AD7709 datasheet(21/33 Pages) AD

21 / 33 page

REV. A

AD7709

–20–

DIGITAL INTERFACE

As previously outlined, AD7709 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 com-

munications to the part must start with a write operation to the

Communications Register. After power-on or reset, the device

expects a write to its Communications Register. The data writ-

ten to this register determines whether the next operation to the

part is a read or a write operation and also determines to which

access to any of the other registers on the part starts with a write

operation to the Communications Register followed by a write

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

on the part (including the output data register) starts with a

write operation to the Communications Register followed by a

read operation from the selected register.

The AD7709 serial interface consists of five signals:

CS, SCLK,

DIN, DOUT, and RDY. The DIN line is used for transferring

data into the on-chip registers, while the DOUT line is used for

accessing data from the on-chip registers. SCLK is the serial

clock input for the device, and all data transfers (either on DIN

or DOUT) take place with respect to this SCLK signal. The

RDY line is used as a status signal to indicate when data is ready

to be read from the AD7709 data register. RDY goes 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 output 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

the device. It can be used to decode the AD7709 in systems where

a number of parts are connected to the serial bus.

Figures 2 and 3 show timing diagrams for interfacing to the

AD7709 with

CS used to decode the part. Figure 3 is for a read

operation from the AD7709 output shift register while Figure 2

shows a write operation to the input shift register. It is possible

to read the same data twice from the output register even though

the RDY line returns high after the first read operation. Care must

be taken, however, to ensure that the read operations have been

completed before the next output update is about to take place.

The AD7709 serial interface can operate in 3-wire mode by

tying the

CS input low. In this case, the SCLK, DIN, and

DOUT lines are used to communicate with the AD7709, and

the status of the RDY bit can be obtained by interrogating 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 bit. For microcontroller interfaces, it is

recommended that the SCLK idles high between data transfers.

The AD7709 can also be operated with

CS used as a frame

synchronization signal. This scheme is suitable for DSP interfaces.

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

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 exercising the

RESET input

on the part. It can also be reset by writing a series of 1s on the

DIN input. If a Logic 1 is written to the AD7709 DIN line for

at least 32 serial clock cycles, the serial interface is reset. This

ensures that in 3-wire systems, if the interface gets lost either via

a software error or by some glitch in the system, it can be reset

back to a known state. This state returns the interface to where

the AD7709 is expecting a write operation to its Communications

power-on reset values.

Some microprocessor or microcontroller serial interfaces have a

single serial data line. In this case, it is possible to connect the

AD7709 DOUT and DIN lines together and connect them to the

single data line of the processor. A 10 k

W pull-up resistor should

be used on this single data line. In this case, if the interface gets

lost, because the read and write operations share the same line,

the procedure to reset it back to a known state is somewhat

different than previously described. It requires a read operation

of 24 serial clocks followed by a write operation where a Logic 1

is written for at least 32 serial clock cycles to ensure that the

serial interface is back into a known state.

MICROCOMPUTER/MICROPROCESSOR INTERFACING

The AD7709 flexible serial interface allows for easy interface to

most microcomputers and microprocessors. The flowchart of

Figure 10 outlines the sequence that should be followed when

interfacing a microcontroller or microprocessor to the AD7709.

Figures 11, 12, and 13 show some typical interface circuits. The

serial interface on the AD7709 is capable of operating from just

three wires and is compatible with SPI interface protocols. The

3-wire operation makes the part ideal for isolated systems where

minimizing the number of interface lines minimizes the number

of opto-isolators required in the system. The serial clock input is

a Schmitt-triggered input to accommodate slow edges from

opto-couplers. The rise and fall times of other digital inputs to

the AD7709 should be no longer than 1

ms.

Some of the registers on the AD7709 are 8-bit registers, which

facilitates easy interfacing to the 8-bit serial ports of microcon-

trollers. The Data Register on the AD7709 is 16 bits and the

Configuration Register is 24 bits, but data transfers to these

registers can consist of multiple 8-bit transfers to the serial port

of the microcontroller. DSP processors and microprocessors

generally transfer 16 bits of data in a serial data operation. Some

of these processors, such as the ADSP-2105, have the facility to

program the amount of cycles in a serial transfer. This allows the

user to tailor the number of bits in any transfer to match the

Even though some of the registers on the AD7709 are only 8 bits

in length, communicating with two of these registers in successive

write operations can be handled as a single 16-bit data transfer if

required. For example, if the Filter Register is to be updated, the

processor must first write to the Communications Register (say-

ing that the next operation is a write to the Filter Register), and

then write 8 bits to the Filter Register. If required, this can all be

done in a single 16-bit transfer because once the eight serial

clocks of the write operation to the Communications Register

have been completed, the part immediately sets itself up for a

write operation to the Filter Register.

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