AD7291

Rev. 0 | Page 17 of 28

COMMAND REGISTER (0x00)

The command register is a 16-bit write-only register that is used to set the operating modes of the AD7291. The bit functions are outlined

in Table 10. A two-byte write is necessary when writing to the command register. MSB denotes the first bit in the data stream. During

power-up, the default content of the command register is all 0s.

Table 10. Command Register Bits and Default Settings at Power-Up

MSB

LSB

Channel Bit

D15 to DB8

D7

D6

D5

D4

D3

D2

D1

D0

Function

CH0 to CH7

TSENSE

Don’t

care

Noise-delayed

bit trial and

sampling

EXT_REF

Polarity of ALERT

pin (active high/

active low)

Clear alert

RESET

Autocycle

mode

Setting

Enable = 1

Disable = 0

Enable = 1

Disable = 0

0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Active low = 1

Active high = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Table 11. Command Register Bit Function Descriptions

Bit

Mnemonic

Comment

D15 to D8

CH0 to CH7

These 8-channel address bits select the analog input channel(s) to be converted. A 1 in any of Bit D15 to

Bit D8 selects a channel for conversion. If more than one channel bit is set to 1, the AD7291 sequences

through the selected channels, starting with the lowest channel. All unused channels should be set to 0. A

channel or sequence of channels for conversion must be selected in the command register, prior to initiating

a conversion.

D7

TSENSE

This bit enables temperature conversions, which occur in the background at 5 ms intervals. The results can be

refer to the Temperature Sensor Operation section.

D6

Don’t care

D5

Noise-

delayed bit

trial and

sampling

When this function is enabled, it delays the critical sampling intervals and bit trials when there is activity on

conversion time may vary. This bit is disabled on power-up, and it is recommended to write a 1 to enable this

feature for normal operation.

D4

EXT_REF

Writing a Logic 1 to this bit enables the use of an external reference. The input voltage range for the external

reference is 2 V to 2.5 V. The external reference should not exceed 2.5 V or the device performance will be

adversely affected. During power-up, the default configuration has the internal reference enabled.

D3

Polarity of

ALERT pin

This bit determines the active polarity of the ALERT pin. The ALERT pin is configured for active low operation

if this bit is set to 1 and active high if this bit is set to 0. The default configuration on power-up is active high (0).

D2

Clear alert

This bit clears the content of the alert status register. Once the content of both alert status registers is cleared,

this bit should be reprogrammed to a Logic 0 to ensure that future alerts are detected.

D1

RESET

Setting this bit resets the contents of all internal registers in the AD7291 to their default states including the

command register itself. This bit is automatically returned to 0 once the reset is completed to enable the

internal registers to be reprogrammed.

D0

Autocycle

mode

Writing a 1 to this bit enables the autocycle mode of operation. In this mode, the channels selected in Bit D15

to Bit D8 are continuously converted by the AD7291. This function is used in conjunction with the limit

registers, which can be programmed to issue an alert if the conversion result exceeds the preset limit for any

channel selected for conversion.