REV. 0

AD9975

–15–

DIGITAL INTERFACE PORT TIMING

The ADIO[9:0] bus accepts input data-words into the transmit

path when the TXEN pin is high, the RXEN pin is low, and a

clock is present on the TXCLK pin. Figure 3 illustrates the

transmit path input timing.

TXCLK

TXEN

ADIO[9:0]

RXEN

TX0

TX1

TX2

TX3

TX4

Figure 3. Transmit Data Input Timing Diagram

It should be noted that to clear the transmit path input buffers,

an additional six clock cycles on the TXCLK input are required

after TXEN goes low. The interpolation filters will be “flushed”

with zeros if the clock signal into the TXCLK pin is present for

48 clock cycles after TXEN goes low (the data on the ADIO

bus being irrelevant over this interval).

The output from the receive path will be driven onto the

ADIO[9:0] bus when the RXEN pin is high, and a clock is present

on the RXCLK pin. When both TXEN and RXEN are low, the

ADIO[9:0] bus is three-stated. Figure 4 illustrates the receive

path output timing.

RXCLK

RXEN

ADIO[9:0]

RX0

RX1

RX2

RX3

Figure 4. Receive Data Output Timing Diagram

SERIAL INTERFACE FOR REGISTER CONTROL

The serial port is a 3-wire serial communications port consisting

of a clock (SCLK), chip select (

SENABLE), and a bidirectional

data (SDATA) signal. The interface allows read/write access to

all registers that configure the AD9975 internal parameters.

Single or multiple byte transfers are supported as well as MSB

first or LSB first transfer formats.

General Operation of the Serial Interface

Serial communication over the serial interface can be from 1 byte

to 5 bytes in length. The first byte is always the instruction byte.

The instruction byte establishes whether the communication is

going to be a read or write access, the number of data bytes to be

transferred, and the address of the first register to be accessed.

The instruction byte transfer is complete immediately upon the

eighth rising edge of SCLK after

SENABLE is asserted. Likewise,

the data registers change immediately upon writing to the eighth

bit of each data byte.

Instruction Byte

The instruction byte contains the information shown in Table III.

Table III. Instruction Byte Bit Definitions

MSB

LSB

I7

I6

I5

I4

I3

I2

I1

I0

R/W

N1

N0

A4

A3

A2

A1

A0

Bit I7 – R/W

This bit determines whether a read or a write data transfer will

occur after the instruction byte write. Logic high indicates a read

operation, and Logic 0 indicates a write operation.

Bits I6:I5 – N1:N0

These two bits determine the number of bytes to be transferred

during the data transfer cycle. The bit decodes are shown in

Table IV.

Table IV. N1:N0 Bit Map

N1:N0

Description

0:0

Transfer 1 Byte

0:1

Transfer 2 Bytes

1:0

Transfer 3 Bytes

1:1

Transfer 4 Bytes

Bits I4:I0 - A4:A0

These bits determine which register is accessed during the data

transfer portion of the communications cycle. For multibyte

transfers, this address is the starting byte address. The remaining

register addresses are generated by the AD9975.

Serial Interface Port Pin Description

SCLK—Serial Clock

The serial clock pin is used to synchronize data transfers to and

from the AD9975 and to run the internal state machines. SCLK

maximum frequency is 25 MHz. All data transmitted to the

AD9975 is sampled on the rising edge of SCLK. All data read

from the AD9975 is validated on the rising edge of SCLK and is

updated on the falling edge.

SENABLE—Serial Interface Enable

The

SENABLE Pin is active low. It enables the serial communi-

cation to the device.

SENABLE select should stay low during

the entire communication cycle. All input on the serial port is

ignored when

SENABLE is inactive.

SDATA—Serial Data I/O

The signal on this line is sampled on the first eight rising edges

of SCLK after

SENABLE goes active. Data is then read from or

written to the AD9975 depending on what was read.

Figures 5 and 6 show the timing relationships between the three

SPI signals.

INSTRUCTION BIT 7

INSTRUCTION BIT 6

SENABLE

SCLK

SDATA

Figure 5. Timing Diagram Register Write to AD9975