_buffer_size)))
         best_solution = None
         for clock in range(min_clock, max_clock+1):
-            width = self.master_clock_rate / frequency / clock
+            width = self.primary_clock_rate / frequency / clock
             nwaves = int(self.awg_sample_buffer_size / width)
             size = int(round(nwaves * width))
-            actualf = self.master_clock_rate * nwaves / size / clock
+            actualf = self.primary_clock_rate * nwaves / size / clock
             if actualf == frequency:
                 Log.debug(f"Exact solution: size={size} nwaves={nwaves} clock={clock}")
                 break
             error = abs(frequency - actualf) / frequency
-            if error < max_error and (best_solution is None or error < best_solution[0]):
+            if error < max_error and (best_solution is None or error < best_solution[0]):  # noqa
                 best_solution = error, size, nwaves, clock, actualf
         else:
             if best_solution is None:
@@ -422,9 +432,9 @@ class Scope(vm.VirtualMachine):
     async def start_clock(self, frequency, ratio=0.5, max_error=1e-4):
         if self._awg_running:
             raise UsageError("Cannot start clock while waveform generator in use")
-        ticks = min(max(2, int(round(self.master_clock_rate / frequency))), vm.Registers.Clock.maximum_value)
+        ticks = min(max(2, int(round(self.primary_clock_rate / frequency))), vm.Registers.Clock.maximum_value)
         fall = min(max(1, int(round(ticks * ratio))), ticks-1)
-        actualf, actualr = self.master_clock_rate / ticks, fall / ticks
+        actualf, actualr = self.primary_clock_rate / ticks, fall / ticks
         if abs(actualf - frequency) / frequency > max_error:
             raise ConfigurationError("No solution to required frequency and max_error")
         async with self.transaction():
@@ -495,11 +505,11 @@ class Scope(vm.VirtualMachine):
         for lo in np.linspace(self.analog_lo_min, 0.5, n, endpoint=False):
             for hi in np.linspace(self.analog_hi_max, 0.5, n):
                 zero, full, offset = await measure(lo, hi, 2e-3 if len(items) % 4 < 2 else 1e-3, len(items) % 2 == 0)
-                if zero > 0.01 and full < 0.99 and full > zero:
+                if 0.01 < zero < full < 0.99:
                     analog_range = self.clock_voltage / (full - zero)
                     items.append((lo, hi, -zero*analog_range, (1-zero)*analog_range, offset*analog_range))
         await self.stop_clock()
-        lo, hi, low, high, offset = np.array(items).T
+        lo, hi, low, high, offset = np.array(items).T  # noqa
 
         def f(params):
             dl, dh = self.calculate_lo_hi(low, high, self.AnalogParams(*params, analog_scale, analog_offset, None, None, None))
@@ -514,7 +524,7 @@ class Scope(vm.VirtualMachine):
             Log.info(f"Calibration succeeded: {result.message}")
             params = self.AnalogParams(*result.x, analog_scale, analog_offset, None, None, None)
 
-            def f(x):
+            def f(x):  # noqa
                 lo, hi = self.calculate_lo_hi(x[0], x[1], params)
                 return np.sqrt((self.analog_lo_min - lo)**2 + (self.analog_hi_max - hi)**2)
 
@@ -536,23 +546,21 @@ class Scope(vm.VirtualMachine):
         return f"<Scope {self.url}>"
 
 
-"""
-$ ipython3 --pylab
-Using matplotlib backend: MacOSX
-
-In [1]: run scope
-
-In [2]: start_waveform(2000, 'triangle')
-Out[2]: 2000.0
-
-In [3]: traces = capture(['A','B'], period=1e-3, low=0, high=3.3)
-
-In [4]: plot(traces.A.timestamps, traces.A.samples)
-Out[4]: [<matplotlib.lines.Line2D at 0x10c782160>]
-
-In [5]: plot(traces.B.timestamps, traces.B.samples)
-Out[5]: [<matplotlib.lines.Line2D at 0x10e6ea320>]
-"""
+# $ ipython3 --pylab
+# Using matplotlib backend: MacOSX
+#
+# In [1]: run scope
+#
+# In [2]: start_waveform(2000, 'triangle')
+# Out[2]: 2000.0
+#
+# In [3]: traces = capture(['A','B'], period=1e-3, low=0, high=3.3)
+#
+# In [4]: plot(traces.A.timestamps, traces.A.samples)
+# Out[4]: [<matplotlib.lines.Line2D at 0x10c782160>]
+#
+# In [5]: plot(traces.B.timestamps, traces.B.samples)
+# Out[5]: [<matplotlib.lines.Line2D at 0x10e6ea320>]
 
 
 async def main():

streams.py

@@ -5,6 +5,8 @@ streams
 Package for asynchronous serial IO.
 """
 
