AD7708BRUZ-REEL View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD7708BRUZ-REEL

8-/10-Channel, Low Voltage, Low Power, Σ-∆ ADCs

Analog Devices 

AD7708/AD7718

DIGITAL INTERFACE

As previously outlined, the AD7708/AD7718’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 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 written to this register determines whether the next operation

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

which register this read or write operation occurs. Therefore,

write 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 AD7708/AD7718s 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 devices’s 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 these devices in

systems where a number of parts are connected to the serial bus.

Figures 2 and 3 show timing diagrams for interfacing to the

AD7708/AD7718 with CS used to decode the part. Figure 3 is

for a read operation from the AD7708/AD7718 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 out-

put 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 serial interface can operate in three-wire mode by tying the

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

are used to communicate with the device and the status of the

RDY bit can be obtained by interrogating the STATUS Regis-

ter. 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 AD7708/AD7718 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 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 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 AD7708/AD7718 DIN

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

This ensures that in three-wire systems, if the interface is 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 ADC is expecting a write operation to its Commu-

nications Register. This operation resets the contents of all

registers to their 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

ADC’s DOUT and DIN lines together and connect them to the

single data line of the processor. A 10 kΩ pull-up resistor should

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

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 flexible serial interface allows for easy interface to most

microcomputers and microprocessors. The flowcharts of Figures

16, 17, and 18 outline the sequence that should be followed

when interfacing a microcontroller or microprocessor to the

AD7708/AD7718. Figures 19, 20, and 21 show some typical

interface circuits.

The serial interface on the AD7708/AD7718 is capable of oper-

ating from just three wires and is compatible with SPI interface

protocols. The three-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 accommo-

date slow edges from optocouplers. The rise and fall times of

other digital inputs to the AD7708/AD7718 should be no slower

than 1 µs.

Most of the registers on the AD7708/AD7718 are 8-bit regis-

ters, which facilitates easy interfacing to the 8-bit serial ports of

microcontrollers. The Data Register on the AD7718 is 24 bits

wide, the ADC data register on the AD7708 is 16 bits wide, and

the offset and gain registers are 16-bit registers on the AD7708

and 24-bit registers on the AD7718; however, data transfers to

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

port of the microcontroller. DSP processors and microproces-

sors 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 register length of the required register in the AD7708/

AD7718.

Even though some of the registers on the AD7708/AD7718 are

only eight 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 (saying that the next operation is a

write to the Filter Register) and then write eight 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 opera-

tion to the Communications Register have been completed,

the part immediately sets itself up for a write operation to the

Filter Register.

–34–

REV. 0

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