DATA COMMUNICATIONS APPLICATION NOTE

DAN 200

APRIL 2009

MULTIDROP/9-BIT MODE FEATURE

1.0 INTRODUCTION

This application note describes the Multidrop/9-bit mode of Exar’s low power and high speed XR16Mxxx single

channel UART family.

2.0 MULTIDROP/9-BIT MODE

A multidrop system is a master and slave system. One master connects with a few slaves in the system. Each

time, the master communicates with one of the slaves. When the master wants to transfer a block of data to a

slave, it first sends out an address byte to identify the target slave. The 9th-bit of the data byte sent from the

master is set to 1 to indicate the address byte while cleared to 0 to indicate the data byte. All the slave systems

will compare the address byte with their own address. Only the target slave will respond to the master. The

master then starts transmitting data bytes to the target slave. The non-addressed slave systems will ignore the

incoming data until a new address byte is received. Figure 1 shows an example of the master and slave sys-

tem block diagram.

Master

Slave

1

Slave

2

Slave

3

Slave

N

Since UARTs are 8-bit devices, the parity bit is used as the 9th-bit. Thus, the master needs to set parity bit be-

fore it sends out data and the slave needs to check the parity bit to decide whether it is an address byte or a

data byte. The XR16Mxxx single channel UART family provides two multidrop mode to simplify the software.