AD9874ABST datasheet(16/40 Pages) AD

16 / 40 page

REV. A

AD9874

–16–

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

CLKOUT and DOUTA output signals 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 (fCLK) divided by the digital

filter’s decimation factor that is programmed in the Decimator

by a Q word, where each word is either 24 bits or 16 bits long

and is given MSB first in twos complement form. Two optional

bytes may also be included within the SSI frame following the

Qword. 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.

16-Bit I AND Q, EAGC = 0, AAGC = X:32 DATA BITS

16-Bit I AND Q, EAGC = 1, AAGC = 0:48 DATA BITS

16-Bit I AND Q, EAGC = 1, AAGC = 1:40 DATA BITS

I (15:0)

Q (15:0)

I (15:0)

Q (15:0)

ATTN (7:0)

I (15:0)

Q (15:0)

I (15:0)

Q (15:0)

RESET COUNT

I (24:0)

Q (24:0)

24-Bit I AND Q, EAGC = 0, AAGC = X: 48 DATA BITS

ATTN (7:1)

SSI(5:1)

SSI(5:0)

I (24:0)

Q (24:0)

SSI(5:0)

RESET COUNT

ATTN (7:0)

24-Bit I AND Q, EAGC = 1, AAGC = 0:64 DATA BITS

Figure 2. SSI Frame Structure

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 field. The reset field contains the number of modula-

tor 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 corre-

sponds to a full-scale signal.

The two optional bytes follow the I and Q data as a 16-bit

word provided 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.

Table III. SSI Control Registers

Name Width Default

Description

SSICRA (ADDR = 0x18)

AAGC

EAGC

EFS

SFST

SFSI

SLFS

SCKT

SCKI

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.

SSICRB (ADDR = 0x19)

FS, CLKOUT, and DOUT Drive

Strength.

Output Bit Rate Divisor

fCLKOUT = fCLK/SSIORD.

SSIORD (ADDR = 0x1A)

I/Q Data-WordWidth (0 = 16 bit, 1 bit–24 bit).

Automatically 16-bit when the AGCV = 1.

DIV

4_SPI

Enable 4-Wire SPI Interface for SPI Read

operation via DOUTB.

The SSIORD register controls the output bit rate (fCLKOUT) of

the serial bit stream. fCLKOUT can be set to equal the modulator

clock frequency (fCLK) or an integer fraction of it. It is equal to

fCLK divided by the contents of the SSIORD register. Note that

fCLKOUT should be chosen such that it does not introduce harm-

ful 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.

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