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AD9284

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SERIAL PORT INTERFACE (SPI)

The AD9284 serial port interface (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,

which 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: SCLK, SDIO, and CSB

(see Table 9). SCLK (a serial clock) is used to synchronize the

read and write data presented from and to the ADC. SDIO

(serial data input/output) is a dual-purpose pin that allows data

to be sent to and read from the internal ADC memory map

registers. CSB (chip select bar) is an active low control that

enables or disables the read and write cycles.

Table 9. Serial Port Interface Pins

Function

SCLK

Serial clock. A serial shift clock input that 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 25.

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

held low indefinitely, which permanently enables the device;

this is called streaming. CSB can stall high between bytes to

allow for additional external timing. When the CSB pin is tied

high, SPI functions are placed in high impedance mode. This

mode turns on any SPI pin secondary functions.

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

Data follows the instruction phase, and its length is determined

by the W0 and W1 bits, as shown in Figure 25.

All data is composed of 8-bit words. The first bit of the first byte

in a multibyte serial data transfer frame indicates whether a read

command or a write command is issued. This allows the serial

data input/output (SDIO) pin to change direction from an input

to an output at the appropriate point in the serial frame.

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, the serial data input/output (SDIO) pin changes

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.

DON’T CARE

SDIO

SCLK

CSB

tDH

tCLK

tDS

R/W

A12

A11

A10

tLOW

tHIGH

Figure 25. Serial Port Interface Timing Diagram

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