+# pylama:ignore=W1203,R0916,W0703
+
 import asyncio
 import logging
 import sys
@@ -20,14 +22,14 @@ Log = logging.getLogger(__name__)
 class SerialStream:
 
     @classmethod
-    def devices_matching(cls, vid=None, pid=None, serial=None):
+    def devices_matching(cls, vid=None, pid=None, serial_number=None):
         for port in comports():
-            if (vid is None or vid == port.vid) and (pid is None or pid == port.pid) and (serial is None or serial == port.serial_number):
+            if (vid is None or vid == port.vid) and (pid is None or pid == port.pid) and (serial_number is None or serial_number == port.serial_number):
                 yield port.device
 
     @classmethod
-    def stream_matching(cls, vid=None, pid=None, serial=None, **kwargs):
-        for device in cls.devices_matching(vid, pid, serial):
+    def stream_matching(cls, vid=None, pid=None, serial_number=None, **kwargs):
+        for device in cls.devices_matching(vid, pid, serial_number):
             return SerialStream(device, **kwargs)
         raise RuntimeError("No matching serial device")
 
@@ -59,15 +61,15 @@ class SerialStream:
             return
         if not self._output_buffer:
             try:
-                n = self._connection.write(data)
+                nbytes = self._connection.write(data)
             except serial.SerialTimeoutException:
-                n = 0
+                nbytes = 0
             except Exception:
                 Log.exception("Error writing to stream")
                 raise
-            if n:
-                Log.debug(f"Write {data[:n]!r}")
-            self._output_buffer = data[n:]
+            if nbytes:
+                Log.debug(f"Write {data[:nbytes]!r}")
+            self._output_buffer = data[nbytes:]
         else:
             self._output_buffer += data
         if self._output_buffer and self._output_buffer_empty is None:
@@ -80,16 +82,16 @@ class SerialStream:
 
     def _feed_data(self):
         try:
-            n = self._connection.write(self._output_buffer)
+            nbytes = self._connection.write(self._output_buffer)
         except serial.SerialTimeoutException:
-            n = 0
-        except Exception as e:
+            nbytes = 0
+        except Exception as exc:
             Log.exception("Error writing to stream")
-            self._output_buffer_empty.set_exception(e)
+            self._output_buffer_empty.set_exception(exc)
             self._loop.remove_writer(self._connection)
-        if n:
-            Log.debug(f"Write {self._output_buffer[:n]!r}")
-            self._output_buffer = self._output_buffer[n:]
+        if nbytes:
+            Log.debug(f"Write {self._output_buffer[:nbytes]!r}")
+            self._output_buffer = self._output_buffer[nbytes:]
         if not self._output_buffer:
             self._loop.remove_writer(self._connection)
             self._output_buffer_empty.set_result(None)
@@ -101,40 +103,39 @@ class SerialStream:
                 data = bytes(self._output_buffer)
                 self._output_buffer_lock.release()
                 try:
-                    n = self._connection.write(data)
+                    nbytes = self._connection.write(data)
                 finally:
                     self._output_buffer_lock.acquire()
-                Log.debug(f"Write {self._output_buffer[:n]!r}")
-                self._output_buffer = self._output_buffer[n:]
+                Log.debug(f"Write {self._output_buffer[:nbytes]!r}")
+                self._output_buffer = self._output_buffer[nbytes:]
             self._output_buffer_empty = None
 
