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AD8284

Data Sheet

Rev. D | Page 18 of 28

SERIAL PORT INTERFACE (SPI)

The AD8284 serial port interface allows the user to configure

the signal chain for specific functions or operations through a

structured register space provided inside the chip. The SPI

offers 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, as

documented in the Memory Map section. Detailed operational

information can be found in the AN-877 Application Note,

Interfacing to High Speed ADCs via SPI.

Four pins define the serial port interface, or SPI: the SCLK, SDI,

SDO, and CS pins. The serial clock pin (SCLK) synchronizes the

read and write data presented to the device. The serial data

input and output pins, SDI and SDO, allow data to be sent to

and read from the internal memory map registers of the device.

The chip select pin (CS) is an active low control that enables or

disables the read and write cycles (see Table 8).

Table 8. Serial Port Interface Pins

Function

SCLK

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

synchronize serial interface reads and writes.

SDI

Serial data input.

SDO

Serial data output.

Chip select (active low). This control 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 sequence. During an

instruction phase, a 16-bit instruction is transmitted, followed by

one or more data bytes, which is determined by Bit Field W0 and

Bit Field W1. See Figure 22 and Table 9 for an example of the

serial timing and its definitions.

In normal operation, CS signals to the device that SPI commands

are about to be received and processed. When CS is brought low,

the device processes SCLK and SDI to process instructions.

Normally, CS remains low until the communication cycle is

complete. However, if the AD8284 is connected to a slow

device, CS can be brought high between bytes, allowing older

microcontrollers enough time to transfer data into the shift

registers. CS can be stalled when transferring one, two, or three

bytes of data.

When W0 and W1 are set to 11, the device enters streaming mode

and continues to process data, either reading or writing,

until CS is taken high to end the communication cycle. This

allows complete memory transfers without the need to provide

additional instructions. Regardless of the mode, if CS is taken

high in the middle of any byte transfer, the SPI state machine is

reset and the device waits for a new instruction.

In addition to the operation modes, the SPI port can be

configured to operate in different manners. For applications

that do not require a control port, the CS line can be tied and

held high. This places the remainder of the SPI pins in their

secondary mode as defined in the AN-877 Application Note,

Interfacing to High Speed ADCs via SPI. CS can also be tied low

to enable 3-wire mode. When CS is tied low, SCLK, SDO, and

SDI are the only pins required for communication. Although

the device is synchronized during power-up, caution must be

exercised when using this mode to ensure that the serial port

remains synchronized with the CS line. When operating in

3-wire mode, it is recommended that a 1-, 2-, or 3-byte transfer

be used exclusively. Without an active CS line, streaming mode

can be entered but not exited.

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

mode is the default at power-up and can be changed by adjusting

the 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 8 constitute the physical interface

between the user’s programming device and the serial port of

the AD8284. The SCLK, SDI, and CS pins function as inputs

when using the SPI interface. The SDO pin is an output during

readback.

This interface is flexible enough to be controlled by either serial-

programmable read-only memory (PROM) or PIC micro-

controllers. This provides the user with alternative means, other

than a full SPI controller, for programming the device (see the

AN-812 Application Note).

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