User manual

LPCXpresso Experiment Kit - User’s Guide

Page 100

7.15 Work with a Serial Bus – UART

In this experiment you will learn how to work with the Universal Asynchronous Receiver/Transmitter, or

UART for short. The term asynchronous refers to the fact that no explicit clock signal is transmitted.

The transmitter and receiver must agree beforehand on the bit rate, i.e., how long time a transmitted bit

shall take. The idle state (no transmission) is a high signal. Transmission begins with a start bit, which

is low. The negative edge is detected by the receiver and 1.5 bit periods after this, bit sampling begins.

Eight data bits are sampled. The least significant bit (LSB) is typically transmitted first. An optional

parity bit is then transmitted (for error checking of the data bits). Often this bit is omitted if the

transmission channel is assumed to be noise free or if there are error checking higher up in the

protocol layers. The transmission is ended by a stop bit. Typically one bit, but 1.5 and 2 bits are

sometimes also used. Most common for inter-board communication is 8N1, meaning 8 data bits, no

parity and one stop bit.

Figure 57 – UART Communication

On a side note, there are methods to determine the bit rate of a received signal but that is out of scope

for this experiment.

An UART channel consists of two signals (besides ground):

 TXD: transmit data, direction from transmitter to receiver. This is an output.

 RXD: receive data, direction from transmitter to receiver. This is an input.

TXD and RXD are crossed between transmitter and receiver, i.e., TXD is connected to RXD and vice

versa. For more information about asynchronous serial communication, see

http://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter and

http://en.wikipedia.org/wiki/Asynchronous_serial_communication.

Note and understand the difference between the signaling method (asynchronous serial

communication) and standards of voltage signaling. The signal drawn in Figure 57illustrates the signal

to/from the UART peripheral inside the LPC111x. It is a 3.3V logic signal. This is common for

communication between units on the same board, or closely mounted boards. RS232 is a common

signaling standard with large voltage swings (+- 3-15V) that is used between units that are physically

apart. RS422 and RS485 are other commonly used signaling standards.

Communication is normally point-to-point, meaning that a transmitter sends data to one receiver. There

are signaling standards that also supports network topologies (for example RS422 and RS485). Higher

protocol layers must then be involved in implementing addressing schemes between the nodes.

Note that this experiment requires a UART-to-USB cable from FTDI (TTL-232R-3V3, Digikey: 768-

1015-ND or Mouser: 895-TTL-232R-3V3). This cable is a bridge between a UART channel and USB

communication. Via USB it creates a virtual COM port on a PC. The UART communication is tunneled

over USB to the PC. When plugging in the USB connector on a PC a driver will be installed. See

FTDI’s installation guides for details how to install the driver for different operating systems:

http://www.ftdichip.com/Support/Documents/InstallGuides.htm

Start D0 D1 D2 D3 D4 D5 D6 D7 Parity Stop

bit (lsb) (msb) bit bit

TXD

Bits

Sampling

1.5 bit 1.0 bit etc.

Duration of one byte (10-12 bit periods)