Swissky
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# UART
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### Summary
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* What is it ?
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* Identifying UART Ports
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* Connect to serial port
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* Detect baudrate
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* Interact with the /dev/ttyUSB0
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* UART over BLE
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* Examples
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## Table of contents
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* [What is it ?](#what-is-it-)
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* [Identifying UART ports](#identifying-uart-ports)
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* [Using a multimeter](#using-a-multimeter)
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* [Using a logic analyzer](#using-a-logic-analyzer)
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* [Connect to serial port](#connect-to-serial-port)
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* [WARNING](#warning)
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* [Examples](#examples)
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* [Connection using a USB to TTL](#connection-using-a-usb-to-ttl)
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* [Detect the baud rate](#detect-the-baud-rate)
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* [Interact with UART](#interact-with-uart)
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* [UART over BLE](#uart-over-ble)
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* [Examples](#examples)
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## What is it ?
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UART stands for Universal asynchronous receiver transmitter. Used for serial communications over a computer or peripheral device serial port.
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UART peripherals are commonly integrated in many embedded devices. UART communication makes use of baud rate to maintain synchronism between two devices. The baud rate is the rate at which information is transferred in a communication channel.
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With access to the UART, a user can see bootloader and operating system logs.
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With access to the UART, a user can see bootloader and operating-system logs.
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Generaly, the line is held high (at a logical 1 value) while UART is in idle state.
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Generally, the line is held high (at a logical 1 value) while UART is in idle state.
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We call the most common configuration 8N1 : eight data bits, no parity, and 1 stop bit.
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We call the most common configuration **8N1**: eight data bits, no parity, and 1 stop bit.
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## Identifying UART Ports
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## Identifying UART ports
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A UART pinout has **four** ports:
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* **TX** (Transmit)
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* **RX** (Receive)
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* **VCC** (Voltage)
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* **GNR** (Ground)
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A UART pinout has four ports :
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- TX (Transmit)
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- Rx (Receive)
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- Vcc (Voltage)
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- GNR (Ground)
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To find UART multiple solution :
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- Search on Internet
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- Labeled on PCB
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- Find candidates
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- Use a multi-meter
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- Follow PCB traces (almost always impossible)
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To find UART multiple solution:
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* Search on Internet
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* Labeled on PCB
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* Find candidates
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* Using a multimeter
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* Using a logic analyzer
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* Follow PCB traces (almost always impossible)
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Keep in mind that some devices emulate UART ports by programming the Generla-Purpose Input/Output (GPIO) pins if there isn't enough space on the board for dedicated hardware UART pins.
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### Use a multimeter
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Keep in mind that some devices **emulate** UART ports by programming the General-Purpose Input/Output (GPIO) pins if there isn't enough space on the board for dedicated hardware UART pins.
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### Using a multimeter
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#### GNR pin
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First identify the GRN pin, by using the multimeter in continuity mode.
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Place the black probe on any grounded metallic surface, be it a part of the tested PCB or not. Then place the red probe on each of the ports. When you hear a beeping sound, you found a GND pin.
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#### VCC pin
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Turn the multimeter to the DC voltage mode in and set it up to 20V of voltage. Keep the black probe on a grounded surface. Place the red probe on a suspeted pin and turn on the device.
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Turn the multimeter to the DC voltage mode in and set it up to 20V of voltage. Keep the black probe on a grounded surface. Place the red probe on a suspected pin and turn on the device.
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If the multimeter measures a constant voltage of either 3.3V or 5V, you've found the VCC pin.
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#### Tx pin
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Keep the multimeter mode at DC voltage of 20V or less, and leave the black probe in a grounded surface. Move the red probe to the suspected pin and power cycle the device. If the voltage fluctuates for a few seconds and then stabilizes at the Vcc value, you've most likely found the Tx pin.
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#### TX pin
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Keep the multimeter mode at DC voltage of 20V or less, and leave the black probe in a grounded surface. Move the red probe to the suspected pin and power cycle the device. If the voltage fluctuates for a few seconds and then stabilizes at the VCC value, you've most likely found the TX pin.
