4

FN8164.1

September 15, 2005

Principles of Operation

The X9241A is a highly integrated microcircuit incorporating

four resistor arrays, their associated registers and counters

and the serial interface logic providing direct communication

between the host and the XDCP potentiometers.

Serial Interface

The X9241A supports a bidirectional bus oriented protocol.

The protocol defines any device that sends data onto the

bus as a transmitter and the receiving device as the receiver.

The device controlling the transfer is a master and the

device being controlled is the slave. The master will always

initiate data transfers and provide the clock for both transmit

and receive operations. Therefore, the X9241A will be

considered a slave device in all applications.

Clock and Data Conventions

Data states on the SDA line can change only during SCL

are reserved for indicating start and stop conditions.

Start Condition

All commands to the X9241A are preceded by the start

condition, which is a HIGH to LOW transition of SDA while

SDA and SCL lines for the start condition and will not

respond to any command until this condition is met.

Stop Condition

All communications must be terminated by a stop condition,

which is a LOW to HIGH transition of SDA while SCL is

HIGH.

Acknowledge

Acknowledge is a software convention used to provide a

positive handshake between the master and slave devices

on the bus to indicate the successful receipt of data. The

transmitting device, either the master or the slave, will

release the SDA bus after transmitting eight bits. The master

generates a ninth clock cycle and during this period the

receiver pulls the SDA line LOW to acknowledge that it

successfully received the eight bits of data. See Figure 7.

The X9241A will respond with an acknowledge after

recognition of a start condition and its slave address and

once again after successful receipt of the command byte. If

the command is followed by a data byte the X9241A will

respond with a final acknowledge.

Array Description

The X9241A is comprised of four resistor arrays. Each array

contains 63 discrete resistive segments that are connected

in series. The physical ends of each array are equivalent to

At both ends of each array and between each resistor

output. Within each individual array only one switch may be

turned on at a time. These switches are controlled by the

Wiper Counter Register (WCR). The six least significant bits

of the WCR are decoded to select, and enable, one of sixty-

four switches.

The WCR may be written directly, or it can be changed by

transferring the contents of one of four associated Data

Registers into the WCR. These Data Registers and the WCR

can be read and written by the host system.

Device Addressing

Following a start condition the master must output the

address of the slave it is accessing. The most significant four

bits of the slave address are the device type identifier (refer

to Figure 1 below). For the X9241A this is fixed as 0101[B].

The next four bits of the slave address are the device

address. The physical device address is defined by the state

of the A0-A3 inputs. The X9241A compares the serial data

stream with the address input state; a successful compare of

all four address bits is required for the X9241A to respond

with an acknowledge.

Acknowledge Polling

The disabling of the inputs, during the internal nonvolatile

write operation, can be used to take advantage of the typical

5ms EEPROM write cycle time. Once the stop condition is

issued to indicate the end of the nonvolatile write command

the X9241A initiates the internal write cycle. ACK polling can

be initiated immediately. This involves issuing the start

condition followed by the device slave address. If the

X9241A is still busy with the write operation no ACK will be

returned. If the X9241A has completed the write operation

an ACK will be returned and the master can then proceed

with the next operation.

1

0

0

A3

A2

A1

A0

Device Type

Identifier

Device Address

1

FIGURE 1. SLAVE ADDRESS

X9241A