Datasheet

Data Sheet AD5231
Rev. D | Page 15 of 28
DIGITAL INPUT/OUTPUT CONFIGURATION
All digital inputs are ESD-protected, high input impedance that
can be driven directly from most digital sources. Active at logic
low,
PR
and
WP
must be tied to V
DD
if they are not used. No
internal pull-up resistors are present on any digital input pins.
The SDO and RDY pins are open-drain digital outputs that
need pull-up resistors only if these functions are used. A resistor
value in the range of 1 kΩ to 10 kΩ is a proper choice that
balances the dissipation and switching speed.
The equivalent serial data input and output logic is shown in
Figure 36. The open-drain output SDO is disabled whenever
chip-select
CS
is in logic high. ESD protection of the digital
inputs is shown in Figure 37 and Figure 38.
COUNTER
SERIAL
REGISTER
CLK
SDI
5V
R
PULL-UP
SDO
GND
PR WP
AD5231
CS
COMMAND
PROCESSOR
AND ADDRESS
DECODE
VALID
COMMAND
02739-035
Figure 36. Equivalent Digital Input-Output Logic
LOGIC
PINS
V
DD
GND
INPUT
300Ω
02739-036
Figure 37. Equivalent ESD Digital Input Protection
GND
WP
V
DD
INPUT
300Ω
02739-037
Figure 38. Equivalent
WP
Input Protection
SERIAL DATA INTERFACE
The AD5231 contains a 4-wire SPI-compatible digital interface
(SDI, SDO,
CS
, and CLK). It uses a 24-bit serial data-word
loaded MSB first. The format of the SPI-compatible word is
shown in Table 6. The chip-select
CS
pin must be held low until
the complete data-word is loaded into the SDI pin. When
CS
returns high, the serial data-word is decoded according to the
instructions in
Table 7. The command bits (Cx) control the
operation of the digital potentiometer. The address bits (Ax)
determine which register is activated. The data bits (Dx) are the
values that are loaded into the decoded register.
The AD5231 has an internal counter that counts a multiple of
24 bits (a frame) for proper operation. For example, AD5231
works with a 48-bit word, but it cannot work properly with a
23-bit or 25-bit word. In addition, AD5231 has a subtle feature
that, if
CS
is pulsed without CLK and SDI, the part repeats the
previous command (except during power-up). As a result, care
must be taken to ensure that no excessive noise exists in the
CLK or
CS
line that might alter the effective number of bits
(ENOB) pattern. Also, to prevent data from mislocking (due
to noise, for example), the counter resets if the count is not a
multiple of four when
CS
goes high.
The SPI interface can be used in two slave modes: CPHA = 1,
CPOL = 1 and CPHA = 0, CPOL = 0. CPHA and CPOL refer to
the control bits that dictate SPI timing in the following
MicroConverters® and microprocessors: ADuC812/ADuC824,
M68HC11, and MC68HC16R1/916R1.
DAISY-CHAIN OPERATION
The serial data output pin (SDO) serves two purposes. It can be
used to read the contents of the wiper setting and EEMEM
values using Instruction 10 and Instruction 9, respectively. The
remaining instructions (0 to 8, 11 to 15) are valid for daisy-
chaining multiple devices in simultaneous operations. Daisy-
chaining minimizes the number of port pins required from
the controlling IC (see Figure 39). The SDO pin contains an
open-drain N-Ch FET that requires a pull-up resistor if this
function is used. As shown in Figure 39, users need to tie the
SDO pin of one package to the SDI pin of the next package.
Users might need to increase the clock period, because the
pull-up resistor and the capacitive loading at the SDO to SDI
interface might require additional time delay between sub-
sequent packages. When two AD5231s are daisy-chained,
48 bits of data are required. The first 24 bits go to U2 and the
second 24 bits go to U1. The
CS
should be kept low until all
48 bits are clocked into their respective serial registers. The
CS
is then pulled high to complete the operation.
SDI SDO
CLK
R
P
2kΩ
µC
SDI SDO
CLK
U1 U2
AD5231 AD5231
CS
CS
+V
02739-038
Figure 39. Daisy-Chain Configuration Using SDO