UART - Part 1

UART stands for Universal Asynchronous Receiver and Transmitter.

Asynchronous means that they don't have a dedicated clock line, since all it has is two data lines (Rx, Tx in the diagram below). This will be become more clear after learning I2C.

The diagram below shows two UARTs wired for communication; each is connected to a local data bus.

graph LR
    subgraph uart2 [UART 2]
        direction TB
        Tx2
        Rx2
    end
    subgraph uart1 [UART 1]
        direction TB
        Tx1
        Rx1
    end
    Tx1--> Rx2
    Tx2--> Rx1
    DB2[Data Bus 2] <--> uart2
    DB1[Data Bus 1] <--> uart1

A UART has two data lines: one transmits data (Tx), one receives data (Rx). Each connects to the complementary line on the other UART. For example, the Tx1 line of UART 1, to the Rx2 line of UART 2.

The Data Bus is either where the transmitter reads data from, or where the receiver writes data to.

Communication

Most communication peripherals take the name from the communication protocol, but not UART. So...which protocol does the UART peripheral use?

UART uses Asynchronous Serial Communication which is an "umbrella" or general term. There isn't a subterm / protocol name. So we say: UART uses Asynchronous Serial Communication. How it works for UART is shown in the steps a bit further down.

The "Asynchronous" part was explained in the first paragraphs. "Serial Communication" means sending one bit after another; that is, in series.

• Example: Sending data from the board to the laptop. This is what happens at a high level:

  1. The sending-UART's Tx line uses START and END signals to mark when the communication starts and ends.
  2. A UART device reads a byte from the data bus.
  3. It serialises it, and wraps it with metadata bits. The whole item is now called a frame.
  4. Then sends the bits one-by-one to the other device (through the Tx line).

The other UART receives the series of bits (in the Rx line), uses the wrapping bits and places the data bits in the data bus.

Since there isn't a clock line, the transmission rate must be set as a parameter, called the baud rate, in bits per second.

Besides the baud rate, the data, the parity, and the stop bits must be set equal on each device. A mismatch of speed or configuration bits will cause the devices to decode information incorrectly.

As an example:

From Asynchronous Communication by Plugwash (Public Domain)

And the description of the image (slightly modified):

In this diagram, two bytes are sent, each consisting of a start bit, followed by eight data bits (bits 0-7), and one stop bit, for a 10-bit character frame.

The last data bit is sometimes used as a parity bit.

The number of data and formatting bits, the order of data bits, the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed (frequency) must be pre-agreed by the communicating parties.

The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter.

At the end of each character, the receiver stops briefly to wait for the next start bit. It is this difference which keeps the transmitter and receiver synchronized.

Speed Calculation

With a set up like:

• 8 data bits, 1 parity, and 1 stop bit (10 bits)
  • Note: this is called a frame.
• baud rate 115200 bits/second

It will send 11520 frames per second. Since it includes 1 byte per frame (10 bits), it's also 11520 byte per second. Or 11520/1024 to get it in KiB (about 11 KiB/s). That's about 1 short essay per second.

Suggested Reading

• A bit more on UART: this is a short article, only looks long due to the comments section.