Data Sheet

AD7791

Rev. A | Page 11 of 20

STATUS REGISTER (RS1, RS0 = 0, 0; POWER-ON/RESET = 0x8C)

The status register is an 8-bit read-only register. To access the ADC status register, the user must write to the communications register,

select the next operation to be a read, and load bits RS1 and RS0 with 0. Table 8 outlines the bit designations for the status register. SR0

through SR7 indicate the bit locations, SR denoting the bits are in the status register. SR7 denotes the first bit of the data stream. The

number in brackets indicates the power-on/reset default status of that bit.

SR7

SR6

SR5

SR4

SR3

SR2

SR1

SR0

RDY(1)

ERR(0)

0(0)

0(0)

1(1)

WL(1)

CH1(0)

CH0(0)

Table 8. Status Register Bit Designations

Bit Location

Bit Name

Description

SR7

RDY

Ready bit for ADC. Cleared when data is written to the ADC data register. The RDY bit is set automatically

after the ADC data register has been read or a period of time before the data register is updated with a

new conversion result to indicate to the user not to read the conversion data. It is also set when the part

is placed in power-down mode. The end of a conversion is indicated by the DOUT/RDY pin also. This pin

can be used as an alternative to the status register for monitoring the ADC for conversion data.

SR6

ERR

ADC Error Bit. This bit is written to at the same time as the RDY bit. Set to indicate that the result written

to the ADC data register has been clamped to all 0s or all 1s. Error sources include overrange, under-

range. Cleared by a write operation to start a conversion.

SR5

0

This bit is automatically cleared.

SR4

0

This bit is automatically cleared.

SR3

1

This bit is automatically set.

SR2

1

This bit is automatically set if the device is an AD7791. It can be used to distinguish between the AD7791

and AD7790, in which the bit is cleared.

SR1–SR0

CH1–CH0

These bits indicate which channel is being converted by the ADC.

MODE REGISTER (RS1, RS0 = 0, 1; POWER-ON/RESET = 0x02)

The mode register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the

ADC for unipolar or bipolar mode, enable or disable the buffer, or place the device into power-down mode. Table 9 outlines the bit desig-

nations for the mode register. MR0 through MR7 indicate the bit locations, MR denoting the bits are in the mode register. MR7 denotes

the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. Any write to the setup regis-

ter resets the modulator and filter and sets the RDY bit.

MR7

MR6

MR5

MR4

MR3

MR2

MR1

MR0

MD1(0)

MD0(0)

0(0)

0(0)

BO(0)

U/B(0)

BUF(1)

0(0)

Table 9. Mode Register Bit Designations

Bit Location

Bit Name

Description

MR7–MR6

MD1–MD0

Mode Select Bits. These bits select between continuous conversion mode, single conversion mode, and

standby mode. In continuous conversion mode, the ADC continuously performs conversions and places

the result in the data register. RDY goes low when a conversion is complete. The user can read these

conversions by placing the device in continuous read mode whereby the conversions are automatically

placed on the DOUT line when SCLK pulses are applied. Alternatively, the user can instruct the ADC to

output the conversion by writing to the communications register. After power-on, the first conversion is

conversion mode, the ADC is placed in power-down mode when conversions are not being performed.

When single conversion mode is selected, the ADC powers up and performs a single conversion, which

ADC returns to power-down mode. The conversion remains in the data register and RDY remains active

(low) until the data is read or another conversion is performed. See Table 10.