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AD9683

Data Sheet

Rev. D | Page 32 of 44

SERIAL PORT INTERFACE (SPI)

The AD9683 SPI allows the user to configure the converter for

specific functions or operations through a structured register

space provided inside the ADC. The SPI gives the user added

flexibility and customization, depending on the application.

Addresses are accessed via the serial port and can be written to

or read from via the port. Memory is organized into bytes that

can be further divided into fields. These fields are documented

in the Memory Map section. For detailed operational information,

see the AN-877 Application Note, Interfacing to High Speed

ADCs via SPI.

CONFIGURATION USING THE SPI

Three pins define the SPI of this ADC: the SCLK pin, the SDIO

pin, and the CS pin (see Table 15). The SCLK (serial clock) pin is

used to synchronize the read and write data presented from/to the

ADC. The SDIO (serial data input/output) pin is a dual-purpose

pin that allows data to be sent to and read from the internal ADC

memory map registers. The CS (chip select bar) pin is an active low

control that enables or disables the read and write cycles.

Table 15. Serial Port Interface Pins

Function

SCLK

Serial clock. The serial shift clock input, which is used to

synchronize the serial interface reads and writes.

SDIO

Serial data input/output. A dual-purpose pin that

typically serves as an input or an output, depending on

the instruction being sent and the relative position in the

timing frame.

Chip select bar. An active low control that gates the read

and write cycles.

The falling edge of CS, in conjunction with the rising edge of

SCLK, determines the start of the framing. An example of the

serial timing and its definitions can be found in Figure 67 and

Table 5.

Other modes involving CSare available. CS can be held low

indefinitely, which permanently enables the device; this is called

streaming. CS can stall high between bytes to allow for additional

external timing. When CS is tied high, SPI functions are placed

in a high impedance mode. This mode turns on any SPI pin

secondary functions.

During an instruction phase, a 16-bit instruction is transmitted.

Data follows the instruction phase, and its length is determined

by the W0 and the W1 bits.

All data is composed of 8-bit words. The first bit of each individual

byte of serial data indicates whether a read or write command is

issued. This allows the SDIO pin to change direction from an

input to an output.

In addition to word length, the instruction phase determines

whether the serial frame is a read or write operation, allowing

the serial port to be used both to program the chip and to read

the contents of the on-chip memory. If the instruction is a readback

operation, performing a readback causes the SDIO pin to change

direction from an input to an output at the appropriate point in

the serial frame.

Data can be sent in MSB first mode or in LSB first mode. MSB

first is the default on power-up and can be changed via the SPI

port configuration register. For more information about this and

other features, see the AN-877 Application Note, Interfacing to

High Speed ADCs via SPI.

HARDWARE INTERFACE

The pins described in Table 15 comprise the physical interface

between the user programming device and the serial port of the

AD9683. The SCLK pin and the CS pin function as inputs when

using the SPI interface. The SDIO pin is bidirectional, functioning

as an input during write phases and as an output during readback.

The SPI interface is flexible enough to be controlled by either

FPGAs or microcontrollers. One method for SPI configuration

is described in detail in the AN-812 Application Note,

Microcontroller-Based Serial Port Interface (SPI) Boot Circuit.

Do not activate the SPI port during periods when the full dynamic

performance of the converter is required. Because the SCLK signal,

the CS signal, and the SDIO signal are typically asynchronous to

the ADC clock, noise from these signals can degrade converter

performance. If the on-board SPI bus is used for other devices, it

may be necessary to provide buffers between this bus and the

AD9683 to prevent these signals from transitioning at the

converter inputs during critical sampling periods.

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