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AD9231

Rev. 0 | Page 28 of 36

SERIAL PORT INTERFACE (SPI)

The AD9231 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: the SCLK, the SDIO,

and the CSB (see Table 13). The SCLK (a serial clock) is used

to synchronize the read and write data presented from and to

the ADC. The SDIO (serial data input/output) 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) is an active-

low control that enables or disables the read and write cycles.

Table 13. 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 58 and

Table 5.

Other modes involving the 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 for additional external timing. When CSB is tied high, SPI

functions are placed in 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 W1 bits as shown in Figure 58.

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, performing a readback causes the serial data input/

output (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.

DON’T CARE

SDIO

SCLK

CSB

tDH

tCLK

tDS

tHIGH

tLOW

R/W

A12

A11

A10

Figure 58. Serial Port Interface Timing Diagram

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