Guide: Serial communication over uart

Written by Julie Zelenski

What is uart?

UART is an acronym for Universal Asynchronous Receiver-Transmitter, a protocol for sequential serial communication. Most processors include hardware support for a uart peripheral (the Mango Pi has six of them in fact!). A uart is a serial communication channel for sending and receiving data between devices. Your laptop can use a uart to talk to your Pi and receive output and provide input.

CP2012 USB to serial breakout board

You will use a CP2102 USB to serial breakout board (hereafter referred to as "usb-serial”) to make a serial connection between your laptop usb and the Pi's uart.

Find the usb-serial in your parts kit. One end has a USB-A connector and the other is a 5-pin or 6-pin header. You will plug the USB-A end into a usb port on your hub and wire jumpers from the header pins to the uart on your Pi. usb-serial

The square IC in the center of the board is the CP2102 chip converts from a usb interface to a serial interface. You will need a CP2102 driver on your laptop.

Got CP2102 driver? If your OS version is relatively recent, your laptop likely has the driver pre-installed. If not, you will need to manually install the CP2102 driver. See CP2102 installation guide for more info.

Connect usb-serial to Pi

  1. First, disconnect power to the Pi.

    Danger Always have the Pi powered down whenever you are fiddling with the wiring. If you leave it plugged in, power is flowing and all wires are live, which makes for a dicey situation. An accidental crossed wire can a short circuit, which could fry your Pi or make your laptop disable the USB port.

  2. Connect transmit, receive, and ground between the usb-serial and the Pi

    • Pick out three female-female jumpers: one blue, one green, and one black. People experienced in electronics choose the color of the wire to indicate what type of signal is being carried. This makes it easier to debug connections and visualize the circuit. By convention, black is used for ground, and blue and green are used for transmit and receive, respectively.
    • On the usb-serial, identify the pins labeled transmit TX, receive RX, and ground GND.
    • Use the pinout to locate the Mango Pi pins for UART TX and UART RX. Also identify an open ground pin.
    • Connect the black ground between ground on the usb-serial and ground on the Pi. Connect the blue jumper from TX of the usb-serial to UART RX on the Pi. Connect the green jumper from RX of the usb-serial to UART TX on the Pi. The correct connections are shown in the photo below: usb-serial to Pi connections

Careful with the connections The connections run from one device's TX to the other's RX, i.e. the data transmitted by your laptop is received by your Pi and vice versa. Connecting TX to TX and RX to RX is not correct! Also note that the pins on your USB-serial may be in different positions or have different label names. Don't just follow the photo blindly.

Here are photos of usb-serial variants you might possibly have in your kit (click to enlarge): V1 V2 V3

Once you have verified that your connections are correct, plug the usb-serial into your laptop or usb hub and then power up the Pi. You now have a serial connection!

Commnication over tty: tio

There are a number of programs that support communication with a tty device (screen, minicom, putty, etc.) Our favorite of the bunch is tio for its simple interface and nice features, so this is our strong recommendation. (If you already very comfortable with another program, ok to keep using instead).

Got tio? See guide on how to install and configure tio.

For uart communication to succeed, the transmitter and receiver must agree on device, baud rate, use of start, parity, and stop bits, and so on. In this course, we will be using these settings:

  • peripheral UART0
  • transmit on GPIO PB8, receive on GPIO PB9
  • baud rate 115200 bps
  • 1 start bit, 8 data bits, no parity bit, 1 stop bit ("8N1" for short)

Troubleshooting

  • Review all three connections to confirm correct (TX->RX, RX->TX, GND->GND).
  • Reseat your jumpers if they have wiggled loose.
  • If jumper cables appear stressed or worn, replace with fresh ones.
  • Confirm that your settings are 115200 8N1.
  • If you try to connect and receive the error Device file is locked by another process, this typically means that tio is already running and connected to the device. Look through your windows to find your existing connnection instead of trying to start another one.