UART communication

AN1753

4/22

DocID9986 Rev 2

1

UART communication

The main features of a standard UART are summarized below.

1.1

Main features

The UART offers a flexible means of full-duplex data exchange with external equipment

requiring an industry standard NRZ asynchronous serial data format.

The UART allows a very wide range of baud rates and different baud rates for transmission

and reception.

In UART communication, a minimum of only two signals are needed, one for transmission

and one for reception. No clock signal is needed as it works in asynchronous mode. Each

device has to have a Transmit Data Output pin (the PA3 pin is used in our example for the

ST7FLITE0) and a Receive Data Input pin (PA7 pin in our example). (Refer to Figure 1.)

Figure 1. ST7 UART communication set-up

You must be very careful to identify the use of each pin. A simple method is to put the device

in transmission and check with an oscilloscope if a transmission frame is present or not.

1.2

Baud rate

Transmission and reception can be driven by their own baud rate generator. However, be

aware that, for a correct communication, the receiver must have a reception baud rate

strictly equal to the transmission baud rate of the transmitter. If not, the communication will

be corrupted. As long as this condition is met, a wide range of baud rates is possible.

1.3

Frame

Any transmission is Least Significant Bit first. A data word is usually 8 bits long. A data

frame begins with a «start bit», which is a ‘0‘ bit and ends with a stop bit, which is a ‘1‘ bit

(see Figure 2).

ST7 system

RDI

TDO

Terminal

device

(TDO) PA3

(RDI) PA7

ST7FLITE0

(host)