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This behavior happens because, during bootup, the device sends serial data through that Tx pin for debugging purposes. Once it finishes booting, the UART line goes idle.
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This behavior happens because, during bootup, the device sends serial data through that TX pin for debugging purposes. Once it finishes booting, the UART line goes idle.
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#### Rx pin
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If you've already identified the rest of the UART pins, the nearby fourth pin is most likely the Rx pin.
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If you've already identified the rest of the UART pins, the nearby fourth pin is most likely the RX pin.
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Otherwise, you can identify it because it has the lowest voltage fluctuation and lowest overall value of all the UART pins.
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### Connect to serial port
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### Using a logic analyzer
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A logic analyzer is an electronic instrument that captures and displays multiple signals from a digital system or digital circuit.
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Connect to UART using an USB to TTL, then find the `/dev/ttyUSB0` device in the `dmesg` command output. You need to create the `dialout` group for Debian or `uucp` for Manjaro :
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To find the UART pins we will connect the pins to a logic analyzer and look for data being transmitted.
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* `sudo usermod -a -G dialout username`
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* `sudo gpasswd -a username uucp`
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#### Hardware setup
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Make sure any system you're testing is **powered off** when you connect the logic analyzer's probes to it **to avoid short-circuiting**.
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#### Detect baudrate
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* Connect the suspected TX pin to any channel of the logic analyzer.
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* Connect one of your logic analyzer's GND pins to the PCB that you're testing GND pins so they **share a common ground**.
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Standard baud rate are `110`, `300`, `600`, `1200`, `2400`, `4800`, `9600`, `14400`, `19200`, `38400`, `57600`, `115200`, `128000` and `256000`.\
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Auto-detect baud rate using the script : [devttys0/baudrate/baudrate.py](https://github.com/devttys0/baudrate/blob/master/baudrate.py)
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#### Software setup
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This setup is for **Saleae based logic analyzer**, if you use a different one referer to the constructor documentation.
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#### Interact with the /dev/ttyUSB0
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* Open the saleae software
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* Create a new analyzer entry by pressing a plus (+) icon and select Async Serial (this is for UART).
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* Select a serial channel (8 channels on Logic Analyser) and keep the default settings.
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* Try with popular baud rates used in IoT devices (9600, 19200, 38400, 57600, 115200). Note that when you don't know the bit rate, you can select "**Use Autobaud**" and let the software work its magic.
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* Save the configurations.
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```powershell
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cu -l /dev/ttyUSB0 -s 9600
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screen port_name 115200
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minicom -b 115200 -o -D Port_Name # to exit GNU screen, type Control-A k.
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microcom -d -s 9600 -p /dev/ttyUSB0
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microcom -d -s 19200 -p /dev/ttyUSB0
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If you want to modify the speed and the duration:
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* As a rule, you should sample digital signals **at least four times faster than their bandwidth**.
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* With serial communications, which are generally very slow, a **50 kS/s** sampling rate is more than enough, although sampling faster than this does no harm.
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* As for the duration, **20 seconds** is enough time for the device to power on and start transmitting data.
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Now try with the popular baud rates with both the suspected pins and try to compare the results. If you find any readable text with one of the pins and the text makes some sense then that’s the TX pin.
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## Connect to serial port
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### WARNING
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It's not a big deal if you confuse the UART RX and TX ports with each other, because you can easily swap the wires connecting to them without any consequences. But confusing the VCC with the GND and connecting wires to them incorrectly **might fry the circuit**.
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### Examples
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### Connection using a USB to TTL
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Once the ports are connected, plug the adapter into your computer. You now need to find the **device file descriptor**. To do that enter the following command : `sudo dmesg`.
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Typically, it will be assigned to `/dev/ttyUSB0` **if you don't have any other peripheral devices attached**.
