Datasheet

AD5372/AD5373
Rev. C | Page 21 of 28
SERIAL INTERFACE
The AD5372/AD5373 contain a high speed SPI operating at
clock frequencies up to 50 MHz (20 MHz for read operations).
To minimize both the power consumption of the device and
on-chip digital noise, the interface powers up fully only when
the device is being written to, that is, on the falling edge of
SYNC
. The serial interface is 2.5 V LVTTL-compatible when
operating from a 2.5 V to 3.6 V DV
CC
supply. It is controlled by
four pins:
SYNC
(frame synchronization input), SDI (serial
data input pin), SCLK (clocks data in and out of the device),
and SDO (serial data output pin for data readback).
SPI WRITE MODE
The AD5372/AD5373 allow writing of data via the serial interface
to every register directly accessible to the serial interface, that is,
all registers except the X2A, X2B, and DAC registers. The X2A
and X2B registers are updated when writing to the X1A, X1B,
M, and C registers, and the DAC data registers are updated by
LDAC
. The serial word (see or ) is 24 bits long:
16 (AD5372) or 14 (AD5373) of these bits are data bits; six bits
are address bits; and two bits are mode bits that determine what
is done with the data. Two bits are reserved on the AD5373.
Table 11 Table 12
The serial interface works with both a continuous and a burst
(gated) serial clock. Serial data applied to SDI is clocked into
the AD5372/AD5373 by clock pulses applied to SCLK. The first
falling edge of
SYNC
starts the write cycle. At least 24 falling
clock edges must be applied to SCLK to clock in 24 bits of data
before
SYNC
is taken high again. If
SYNC
is taken high before
the 24
th
falling clock edge, the write operation is aborted.
If a continuous clock is used,
SYNC
must be taken high before the
25
th
falling clock edge. This inhibits the clock within the AD5372/
AD5373. If more than 24 falling clock edges are applied before
SYNC
is taken high again, the input data becomes corrupted. If
an externally gated clock of exactly 24 pulses is used,
SYNC
can
be taken high any time after the 24
th
falling clock edge.
The input register addressed is updated on the rising edge of
SYNC
. For another serial transfer to take place,
SYNC
must be
taken low again.
SPI READBACK MODE
The AD5372/AD5373 allow data readback via the serial
interface from every register directly accessible to the serial
interface, that is, all registers except the X2A, X2B, and DAC
data registers. To read back a register, it is first necessary to tell
the AD5372/AD5373 which register is to be read. This is achieved
by writing a word whose first two bits are the Special Function
Code 00 to the device. The remaining bits then determine which
register is to be read back.
If a readback command is written to a special function register,
data from the selected register is clocked out of the SDO pin
during the next SPI operation. The SDO pin is normally three-
stated but becomes driven as soon as a read command is issued.
The pin remains driven until the register data is clocked out.
See Figure 5 for the read timing diagram. Note that due to the
timing requirements of t
22
(25 ns), the maximum speed of the
SPI interface during a read operation should not exceed 20 MHz.
REGISTER UPDATE RATES
The value of the X2A register or the X2B register is calculated
each time the user writes new data to the corresponding X1, C,
or M register. The calculation is performed by a three-stage
process. The first two stages take approximately 600 ns each, and
the third stage takes approximately 300 ns. When the write to an
X1, C, or M register is complete, the calculation process begins.
If the write operation involves the update of a single DAC
channel, the user is free to write to another register, provided
that the write operation does not finish until the first-stage
calculation is complete (that is, 600 ns after the completion of
the first write operation). If a group of channels is being updated
by a single write operation, the first-stage calculation is repeated
for each channel, taking 600 ns per channel. In this case, the
user should not complete the next write operation until this time
has elapsed.
Table 11. AD5372 Serial Word Bit Assignment
I23 I22 I21 I20 I19 I18 I17 I16 I15 I14 I13 I12 I11 I10 I9 I8 I7 I6 I5 I4 I3 I2 I1 I0
M1 M0 A5 A4 A3 A2 A1 A0 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Table 12. AD5373 Serial Word Bit Assignment
I23 I22 I21 I20 I19 I18 I17 I16 I15 I14 I13 I12 I11 I10 I9 I8 I7 I6 I5 I4 I3 I2 I1
1
I0
1
M1 M0 A5 A4 A3 A2 A1 A0 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 0 0
1
Bit I1 and Bit I0 are reserved for future use and should be 0 when writing the serial word. These bits read back as 0.