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AD7879

Rev. 0 | Page 30 of 36

SERIAL INTERFACE

The AD7879 is available with an serial peripheral interface

(SPI). The AD7879-1 is available with an I2C®-compatible

interface. Both parts are the same, with the exception of the

serial interface. It is recommended not to write to addresses

outside the register map.

SPI INTERFACE

The AD7879 has a 4-wire SPI. The SPI has a data input pin

(DIN) for inputting data to the device, a data output pin

(DOUT) for reading data back from the device, and a data

clock pin (SCL) for clocking data into and out of the device.

A chip select pin (CS) enables or disables the serial interface.

CS is required for correct operation of the SPI interface. Data

is clocked out of the AD7879 on the negative edge of SCL and

data is clocked into the device on the positive edge of SCL.

SPI Command Word

All data transactions on the SPI bus begin with the master

taking CS from high to low and sending out the command

word. This indicates to the AD7879 whether the transaction

is a read or a write, and gives the address of the register from

which to begin the data transfer. The bit map in

shows

the SPI command word.

Table 22

Table 22.

MSB

LSB

9:0

R/W

Bits[15:11] of the command word must be set to 11100 to

successfully begin a bus transaction.

Bit 10 is the read/write bit; 1 indicates a read, and 0 indicates

a write.

Bits[9:0] contain the target register address. When reading or

writing to more than one register, this address indicates the

address of the first register to be written to or read from.

Writing Data

Data is written to the AD7879 in 16-bit words. The first word

written to the device is the command word, with the read/write

bit set to 0. The master then supplies the 16-bit input data-word

on the DIN line. The AD7879 clocks the data into the register

addressed in the command word. If there is more than one word

of data to be clocked in, the AD7879 automatically increments

the address pointer and clocks the next data-word into the

following register.

The AD7879 continues to clock in data on the SDA line until

either the master finishes the write transition by pulling CS

high, or until the address pointer reaches its maximum value.

The AD7879 address pointer does not wrap around. When it

reaches its maximum value, any data provided by the master

on the DIN line is ignored by the AD7879.

NOTES

1. DATA BITS ARE LATCHED ON SCL RISING EDGES. SCL CAN IDLE HIGH OR LOW BETWEEN WRITE OPERATIONS.

2. ALL 32 BITS MUST BE WRITTEN: 16 BITS FOR CONTROL WORD AND 16 BITS FOR DATA.

3. 16-BIT COMMAND WORD SETTINGS FOR SERIAL WRITE OPERATION:

CW[15:11] = 11100 (ENABLE WORD)

CW[10] = 0 (R/W)

CW[9:0] = [AD9, AD8, AD7, AD6, AD5, AD4, AD3, AD2, AD1, AD0] (10-BIT MSB JUSTIFIED REGISTER ADDRESS)

DIN

16-BIT COMMAND WORD

16-BIT DATA

1516

3031

SCL

1234

D15

D14

D13

ENABLE WORD

R/W

Figure 38. Single Register Write, SPI Timing

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