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Data Sheet

AD9690

SERIAL PORT INTERFACE

The AD9690 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 Serial Control Interface Standard (Rev. 1.0).

CONFIGURATION USING THE SPI

Three pins define the SPI of the AD9690 ADC: the SCLK pin,

the SDIO pin, and the CSB pin (see Table 34). 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 and read from the

internal ADC memory map registers. The CSB (chip select bar)

pin is an active low control that enables or disables the read and

write cycles.

Table 34. Serial Port Interface Pins

Function

SCLK

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

synchronize 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.

CSB

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

and write cycles.

The falling edge of CSB, 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 4 and

Table 5.

Other modes involving CSB are available. The CSB can be held

low indefinitely, which permanently enables the device; this is

called streaming. The CSB can stall high between bytes to allow

additional external timing. When CSB is tied high, SPI functions

are placed in a high impedance mode. This mode turns on any

SPI pin secondary functions.

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, which 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 Serial Control Interface Standard

(Rev. 1.0).

HARDWARE INTERFACE

The pins described in Table 34 comprise the physical interface

between the user programming device and the serial port of the

AD9690. The SCLK pin and the CSB 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 CSB 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 AD9690 to prevent these signals from

transitioning at the converter inputs during critical sampling

periods.

SPI ACCESSIBLE FEATURES

Table 35 provides a brief description of the general features that

are accessible via the SPI. These features are described in detail

in the Serial Control Interface Standard (Rev. 1.0). The AD9690

device-specific features are described in the Memory Map section.

Table 35. Features Accessible Using the SPI

Feature

Name

Description

Mode

Allows the user to set either power-down mode or

standby mode.

Clock

Allows the user to access the clock divider via

the SPI.

DDC

Allows the user to set up decimation filters for

different applications.

Test

Input/Output

Allows the user to set test modes to have known

data on output bits.

Output Mode

Allows the user to set up outputs.

SERDES

Output

Setup

Allows the user to vary SERDES settings such as

swing and emphasis.

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