-    async def read(self, n=None):
+    async def read(self, nbytes=None):
         if self._use_threads:
-            return await self._loop.run_in_executor(None, self._read_blocking, n)
+            return await self._loop.run_in_executor(None, self._read_blocking, nbytes)
         while True:
-            w = self._connection.in_waiting
-            if w:
-                data = self._connection.read(w if n is None else min(n, w))
+            nwaiting = self._connection.in_waiting
+            if nwaiting:
+                data = self._connection.read(nwaiting if nbytes is None else min(nbytes, nwaiting))
                 Log.debug(f"Read {data!r}")
                 return data
-            else:
-                future = self._loop.create_future()
-                self._loop.add_reader(self._connection, future.set_result, None)
-                try:
-                    await future
-                finally:
-                    self._loop.remove_reader(self._connection)
+            future = self._loop.create_future()
+            self._loop.add_reader(self._connection, future.set_result, None)
+            try:
+                await future
+            finally:
+                self._loop.remove_reader(self._connection)
 
-    def _read_blocking(self, n=None):
+    def _read_blocking(self, nbytes=None):
         data = self._connection.read(1)
-        w = self._connection.in_waiting
-        if w and (n is None or n > 1):
-            data += self._connection.read(w if n is None else min(n-1, w))
+        nwaiting = self._connection.in_waiting
+        if nwaiting and (nbytes is None or nbytes > 1):
+            data += self._connection.read(nwaiting if nbytes is None else min(nbytes-1, nwaiting))
         Log.debug(f"Read {data!r}")
         return data
 
-    async def readexactly(self, n):
+    async def readexactly(self, nbytes):
         data = b''
-        while len(data) < n:
-            data += await self.read(n-len(data))
+        while len(data) < nbytes:
+            data += await self.read(nbytes-len(data))
         return data

test.py

@@ -1,26 +1,12 @@
 
-import numpy as np
 from pylab import figure, plot, show
 
 from analysis import annotate_series
 from scope import await_, capture, main
-from utils import DotDict
 
 
 await_(main())
-
-# o = 400
-# m = 5
-# n = o * m
-# samples = square_wave(o)
-# samples = np.hstack([samples] * m) * 2
-# samples = np.hstack([samples[100:], samples[:100]])
-# samples += np.random.normal(size=n) * 0.1
-# samples += np.linspace(4.5, 5.5, n)
-# series = DotDict(samples=samples, sample_rate=1000000)
-
-data = capture(['A'], period=20e-3, nsamples=2000)
-series = data.A
+series = capture(['A'], period=20e-3, nsamples=2000).A
 
 figure(1)
 plot(series.timestamps, series.samples)
@@ -35,6 +21,7 @@ if annotate_series(series):
         print(f"Found {waveform.frequency:.0f}Hz {waveform.shape} wave, "
               f"with amplitude ±{waveform.amplitude:.2f}V and offset {waveform.offset:.2f}V")
 
-    plot(waveform.timestamps + waveform.capture_start - series.capture_start, waveform.samples * waveform.amplitude + waveform.offset)
+    plot(waveform.timestamps + waveform.capture_start - series.capture_start,
+         waveform.samples * waveform.amplitude + waveform.offset)
 
 show()

utils.py

@@ -1,3 +1,9 @@
+"""
+utils
+=====
+
+Random utility classes/functions.
+"""
 
 
 class DotDict(dict):

vm.py

@@ -1,4 +1,3 @@
-
 """
 vm
 ==
@@ -7,12 +6,14 @@ Package capturing BitScope VM specification, including registers, enumerations,
 commands and logic for encoding and decoding virtual machine instructions and data.
 
 All names and descriptions copyright BitScope and taken from their [VM specification
-document][VM01B].
+document][VM01B] (with slight changes).
 
 [VM01B]: https://docs.google.com/document/d/1cZNRpSPAMyIyAvIk_mqgEByaaHzbFTX8hWglAMTlnHY
 
 """
 
