AD5370
Rev. 0 | Page 21 of 28
SERIAL INTERFACE
The AD5370 contains a high speed SPI-compatible serial interface
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 con-
trolled 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 AD5370 allows writing of data via the serial interface to
every register directly accessible to the serial interface, which is
all registers except the X2A and X2B registers and the DAC
registers. The X2A and X2B registers are updated when the user
writes to the X1A, X1B, M, or C register, and the DAC registers
are updated by
LDAC
.
The serial word (see
Table 10) is 24 bits long; 16 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.
The serial interface works with both a continuous and a burst
(gated) serial clock. Serial data applied to SDI is clocked into
the AD5370 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
AD5370. 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 AD5370 allows data readback via the serial interface from
every register directly accessible to the serial interface, which is
all registers except the X2A, X2B, and DAC registers. To read
back a register, it is first necessary to tell the AD5370 which
register to 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
5
(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 or 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 the 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 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.
CHANNEL ADDRESSING AND SPECIAL MODES
If the mode bits are not 00, the data-word for D13 to D0 is
written to the device. Address Bit A5 to Address Bit A0
determine which channels are written to, whereas the mode bits
determine the register (X1A, X1B, C, or M) to which the data is
written, as shown in
Table 9. If data is to be written to the X1A or
X1B register, the setting of the
A
/B bit in the control register
determines the register to which the data is written (that is,
0 → X1A, 1 → X1B).
Table 9. Mode Bits
M1 M0 Action
1 1
Writes to the DAC input data (X) register,
depending on the control register
A/B bit
1 0 Writes to the DAC offset (C) register
0 1 Writes to the DAC gain (M) register
0 0
Special function, used in combination
with other bits of the data-word
Table 10. 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
