openbci_cyton 0.2.0

The OpenBCI Cyton (and Cyton + Daisy) biosensing board over its USB dongle, in pure Dart: commands, channel settings, samples in µV, markers, impedance, and SD card recordings (also as XDF).

openbci_cyton

openbci_cyton connects to an OpenBCI Cyton biosensing board, with or without the Daisy module, through its USB dongle. It configures the board and delivers the streamed samples, with EEG in microvolts and the accelerometer in g, decoded in the same way as BrainFlow. It also sets the sampling rate, sends markers, measures electrode impedance, records to the SD card of the board and reads those recordings. The package is pure Dart and uses serial_transport for the serial connection, so it runs on desktop platforms and, with WebSerial, in Chrome and Edge.

API documentation

Installation

dart pub add openbci_cyton serial_transport

Usage

import 'package:openbci_cyton/openbci_cyton.dart';
import 'package:serial_transport/serial_transport.dart';

Future<void> main() async {
  final provider = SerialPortProvider.platform();
  final ports = await provider.listPorts();
  if (ports.isEmpty) return;
  final transport = await provider.open(ports.first, baudRate: cytonBaudRate);

  final board = await CytonBoard.connect(transport);
  print('${board.channelCount} channels at ${board.rate} Hz');

  board.samples.listen((sample) => print(sample.eeg));
  await board.start();
  await Future<void>.delayed(const Duration(seconds: 5));
  await board.stop();
  await board.close();
}

CytonBoard.connect stops any stream left running from an earlier session, resets the board, detects a Daisy module and restores the default channel settings. With a Daisy the board provides 16 channels at 125 Hz, and otherwise 8 channels at 250 Hz. If the board does not reply, the board must be switched on and the switch of the dongle set to GPIO_6. A reset keeps the sampling rate and the board mode, so connect sets them when they are given (sampleRate, boardMode) and reads them from the board otherwise.

Each CytonSample has the sample number, the EEG values, the accelerometer values, the six aux bytes, a marker if one was sent, and the number of samples lost before it, derived from the packet counter of the board. CytonBoard.dropped counts packets that could not be decoded.

Channels

configureChannel sets the power, gain, input, bias and reference (SRB1, SRB2) of a channel. CytonChannelSettings.bipolar removes the SRB2 reference, as for EMG on some of the channels of an EEG cap. setChannelEnabled switches a channel on or off, and resetChannels restores the defaults. The settings are retained (channels, gains), and the gains scale the samples. The board accepts these commands while it streams, without a reply, so a change of gain applies to the samples that follow.

setTestSignal connects every channel to one of the internal test signals of the ADS1299, such as a square wave of about 1.86 mV, and with null restores the previous settings.

Sampling rate and board mode

setSampleRate sets the rate of the ADS1299 from 250 Hz to 16 kHz. The radio link of the dongle carries 250 Hz at most, so at higher rates the board does not stream and only an SD card recording receives the samples. setBoardMode chooses what the aux bytes carry: the accelerometer (the default), the analog or digital pins, or markers.

Markers

In CytonBoardMode.marker, marker places a value in the next sample, as CytonSample.marker and in an SD card recording; the accelerometer is off. The board receives a marker as a backtick followed by the value. If the radio loses the backtick, the board takes the value for a command, and values such as A (start an SD card recording), b (start streaming) or 1 (switch channel 1 off) then change its state. marker therefore accepts only values that are not commands: 128 to 239, and the characters 9fghklmnoBMNOP()+,._| (cytonSafeMarkerChars). The board holds one marker per sample, so markers are sent at least two samples apart. Markers sent in the first moments after streaming starts can be lost.

Electrode impedance

measureImpedance and measureImpedances measure electrodes one at a time while the board streams, as the OpenBCI GUI does. The channel is set to gain 1 without a reference, a lead-off current of 6 nA at 31.25 Hz is applied to its N input (the usual electrode input with SRB2 referencing) or its P input, and the impedance follows from the amplitude of the signal at 31.25 Hz, less the series resistance of 2.2 kΩ. The channel's settings are restored afterwards. With a Daisy, the board streams the mean of two consecutive samples, which reduces the amplitude at 31.25 Hz to 0.92 of its value; the measurement corrects for this, whereas the OpenBCI GUI reports impedances about 8% lower.

SD card recordings

startSdRecording reserves space on the card for a duration (CytonSdDuration, from 5 minutes to 24 hours at 250 Hz; CytonSdDuration.forMinutes chooses one for a rate) and starts streaming, during which the board writes its samples to the file it names, such as OBCI_2A.TXT. stopSdRecording closes the file. At 250 Hz the samples are streamed as well. The files hold 250 samples per second with a Daisy, as the board does not average them for the card.

CytonSdFile.parse reads such a file, and CytonSdReader reads one in pieces. The file does not record the sampling rate or the gains: the rate is estimated from the accelerometer, which the board reads at 25 Hz, and the gains are taken to be 24, unless they are given. In the marker board mode, the aux values of each line carry the markers. The library package:openbci_cyton/xdf.dart converts a recording to XDF as it is read (convertCytonSdToXdf), so that large files need not be held in memory.

The replies of the board were verified with firmware v3.1.2. Streaming during an SD card recording, and SD card recordings above 250 Hz, were verified with a fork of the firmware that corrects SD card logging.

Testing without a board

package:openbci_cyton/testing.dart provides FakeCyton, a simulated board that is attached to the far end of a serial connection, such as one side of a pseudo-terminal pair.

Related tools

LSL Viewer connects to a Cyton under Serial > Connect an OpenBCI Cyton…, with its settings, markers, impedance and SD card recording in the Cyton menu, and opens SD card recordings. The command lsl cyton <port> of lsl_tools publishes a board as LSL streams, and lsl cyton-sd converts an SD card recording to XDF.