+# pylama:ignore=E221,C0326,R0904,W1203
+
 import array
 from collections import namedtuple
 from enum import IntEnum
@@ -35,30 +36,30 @@ class Register(namedtuple('Register', ['base', 'dtype', 'description'])):
         else:
             width = int(self.dtype[1:])
         if sign == 'U':
-            n = 1 << width
-            value = max(0, min(value, n-1))
-            bs = struct.pack('<I', value)
+            max_value = (1 << width) - 1
+            value = min(max(0, value), max_value)
+            data = struct.pack('<I', value)
         elif sign == 'S':
-            n = 1 << (width - 1)
-            value = max(-n, min(value, n-1))
-            bs = struct.pack('<i', value)
+            max_value = (1 << (width - 1))
+            value = min(max(-max_value, value), max_value - 1)
+            data = struct.pack('<i', value)
         else:
             raise TypeError("Unrecognised dtype")
-        return bs[:width//8]
+        return data[:width//8]
 
-    def decode(self, bs):
-        if len(bs) < 4:
-            bs = bs + bytes(4 - len(bs))
+    def decode(self, data):
+        if len(data) < 4:
+            data = data + bytes(4 - len(data))
         sign = self.dtype[0]
         if sign == 'U':
-            value = struct.unpack('<I', bs)[0]
+            value = struct.unpack('<I', data)[0]
         elif sign == 'S':
-            value = struct.unpack('<i', bs)[0]
+            value = struct.unpack('<i', data)[0]
         else:
             raise TypeError("Unrecognised dtype")
         if '.' in self.dtype:
-            whole, fraction = map(int, self.dtype[1:].split('.', 1))
-            value = value / (1 << fraction)
+            fraction = int(self.dtype.split('.', 1)[1])
+            value /= (1 << fraction)
         return value
 
     @property
@@ -69,15 +70,14 @@ class Register(namedtuple('Register', ['base', 'dtype', 'description'])):
             whole, fraction = int(self.dtype[1:]), 0
         if self.dtype[0] == 'S':
             whole -= 1
-        n = (1 << (whole+fraction)) - 1
-        return n / (1 << fraction) if fraction else n
+        max_value = (1 << (whole+fraction)) - 1
+        return max_value / (1 << fraction) if fraction else max_value
 
     @property
     def width(self):
         if '.' in self.dtype:
             return sum(map(int, self.dtype[1:].split('.', 1))) // 8
-        else:
-            return int(self.dtype[1:]) // 8
+        return int(self.dtype[1:]) // 8
 
 
 Registers = DotDict({
@@ -90,8 +90,8 @@ Registers = DotDict({
     "TriggerOutro": Register(0x34, 'U16', "Edge trigger outro filter counter (samples/2)"),
     "TriggerValue": Register(0x44, 'S0.16', "Digital (comparator) trigger (signed)"),
     "TriggerTime": Register(0x40, 'U32', "Stopwatch trigger time (ticks)"),
-    "ClockTicks": Register(0x2e, 'U16', "Master Sample (clock) period (ticks)"),
-    "ClockScale": Register(0x14, 'U16', "Clock divide by N (low byte)"),
+    "ClockTicks": Register(0x2e, 'U16', "Sample period (ticks)"),
+    "ClockScale": Register(0x14, 'U16', "Sample clock divider"),
     "TraceOption": Register(0x20, 'U8', "Trace Mode Option bits"),
     "TraceMode": Register(0x21, 'U8', "Trace Mode (see Trace Mode Table)"),
     "TraceIntro": Register(0x26, 'U16', "Pre-trigger capture count (samples)"),
@@ -151,13 +151,11 @@ Registers = DotDict({
     "Map5": Register(0x99, 'U8', "Peripheral Pin Select Channel 5"),
     "Map6": Register(0x9a, 'U8', "Peripheral Pin Select Channel 6"),
     "Map7": Register(0x9b, 'U8', "Peripheral Pin Select Channel 7"),
-    "MasterClockN": Register(0xf7, 'U8', "PLL prescale (DIV N)"),
-    "MasterClockM": Register(0xf8, 'U16', "PLL multiplier (MUL M)"),
+    "PrimaryClockN": Register(0xf7, 'U8', "PLL prescale (DIV N)"),
+    "PrimaryClockM": Register(0xf8, 'U16', "PLL multiplier (MUL M)"),
 })
 
 
-# pylama:ignore=E221
-
 class TraceMode(IntEnum):
     Analog         = 0
     Mixed          = 1
@@ -254,12 +252,12 @@ class VirtualMachine:
     class Transaction:
         def __init__(self, vm):
             self._vm = vm
+            self._data = b''
 
         def append(self, cmd):
             self._data += cmd
 
         async def __aenter__(self):
-            self._data = b''
             self._vm._transactions.append(self)
             return self
 
