Datasheet

AD5320
Rev. C | Page 12 of 20
SERIAL INTERFACE
The AD5320 has a 3-wire serial interface (
SYNC
, SCLK, and
DIN) that is compatible with SPI®, QSPI
TM
, and
MICROWIRE
TM
interface standards as well as most DSPs. See
Figure 2 for a timing diagram of a typical write sequence.
The write sequence begins by bringing the
SYNC
line low. Data
from the DIN line is clocked into the 16-bit shift register on the
falling edge of SCLK. The serial clock frequency can be as high
as 30 MHz, making the AD5320 compatible with high speed
DSPs. On the 16th falling clock edge, the last data bit is clocked
in and the programmed function is executed (that is, a change
in DAC register contents and/or a change in the mode of
operation). At this stage, the
SYNC
line can be kept low or be
brought high. In either case, it must be brought high for a
minimum of 33 ns before the next write sequence so that a
falling edge of
SYNC
can initiate the next write sequence.
Because the
SYNC
buffer draws more current when V
IN
= 2.4 V
than it does when V
IN
= 0.8 V,
SYNC
should be idled low
between write sequences for even lower power operation of the
part. As previously mentioned,
SYNC
must be brought high
again just before the next write sequence.
INPUT SHIFT REGISTER
The input shift register is 16 bits wide (see Figure 25). The first two
bits are “don’t cares.” The next two are control bits that control
which mode of operation the part is in (normal mode or any one of
three power-down modes). There is a more complete description of
the various modes in the Power-Down Modes section. The next
twelve bits are the data bits. These are transferred to the DAC
register on the 16th falling edge of SCLK.
SYNC INTERRUPT
In a normal write sequence, the
SYNC
line is kept low for at
least 16 falling edges of SCLK and the DAC is updated on the
16th falling edge. However, if
SYNC
is brought high before the
16th falling edge, then this acts as an interrupt to the write
sequence. The shift register is reset and the write sequence is
seen as invalid. Neither an update of the DAC register contents
nor a change in the operating mode occurs (see
Figure 26).
POWER-ON RESET
The AD5320 contains a power-on reset circuit that controls the
output voltage during power-up. The DAC register is filled with
zeros and the output voltage is 0 V. It remains there until a valid
write sequence is made to the DAC. This is useful in applica-
tions where it is important to know the state of the output of the
DAC while it is in the process of powering up.
DB15 (MSB) DB0 (LSB)
X PD0 D11 D10 D9 D8 D7 D6 D5 D4PD1X D3 D2 D1 D0
DATA BITS
00934-025
0
0
1
1
0
1
0
1
NORMAL OPERATION
1kΩ TO GND
100kΩ TO GND
THREE–STATE
POWER-DOWN MODES
Figure 25. Input Register Contents
DB15 DB0 DB15
SCLK
S
YNC
DIN
INVALID WRITE SEQUENCE:
SYNC HIGH BEFORE 16TH FALLING EDGE
VALID WRITE SEQUENCE, OUTPUT UPDATES
ON THE 16TH FALLING EDGE
DB0
00934-028
Figure 26.
SYNC
Interrupt Facility