AD9956SLASHPCB datasheet(23/32 Pages) AD

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AD9956

Rev. 0 | Page 23 of 32

INSTRUCTION BYTE

The instruction byte contains the following information:

Table 4.

R/Wb

R/Wb—Bit 7 of the instruction byte determines whether a read

or write data transfer occurs after the instruction byte write.

Logic 1 indicates a read operation. Logic 0 indicates a write

operation.

X, X—Bits 6 and 5 of the instruction byte are Don’t Care.

A4 to A0—Bits 4 to 0 of the instruction byte determine which

communications cycle.

SERIAL INTERFACE PORT PIN DESCRIPTION

SCLK—Serial Clock. The serial clock pin is used to synchronize

data to and from the AD9956 and to run the internal state

machines. The SCLK maximum frequency is 25 MHz.

CS—Chip Select Bar. CS is an active low input that allows more

than one device on the same serial communications line. The

SDO and SDI/O pins go to a high impedance state when this

input is high. If driven high during any communications cycle,

that cycle is suspended until CS is reactivated low. Chip select

can be tied low in systems that maintain control of SCLK.

SDI/O—Serial Data Input/Output. Data is always written to the

AD9956 on this pin. However, this pin can be used as a bidirec-

tional data line. CFR1<7> controls the configuration of this pin.

The default value (0) configures the SDI/O pin as bidirectional.

SDO—Serial Data Out. Data is read from this pin for protocols

that use separate lines for transmitting and receiving data. When

the AD9956 operates in a single bidirectional I/O mode, this pin

does not output data and is set to a high impedance state.

I/O_RESET—A high signal on this pin resets the I/O port state

machines without affecting the addressable registers’ contents.

An active high input on the I/O_RESET pin causes the current

communication cycle to abort. After I/O_RESET returns low

(0), another communication cycle can begin, starting with the

instruction byte write. Note that when not in use, this pin

should be forced low, because it floats to the threshold value.

MSB/LSB TRANSFERS

The AD9956 serial port can support both most significant bit

(MSB) first or least significant bit (LSB) first data formats. This

functionality is controlled by the LSB first bit in Control

(MSB first). When CFR1 <15> is set high, the AD9956 serial

port is in LSB first format. The instruction byte must be written

in the format indicated by CFR1 <15>. If the AD9956 is in LSB

first mode, the instruction byte must be written from least

significant bit to most significant bit. However, the instruction

byte phase of the communications cycle still precedes the data

transfer cycle.

For MSB first operation, all data written to (read from) the

AD9956 are in MSB first order. If the LSB mode is active, all

data written to (read from) the AD9956 are in LSB first order.

SCLK

SDI/O

TPRE

TDSU

TSCLKW

TDHLD

SECOND BIT

FIRST BIT

SYMBOL

TPRE

TSCLKW

TDSU

TDHLD

MIN

6ns

40ns

6.5ns

0ns

DEFINITION

CS SETUP TIME

PERIOD OF SERIAL DATA CLOCK (WRITE)

SERIAL DATA SETUP TIME

SERIAL DATA HOLD TIME

Figure 33. Timing Diagram for Data Write to AD9956

TDV

FIRST BIT

SECOND BIT

SDI/O

SDO

SCLK

SYMBOL

TDV

TSCLKR

MAX

40ns

400ns

DEFINITION

DATA VALID TIME

PERIOD OF SERIAL DATA CLOCK (READ)

TSCLKR

Figure 34. Timing Diagram for Data Read to AD9956

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