3

FN8195.0

March 7, 2005

PIN NAMES

PRINCIPLES OF OPERATION

The

X9420

is

a

highly

integrated

microcircuit

incorporating a resistor array and associated registers

and counter and the serial interface logic providing

direct communication between the host and the XDCP

potentiometer.

Serial Interface

The X9420 supports the SPI interface hardware

conventions. The device is accessed via the SI input

with data clocked in on the rising SCK. CS must be

LOW and the HOLD and WP pins must be HIGH

during the entire operation.

The SO and SI pins can be connected together, since

they have three state outputs. This can help to reduce

system pin count.

Array Description

The X9420 is comprised of one resistor array

containing 63 discrete resistive segments that are

connected in series. The physical ends of each array

are equivalent to the fixed terminals of a mechanical

At both ends of the array and between each resistor

segment is a CMOS switch connected to the wiper

switch may be turned on at a time.

These switches are controlled by a Wiper Counter

Register (WCR). The six bits of the WCR are decoded to

select, and enable, one of sixty-four switches. The block

diagram of the potentiometer is shown in Figure 1.

Wiper Counter Register (WCR)

The X9420 contains a Wiper Counter Register. The

WCR can be envisioned as a 6-bit parallel and serial

load counter with its outputs decoded to select one of

sixty-four switches along its resistor array. The

contents of the WCR can be altered in four ways: it

may be written directly by the host via the Write Wiper

Counter Register instruction (serial load); it may be

written indirectly by transferring the contents of one of

four associated data registers via the XFR Data

Register instruction (parallel load); it can be modified

one step at a time by the Increment/ Decrement

instruction. Finally, it is loaded with the contents of its

data register zero (DR0) upon power-up.

The Wiper Counter Register is a volatile register; that

is, its contents are lost when the X9420 is powered-

down. Although the register is automatically loaded

with the value in DR0 upon power-up, this may be

different from the value present at power-down.

Data Registers

The potentiometer has four 6-bit nonvolatile Data

Registers. These can be read or written directly by the

host. Data can also be transferred between any of the

four Data Registers and the WCR. It should be noted all

operations changing data in one of the Data Registers is

a nonvolatile operation and will take a maximum of 10ms.

If the application does not require storage of multiple

settings for the potentiometer, the Data Registers can

be used as regular memory locations for system

parameters or user preference data.

Register Descriptions

Table 1. Data Registers, (6-bit), Nonvolatile

There are four 6-bit Data Registers associated with the

potentiometer.

– {D5~D0}: These bits are for general purpose Non-

volatile data storage or for storage of up to four dif-

ferent wiper values.

Table 2. Wiper Counter Register, (6-bit), Volatile

– {WP5~WP0}: These bits specify the wiper position

of the potentiometer.

Symbol

Description

SCK

Serial Clock

SI, SO

Serial Data

A0

Device Address

Potentiometer Pins (terminal equivalent)

Potentiometer Pins (wiper equivalent)

WP

Hardware Write Protection

HOLD

Serial Communication Pause

V+,V-

Analog Supplies

System Supply Voltage

System Ground

NC

No Connection

0

0

D5

D4

D3

D2

D1

D0

(MSB)

(LSB)

0

0

WP5 WP4 WP3 WP2 WP1 WP0

(MSB)

(LSB)

X9420