The write protect (WP) pin disables any changes to the
scratchpad register contents, except for the EEMEM setting,
which can still be restored using Instruction 1, Instruction 8,
and the PR pulse. Therefore, WP can be used to provide a
hardware EEMEM protection feature.
DIGITAL INPUT AND 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 VDD, if they are not used. No
internal pull-up resistors are present on any digital input pins.
To avoid floating digital pins that might cause false triggering
in a noisy environment, add pull-up resistors. This is applicable
when the device is detached from the driving source when it is
The SDO and RDY pins are open-drain digital outputs that only
need pull-up resistors if these functions are used. To optimize
the speed and power trade-off, use 2.2 kΩ pull-up resistors.
The equivalent serial data input and output logic is shown in
Figure 38. 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 39 and Figure 40.
Figure 38. Equivalent Digital Input and Output Logic
Figure 40. Equivalent WP Input Protection
SERIAL DATA INTERFACE
The AD5235 contains a 4-wire SPI-compatible digital interface
(SDI, SDO, CS, and CLK). The 24-bit serial data-word must be
loaded with MSB first. The format of the word is shown in Table 6.
The command bits (C0 to C3) control the operation of the digital
potentiometer according to the command shown in Table 7. A0
to A3 are the address bits. A0 is used to address RDAC1 or RDAC2.
Address 2 to Address 14 are accessible by users for extra EEMEM.
Address 15 is reserved for factory usage. Table 9 provides an
address map of the EEMEM locations. D0 to D9 are the values
for the RDAC registers. D0 to D15 are the values for the EEMEM
The AD5235 has an internal counter that counts a multiple of
24 bits (a frame) for proper operation. For example, AD5235
works with a 24-bit or 48-bit word, but it cannot work properly
with a 23-bit or 25-bit word. 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 but remains in the register if it
is multiple of four. In addition, the AD5235 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 pattern.
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, MC68HC16R1, and MC68HC916R1.
Figure 39. Equivalent ESD Digital Input Protection
Rev. E | Page 17 of 32