AD6657EBZ datasheet(25/32 Pages) AD

25 / 32 page

AD6657

Rev. 0 | Page 25 of 32

SERIAL PORT INTERFACE (SPI)

The AD6657 serial port interface (SPI) allows the user to con-

figure the receiver for specific functions or operations through a

structured internal register space. The SPI provides 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 Application

Note AN-877, Interfacing to High Speed ADCs via SPI.

CONFIGURATION USING THE SPI

Three pins define the SPI of the AD6657: SCLK, SDIO, and CSB

(see Table 12). SCLK (a serial clock) is used to synchronize the

read and write data presented from and to the AD6657. SDIO

(serial data input/output) is a bidirectional pin that allows data

to be sent to and read from the internal memory map registers.

CSB (chip select bar) is an active low control that enables or

disables the read and write cycles.

Table 12. Serial Port Interface Pins

Function

SCLK

Serial clock. Serial shift clock input. SCLK is used to

synchronize serial interface reads and writes.

SDIO

Serial data input/output. Bidirectional pin that 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 (active low). This control gates the read

and write cycles.

The falling edge of the CSB pin, in conjunction with the rising

edge of the SCLK pin, determines the start of the framing. An

example of the serial timing can be found in Figure 51 (for

symbol definitions, see Table 5).

CSB 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 CSB is tied

high, SPI functions are placed in high impedance mode.

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

The first bit of the first byte in a serial data transfer frame indicates

whether a read command or a write command is issued. Data

follows the instruction phase, and its length is determined by

the W0 and W1 bits. All data is composed of 8-bit words.

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 read 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 mode on power-up and can be changed

via the SPI port configuration register. For more information

about this and other features, see Application Note AN-877,

Interfacing to High Speed ADCs via SPI.

HARDWARE INTERFACE

The pins described in Table 12 constitute the physical interface

between the user programming device and the serial port of the

AD6657. 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 the write phase 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 Application Note AN-812, Micro-

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

The SPI port should not be active during periods when the full

dynamic performance of the AD6657 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 AD6657 performance. If the on-board SPI bus is used

for other devices, it may be necessary to provide buffers between

this bus and the AD6657 to prevent these signals from transi-

tioning at the receiver inputs during critical sampling periods.

DON’T CARE

DON’T

CARE

DON’T

CARE

SDIO

SCLK

CSB

tDH

tCLK

tDS

R/W

A12

A11

A10

A9A8

D3D2D1

tLOW

tHIGH

Figure 51. Serial Port Interface Timing Diagram

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