AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 15

• Manufacturer Part Number: AD9874BST
• Description: IF Digitizing Subsystem
• Manufacturer: AD [Analog Devices]

SYNCHRONOUS SERIAL INTERFACE (SSI)
The AD9874 provides a high degree of programmability of its
SSI output data format, control signals, and timing parameters
to accommodate various digital interfaces. In a 3-wire digital
interface, the AD9874 provides a frame sync signal (FS), a
clock output (CLKOUT), and a serial data stream (DOUTA)
signal to the host device. In a 2-wire interface, the frame sync
information is embedded into the data stream, thus only a
CLKOUT and DOUTA output signal are provided to the host
device. The SSI control registers are SSICRA, SSICRB, and
SSIORD. Table III shows the different bit fields associated with
these registers.
The primary output of the AD9874 is the converted I and Q
demodulated signal available from the SSI port as a serial bit
stream contained within a frame. The output frame rate is equal
to the modulator clock frequency (f
filter’s decimation factor that is programmed in the Decimator
Register (0x07). The bit stream consists of an I word followed
by a Q word, where each word is either 24 bits or 16 bits long
and is given MSB first in two’s complement form. Two optional
bytes may also be included within the SSI frame following the
Q word. One byte contains the AGC attenuation and the other
byte contains both a count of modulator reset events and an
estimate of the received signal amplitude (relative to full scale
of the AD9874’s ADC). Figure 2 illustrates the structure of the
SSI data frames in a number of SSI modes.
The two optional bytes are output if the EAGC bit of SSICRA
is set. The first byte contains the 8-bit attenuation setting (0 = no
attenuation, 255 = 24 dB of attenuation), while the second byte
contains a 2-bit reset field and 6-bit received signal strength
signal field. The reset field contains the number of modulator
reset events since the last report, saturating at 3. The received
signal strength (RSSI) field is a linear estimate of the signal
strength at the output of the first decimation stage; 60 corresponds
to a full-scale signal.
REV. 0
16-Bit I AND Q, EAGC = 1, AAGC = 1:40 DATA BITS
16-Bit I AND Q, EAGC = 1, AAGC = 0:48 DATA BITS
16-Bit I AND Q, EAGC = 0, AAGC = X:32 DATA BITS
24-Bit I AND Q, EAGC = 0, AAGC = X: 48 DATA BITS
24-Bit I AND Q, EAGC = 1, AAGC = 0:64 DATA BITS
I (15:0)
I (15:0)
I (15:0)
I (15:0)
I (24:0)
I (24:0)
Figure 2. SSI Frame Structure
RESET COUNT
Q (15:0)
Q (15:0)
Q (15:0)
Q (15:0)
ATTN (7:1)
ATTN (7:0)
Q (24:0)
Q (24:0)
SSI(5:1)
CLK
0
1
) divided by the digital
SSI(5:0)
RESET COUNT
ATTN (7:0)
SSI(5:0)
–15–
The two optional bytes follow the I and Q data as a 16-bit word
providing that the AAGC bit of SSICRA is not set. If the
AAGC bit is set, the two bytes follow the I and Q data in an
alternating fashion. In this alternate AGC data mode, the LSB
of the byte containing the AGC attenuation is a 0, while the LSB
of the byte containing reset and RSSI information is always a 1.
In a 2-wire interface, the embedded frame sync bit (EFS) within
the SSICRA Register is set to 1. In this mode, the framing infor-
mation is embedded in the data stream with each eight bits of
data surrounded by a start bit (low) and a stop bit (high), and
each frame ends with at least 10 high bits. FS remains either
low or three-stated (default) depending on the state of the
SFST bit. Other control bits can be used to invert the frame
sync (SFSI), to delay the frame sync pulse by one clock
period (SLFS), to invert the clock (SCKI), or to three-state the
clock (SCKT). Note that if EFS is set, SLFS is a don’t care.
The SSIORD Register controls the output bit rate (f
the serial bit stream. f
clock frequency (f
f
f
spurs within the pass band of the target signal. Users must verify
that the output bit rate is sufficient to accommodate the required
number of bits per frame for a selected word size and decimation
factor. Idle (high) bits are used to fill out each frame.
CLK
CLKOUT
AAGC
EAGC
EFS
SFST
SFSI
SLFS
SCKT
SCKI
DW
DS
Name Width Default
4_SPI
DIV
SSICRA (ADDR = 0x18)
SSICRB (ADDR = 0x19)
SSIORD (ADDR = 0x1A)
divided by the contents of the SSIORD Register. Note,
should be chosen such that it does not introduce harmful
4
3
1
1
1
1
1
1
1
1
1
1
Table III. SSI Control Registers
CLK
1
0
0
7
0
1
0
0
0
0
1
0
) or an integer fraction of it. It is equal to
CLKOUT
Output Bit Rate Divisor
f CLKOUT = f CLK /SSIORD
I/Q data-word width (0 = 16 bit, 1 bit–24 bit)
Automatically 16-bit when the AGCV=1
Enable 4-Wire SPI Interface for SPI Read
operation via DOUTB
FS, CLKOUT, and DOUT Drive
Strength
Alternate AGC Data Bytes
Embed AGC Data
Embed Frame Sync
Three-State Frame Sync
Invert Frame Sync
Late Frame Sync (1 = Late, 0 = Early)
Three-State CLKOUT
Invert CLKOUT
Description
can be set to equal the modulator
AD9874
CLKOUT
) of