Code examples

For using these code snippets, you will have to have your hardware (e.g., based on the Arduino Leonardo) connected to a USB port of your computer. Next, you will have to find out the port that your computer assigned to the USB device. This requires a different procedure, depending on the operating system.

  • Windows: Open the “Device Manager” and look at the Ports list (COM & LPT). The ports usually have names like "COM*".

  • Linux and OSX: Check the devices in /dev/ through ls /dev/*. The port names differ between Linux and OSX:

    • Linux: usually /dev/ttyUSB* or /dev/ttyACM*

    • OSX: usually /dev/tty.usbmodem* or /dev/tty.usbserial*

The Python section below shows a way how to find the ports using the pyserial library.

Python

You will need to install the pyserial package via pip install pyserial.

Find the port

We can use pyserial to find the port of our USB device

import serial  # imports pyserial
import serial.tools.list_ports as list_ports

# List all comports
all_ports = list_ports.comports()
print(all_ports)

# Each entry in the `all_ports` list is a serial device. Check it's
# description and device attributes to learn more
first_serial_device = all_ports[0]
print(first_serial_device.device)  # the `port_name`
print(first_serial_device.description)  # perhaps helpful to know if this is your device

# continue until you found your device, then note down the `port_name`

Send information

Now open up a serial connection and send some information.

import serial

port_name = "COM4"  # the name / address we found for our device

ser = serial.Serial(
    port=port_name,
    baudrate=115200,
    bytesize=serial.EIGHTBITS,  # set this to the amount of data you want to send
    )

# the information we want to send: 8 bits = 1 byte
byte_to_send = bytes([1])  # send a "1"
ser.write(byte_to_send)

# After measurements are done, close the connection
ser.close()

Consider sampling rate of recording device

For event marking, we usually want to start with a state of 0, then set the data to some byte (e.g., 1), and finally reset the state to 0 again. Notably, the time that the data is set to a byte different from 0 needs to be long enough for our recording device to notice. How long it “long enough” depends on the sampling rate: At a sampling rate of 1000Hz, the data needs to be “switched on” for at least 1ms (at a sampling rate of 250Hz, for 4ms, etc.).

When using the firmware we recommend in this project for running your usb-to-ttl device, all of this is taken care of in the microcontroller, see Firmware:

  // Set the 8 pins to correspond to the byte
  setOutputs(inChar);

  // Leave the pins ON for long enough to be detected
  // This depends on the sampling rate of the device that receives the byte
  delay(2000);

  // Then clear again and wait shortly
  clearOutputs();

This part of the firmware sets the data, then delays by a bit, and then resets to 0. Of course, handling this issue in the microcontroller also limits the frequency with which you can send signals (every 2000 milliseconds in our firmware example), so don’t forget to adjust the delay to your needs.