EVAL-AD7791EBZ datasheet(16/21 Pages) AD

16 / 21 page

Data Sheet

AD7791

Rev. A | Page 15 of 20

DIGITAL INTERFACE

As previously outlined, the AD7791’s programmable functions

are controlled using a set of on-chip registers. Data is written to

these registers via the part’s serial interface and read access to

the on-chip registers is also provided by this interface. All

communications with the part must start with a write to the

communications register. After power-on or reset, the device

expects a write to its communications register. The data written

to this register determines whether the next operation is a read

operation or a write operation and also determines to which

access to any of the other registers on the part begins with a

write operation to the communications register followed by a

write to the selected register. A read operation from any other

with a write to the communications register followed by a read

operation from the selected register.

The AD7791’s serial interface consists of four signals: CS, DIN,

SCLK, and DOUT/RDY. The DIN line is used to transfer data

into the on-chip registers while DOUT/RDY is used for access-

ing from the on-chip registers. SCLK is the serial clock input for

the device and all data transfers (either on DIN or DOUT/RDY)

occur with respect to the SCLK signal. The DOUT/ RDY pin

operates as a Data Ready signal also, the line going low when a

new data-word is available in the output register. It is reset high

when a read operation from the data register is complete. It also

goes high prior to the updating of the data register to indicate

when not to read from the device to ensure that a data read is

not attempted while the register is being updated. CS is used to

select a device. It can be used to decode the AD7791 in systems

where several components are connected to the serial bus.

Figure 3 and Figure 4 show timing diagrams for interfacing to

the AD7791 with CS being used to decode the part. Figure 3

shows the timing for a read operation from the AD7791’s out-

put

shift register while Figure 4 shows the timing for a write opera-

tion to the input shift register. In all modes except continuous

read mode, it is possible to read the same word from the data

high after the first read operation. However, care must be taken

to ensure that the read operations have been completed before

the next output update occurs. In continuous read mode, the

data register can be read only once.

The serial interface can operate in 3-wire mode by tying CS low.

In this case, the SCLK, DIN, and DOUT/RDY lines are used to

communicate with the AD7791. The end of the conversion can

be monitored using the RDY bit in the status register. This

scheme is suitable for interfacing to microcontrollers. If CS is

required as a decoding signal, it can be generated from a port

pin. For microcontroller interfaces, it is recommended that

SCLK idles high between data transfers.

The AD7791 can be operated with CS being used as a frame

synchronization signal. This scheme is useful for DSP interfac-

es. In this case, the first bit (MSB) is effectively clocked out by

CS since CS would normally occur after the falling edge of

SCLK in DSPs. The SCLK can continue to run between data

transfers, provided the timing numbers are obeyed.

The serial interface can be reset by writing a series of 1s on the

DIN input. If a Logic 1 is written to the AD7791 line for at least

32 serial clock cycles, the serial interface is reset. This ensures

that in 3-wire systems, the interface can be reset to a known

state if the interface gets lost due to a software error or some

glitch in the system. Reset returns the interface to the state in

which it is expecting a write to the communications register.

This operation resets the contents of all registers to their power-

on values.

The AD7791 can be configured to continuously convert or to

perform a single conversion. See Figure 13 through Figure 15.

04227-0-007

DIN

SCLK

DOUT/RDY

0x10

0x38

0x82

DATA

Figure 13. Single Conversion

EVAL-AD7791EBZ Datasheet(PDF) 16 Page - Analog Devices

EVAL-AD7791EBZ Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL