AD7779 datasheet(43/97 Pages) AD

43 / 97 page

Data Sheet

AD7779

Rev. 0 | Page 43 of 97

The SPI interface can operate in multiples of eight bits. For

example, in SPI control mode, if the SDO pin is used to read

back the data from the internal register or the SAR ADC, the data

frame is 16 bits wide (CRC disabled), as shown in Figure 101, or

24 bits wide (CRC enabled), as shown in Figure 102. In this

case, the controller can generate one frame of 16 bits/24 bits

(with and without the CRC enabled), or 2/3 frames of 8 bits

(with and without the CRC enabled). When the SDO pin is

used to read back the data from the -Δ channels, 64 bits must

be read back from the controller (in this case, the controller can

generate a frame of 64 bits—either 2 × 32 bits, 4 × 16 bits, or 8 ×

8 bits).

SPI CRC—Checksum Protection (SPI Control Mode)

The AD7779 has a checksum mode that improves SPI interface

robustness in SPI control mode. Using the checksum ensures

that only valid data is written to a register and allows data read

from the device to be validated. The SPI CRC can be enabled by

setting the SPI_CRC_TEST_EN bit. If an error occurs during a

Enabling the SPI_CRC_TEST_EN bit results in a CRC checksum

being performed on all the R/W operations. When SPI_

CRC_TEST_EN is enabled, an 8-bit CRC word is appended

to every SPI transaction for SAR and register map operations.

For more information on -Δ readback operations, see the

CRC Header section.

To ensure that the register write is successful, it is recommended to

read back the register and verify the checksum.

For CRC checksum calculations, the following polynomial is

always used: x8 + x2 + x + 1. See the SPI Control Mode

Checksum section for more information.

SPI Read/Write Register Mode (SPI Control Mode)

The AD7779 has on-board registers to configure and control

the device.

The registers have 7-bit addresses—the 7-bit register address on

the SDI line selects the register for the read/write function. The

7-bit register address follows the R/W bit in the SDI data. The

8 bits on the SDI line following the 7-bit register address are the

data to be written to the selected register if the SPI is a write

transfer. Data on the SDI line is clocked into the AD7779 on

the rising edge of SCLK, as shown in Figure 3.

The data on the SDO line during the SPI transfer contains the

8-bit 0010 0000 header: 8 bits of register data in the case of a read

(R) operation, or 8 zeros in the case of a write (W) operation.

With the CRC disabled, the basic data frame on the SDI line

during the transfer is 16 bits long, as shown in Figure 101.

When the CRC is enabled, a minimum frame length of 24 SCLKs is

required on SPI transfers. The 24 bits of data on the SDO line

consist of an 8-bit header (0010 0000), 8 bits of data, and an 8-bit

CRC (see Figure 102).

R/W

A6A5A4A3A2A1A0

SDO

SCLK

SDI

Figure 101. 16-Bit SPI Transfer—CRC Disabled

R/W

A6A5A4

A2A1

D0 ICRC7 ICRC6 ICRC5 ICRC4 ICRC3 ICRC2 ICRC1 ICRC0

SDO

SCLK

SDI

00000

R0 ICRC7 ICRC6 ICRC5 ICRC4 ICRC3 ICRC2 ICRC1 ICRC0

Figure 102. 24-Bit SPI Transfer—CRC Enabled

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