UART

What is it?

UART stands for Universal Asynchronous Receiver-Transmitter is a hardware device for communicating over a serial connection.

Universal – That one’s obvious

Asynchronous – Because data is transferred without an external clock

Receiver/Transmitter – Because 2 way communication is possible.

Essentially a UART takes parallel data and turns it into a serial stream sequentially in order to transmit the data, once it arrives it’s then re-ordered back into parallel data, the key here is that it allows data to be sent across a single wire (or whatever medium) rather than setting up parallel communication across multiple wires.

What that means in real terms is that by connecting to a UART you have a way of communicating directly with a device.

How to find it?

The first step is to take a look on the board for some pins that look like they might be a UART, often you’ll be looking for 4 pins in a line.

These will most likely be:

• VCC
• GND
• TX
• RX

Now we have something that looks like it might be what we’re looking for we can start testing to see whether we’re in luck.

The pin on the right is labelled as pin 1, so from now on we can refer to them in that order, 4321 left to right.

Sometimes a visual inspection can shed some light onto the situation, often you’ll be able to see traces on the pins, as a general rule a thick line indicates power and a thin line indicates data.

In our example it’s pretty hard to see anything although pin 3 does seem to have an ‘X’ shape which may indicate that it’s a GND, we can use a multimeter to help identify the pins to work out whether this is likely to be a UART, and if so what pin is what.

The first step is to find the ground, the easiest way to do this is to use the multimeter’s continuity tester, we’ll place one probe on a metal shield on the board to act as a ground and place the other probe on each pin in turn, if the continuity tester makes a continuous tone then that pin must be GND, it turns out that our hunch was correct and pin 3 is the GND.

 

Next we’ll power the device up and change the multimeter setting to read voltage, with one probe on the ground, either the metal shield or the ground pin we already identified we can probe the other pins.

The VCC pin should have a constant reading of 3.3v or 5v depending on the operating voltage of the device, here we can see the VCC pin has been identified at 3.3v.

Identifying the TX and RX pin is slightly more difficult, and differentiating is harder still. Often it can be done by measuring the voltage, since the TX pin is sending data it is effectively going from 0 to 3.3v over and over again, using a multimeter this will often be represented as an average voltage of something between 1.5 and 2.5v.

Following the same principal, the RX pin should be at 0v effectively waiting for a signal input, in this instance however both remaining pins measure at around 2.5v so we’ll just use some trial and error to determine which is which.

*After some trial and error with the TX/RX it was determined that the order of pins is:

1 – TX

2 – RX

3 – GND

3 – VCC