@@ -281,6 +279,8 @@ class VirtualMachine:
             self._writer.close()
             self._writer = None
             self._reader = None
+            return True
+        return False
 
     __del__ = close
 
@@ -300,12 +300,12 @@ class VirtualMachine:
         else:
             self._transactions[-1].append(cmd)
 
-    async def read_replies(self, n):
+    async def read_replies(self, nreplies):
         if self._transactions:
             raise TypeError("Command transaction in progress")
         replies = []
         data, self._reply_buffer = self._reply_buffer, b''
-        while len(replies) < n:
+        while len(replies) < nreplies:
             index = data.find(b'\r')
             if index >= 0:
                 reply = data[:index]
@@ -331,25 +331,25 @@ class VirtualMachine:
 
     async def set_registers(self, **kwargs):
         cmd = ''
-        r0 = r1 = None
+        register0 = register1 = None
         for base, name in sorted((Registers[name].base, name) for name in kwargs):
             register = Registers[name]
-            bs = register.encode(kwargs[name])
-            Log.debug(f"{name} = 0x{''.join(f'{b:02x}' for b in reversed(bs))}")
-            for i, byte in enumerate(bs):
+            data = register.encode(kwargs[name])
+            Log.debug(f"{name} = 0x{''.join(f'{b:02x}' for b in reversed(data))}")
+            for i, byte in enumerate(data):
                 if cmd:
                     cmd += 'z'
-                    r1 += 1
+                    register1 += 1
                 address = base + i
-                if r1 is None or address > r1 + 3:
+                if register1 is None or address > register1 + 3:
                     cmd += f'{address:02x}@'
-                    r0 = r1 = address
+                    register0 = register1 = address
                 else:
-                    cmd += 'n' * (address - r1)
-                    r1 = address
-                if byte != r0:
+                    cmd += 'n' * (address - register1)
+                    register1 = address
+                if byte != register0:
                     cmd += '[' if byte == 0 else f'{byte:02x}'
-                    r0 = byte
+                    register0 = byte
         if cmd:
             await self.issue(cmd + 's')
 
@@ -382,22 +382,21 @@ class VirtualMachine:
     async def issue_triggered_trace(self):
         await self.issue(b'D')
 
-    async def read_analog_samples(self, n, sample_width):
+    async def read_analog_samples(self, nsamples, sample_width):
         if self._transactions:
             raise TypeError("Command transaction in progress")
         if sample_width == 2:
-            data = await self._reader.readexactly(2*n)
+            data = await self._reader.readexactly(2 * nsamples)
             return array.array('f', ((value+32768)/65536 for (value,) in struct.iter_unpack('>h', data)))
-        elif sample_width == 1:
-            data = await self._reader.readexactly(n)
+        if sample_width == 1:
+            data = await self._reader.readexactly(nsamples)
             return array.array('f', (value/256 for value in data))
-        else:
-            raise ValueError(f"Bad sample width: {sample_width}")
+        raise ValueError(f"Bad sample width: {sample_width}")
 
-    async def read_logic_samples(self, n):
+    async def read_logic_samples(self, nsamples):
         if self._transactions:
             raise TypeError("Command transaction in progress")
-        return await self._reader.readexactly(n)
+        return await self._reader.readexactly(nsamples)
 
     async def issue_cancel_trace(self):
         await self.issue(b'K')
@@ -411,15 +410,15 @@ class VirtualMachine:
     async def issue_wavetable_read(self):
         await self.issue(b'R')
 
-    async def wavetable_read_bytes(self, n):
+    async def wavetable_read_bytes(self, nbytes):
         if self._transactions:
             raise TypeError("Command transaction in progress")
-        return await self._reader.readexactly(n)
+        return await self._reader.readexactly(nbytes)
 
-    async def wavetable_write_bytes(self, bs):
+    async def wavetable_write_bytes(self, data):
         cmd = ''
         last_byte = None
-        for byte in bs:
+        for byte in data:
             if byte != last_byte:
                 cmd += f'{byte:02x}'
             cmd += 'W'