X9221
DETAILED OPERATION
The WCR is a volatile register; that is, its contents are
lost when the X9221 is powered-down. Although the
register is automatically loaded with the value in R0
upon power-up, it should be noted this may be different
from the value present at power-down.
Both XDCP potentiometers share the serial interface
and share a common architecture. Each potentiometer
is comprised of a resistor array, a wiper counter regis-
ter and four data registers. A detailed discussion of the
register organization and array operation follows.
Data Registers
Each potentiometer has four nonvolatile data registers.
These can be read or written directly by the host and
data can be transferred between any of the four data
registers and the WCR. It should be noted all opera-
tions changing data in one of these registers is a non-
volatile operation and will take a maximum of 10ms.
ꢀiper Counter Register
The X9221 contains two wiper counter registers
(WCR), one for each XDCP potentiometer. 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 WCR 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 con-
tents of its data register zero (R0) upon power-up.
If the application does not require storage of multiple
settings for the potentiometer, these registers can be
used as regular memory locations that could possibly
store system parameters or user preference data.
Figure 8. Detailed Potentiometer Block Diagram
Serial Data Path
Serial
Bus
Input
V /R
H
H
From Interface
Circuitry
Register 0
Register 2
Register 1
Register 3
Parallel
Bus
Input
8
6
C
o
u
n
t
e
r
Wiper
Counter
Register
D
e
c
o
d
e
INC/DEC
Logic
If WCR = 00[H] then V /R = V /R
L
W
W
L
UP/DN
UP/DN
If WCR = 3F[H] then V /R = V /R
H
W
W
H
V /R
L
Modified SCL
L
CLK
V
/R
W
W
Characteristics subject to change without notice. 7 of 15
REV 1.1 8/8/02
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