X9421 datasheet(5/20 Pages) RENESAS

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X9421

FN8196 Rev.1.00

Page 5 of 20

January 14, 2009

communication with the X9421. A maximum of two devices

may occupy the SPI serial bus.

Potentiometer Pins

VH/RH, VL/RL

The VH/RH and VL/RL inputs are equivalent to the terminal

connections on either end of a mechanical potentiometer.

VW/RW

The wiper output is equivalent to the wiper output of a

mechanical potentiometer.

HARDWARE WRITE PROTECT INPUT (WP)

The WP pin when LOW prevents nonvolatile writes to the Data

Registers. Writing to the Wiper Counter Register is not

restricted.

SYSTEM/DIGITAL SUPPLY (VCC)

VCC is the supply voltage for the system/digital section. VSS is

the system ground.

Principles of Operation

The X9421 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 X9421 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 X9421 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 potentiometer (VH/RH and VL/RL

inputs).

At both ends of the array and between each resistor segment

is a CMOS switch connected to the wiper (VW/RW) output.

Within the individual array only one 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 X9421 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 X9421 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.

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 Nonvolatile

data storage or for storage of up to four different wiper

values.

TABLE 2. WIPER COUNTER REGISTER, (6-BIT), VOLATILE

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

potentiometer.

(MSB)

(LSB)

WP5WP4

WP3WP2

WP1WP0

(MSB)

(LSB)

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