AD7731

–13–

REV. 0

Communications Register (RS2-RS0 = 0, 0, 0)

The Communications Register is an 8-bit write-only register. All communications to the part must start with a write operation to the

Communications Register. The data written to the Communications Register determines whether the next operation is a read or

write operation, the type of read operation and to which register this operation takes place. For single-shot read or write operations,

once the subsequent read or write operation to the selected register is complete, the interface returns to where it expects a write op-

eration to the Communications Register. This is the default state of the interface, and on power-up or after a

RESET, the AD7731 is

in this default state waiting for a write operation to the Communications Register. In situations where the interface sequence is lost, a

write operation of at least 32 serial clock cycles with DIN high, returns the AD7731 to this default state by resetting the part. Table

VI outlines the bit designations for the Communications Register. CR0 through CR7 indicate the bit location, CR denoting the bits

are in the Communications Register. CR7 denotes the first bit of the data stream.

Table VI. Communications Register

7

R

C

6

R

C5

R

C4

R

C3

R

C2

R

C1

R

C0

R

C

N

E

W

O

R

E

Z1

W

R0

W

RO

R

E

Z2

S

R1

S

R0

S

R

Bit

Bit

Location

Mnemonic

Description

CR7

WEN

Write Enable Bit. A 0 must be written to this bit so the write operation to the Communica-

tions Register actually takes place. If a 1 is written to this bit, the part will not clock on to

subsequent bits in the register. It will stay at this bit location until a 0 is written to this bit.

Once a 0 is written to the

WEN bit, the next seven bits will be loaded to the Communica-

tions Register.

CR6

ZERO

A zero must be written to this bit to ensure correct operation of the AD7731.

CR5, CR4

RW1, RW0

Read Write Mode Bits. These two bits determine the nature of the subsequent read/write

operation. Table VII outlines the four options.

Table VII. Read/Write Mode

RW1

RW0

Read/Write Mode

0

0

Single Write to Specified Register

0

1

Single Read of Specified Register

1

0

Start Continuous Read of Specified Register

1

1

Stop Continuous Read Mode

With 0, 0 written to these two bits, the next operation is a write operation to the register

specified by bits RS2, RS1, RS0. Once the subsequent write operation to the specified regis-

ter has been completed, the part returns to where it is expecting a write operation to the

Communications Register.

With 0, 1 written to these two bits, the next operation is a read operation of the register specified

by bits RS2, RS1, RS0. Once the subsequent read operation to the specified register has been

completed, the part returns to where it is expecting a write operation to the Communications

Register.

Writing 1, 0 to these bits, sets the part into a mode of continuous reads from the register

specified by bits RS2, RS1, RS0. The most likely registers which the user will want to use this

function with are the Data Register and the Status Register. Subsequent operations to the

part will consist of read operations to the specified register without any intermediate writes to

the Communications Register. This means that once the next read operation to the specified

register has taken place, the part will be in a mode where it is expecting another read from

that specified register. The part will remain in this continuous read mode until 30 Hex has

been written to bits RW1 and RW0.

When 1, 1 is written to these bits (and 0 written to bits CR3 through CR0), the continuous

read mode is stopped and the part returns to where it is expecting a write operation to the

Communications Register. Note, the part continues to look at the DIN line on each SCLK

edge during the continuous read mode so that it can determine when to stop the continuous

read mode. Therefore, the user must be careful not to inadvertently exit the continuous read

mode or reset the part by writing a series of 1s to the part. The easiest way to avoid this is to

place a logic 0 on the DIN line while the part is in continuous read mode.