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Under Ubuntu or Debian, a non-root user cannot have access to serial ports such as ttyS0 or ttyUSB0 if he is not a member of the **dialout** group ! The equivalent group on Arch based distributions is **uucp**. In other words, you just have to add yourself to this group to have access.
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Ubuntu or Debian: `sudo usermod -a -G dialout $USER`
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Arch based: `sudo usermod -a -G uucp $USER`
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### Detect the baud rate
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#### Most common baud rate
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The most common baud rates for UART are `9600`, `19200`, `38400`, `57600` and `115200`.
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A table of other used but less common baud rates can be found here: [Here](https://lucidar.me/en/serialib/most-used-baud-rates-table/)
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#### Autodetect the baud rate using a script
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Link: [baudrate.py](https://github.com/devttys0/baudrate/blob/master/baudrate.py)
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```bash
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# Download the script
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wget https://raw.githubusercontent.com/devttys0/baudrate/master/baudrate.py
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# Install serial dependency
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pip2.7 install serial
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# Run the script on "/dev/ttyUSB0"
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python2.7 baudrate.py -p /dev/ttyUSB0
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```
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### Interact with UART
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Different command line tools to interact with UART:
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```powershell
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cu -l /dev/ttyUSB0 -s 115200
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microcom -d -s 115200 -p /dev/ttyUSB0
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minicom -b 115200 -o -D /dev/ttyUSB0 # To exit GNU screen, type Control-A k
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screen /dev/ttyUSB0 115200
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```
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Script to brute force a password protected UART:
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```python
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import serial, time
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port = "/dev/ttyUSB0"
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baud = 9600
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baud = 115200
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s = serial.Serial(port)
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s.baudrate = baud
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@ -100,22 +163,18 @@ with open('/home/audit/Documents/IOT/passwords.lst', 'r') as f:
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time.sleep(10)
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```
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### UART over BLE
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## UART over BLE
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It’s an emulation of serial port over BLE. The UUID of the Nordic UART Service is `6E400001-B5A3-F393-E0A9-E50E24DCCA9E`. This service exposes two characteristics: one for transmitting and one for receiving.
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* **RX Characteristic** (UUID: 6E400002-B5A3-F393-E0A9-E50E24DCCA9E) : The peer can send data to the device by writing to the RX Characteristic of the service. ATT Write Request or ATT Write Command can be used. The received data is sent on the UART interface.
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* **TX Characteristic** (UUID: 6E400003-B5A3-F393-E0A9-E50E24DCCA9E) : If the peer has enabled notifications for the TX Characteristic, the application can send data to the peer as notifications. The application will transmit all data received over UART as notifications.
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* **RX Characteristic (UUID: 6E400002-B5A3-F393-E0A9-E50E24DCCA9E)** :
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* The peer can send data to the device by writing to the RX Characteristic of the service. ATT Write Request or ATT Write Command can be used. The received data is sent on the UART interface.
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* **TX Characteristic (UUID: 6E400003-B5A3-F393-E0A9-E50E24DCCA9E)** :
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* If the peer has enabled notifications for the TX Characteristic, the application can send data to the peer as notifications. The application will transmit all data received over UART as notifications.
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### Examples
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* [nRF UART 2.0 - Nordic Semiconductor ASA](https://play.google.com/store/apps/details?id=com.nordicsemi.nrfUARTv2)
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* [UART/Serial Port Emulation over BLE](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v14.0.0%2Fble_sdk_app_nus_eval.html)
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* [UART Over Bluetooth Low Energy](https://thejeshgn.com/2016/10/01/uart-over-bluetooth-low-energy/)
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Example with Micro::bit :
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* [https://makecode.microbit.org/v1/98535-28913-33692-07418](https://makecode.microbit.org/v1/98535-28913-33692-07418)
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* [Using the micro:bit Bluetooth Low Energy UART (serial over Bluetooth)](https://support.microbit.org/support/solutions/articles/19000062330-using-the-micro-bit-bluetooth-low-energy-uart-serial-over-bluetooth-)
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### Examples
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