AD7291

Preliminary Technical Data

Rev. PrC | Page 12 of 21

COMMAND REGISTER

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 8. A two-byte write is necessary when writing to the configuration register. MSB denotes the first bit in the data stream. On

power up the default content of the command register is all zero’s.

Table 8. Command Register Bits and Default Settings at Power-Up

MSB

Channel Bit

D15

D14

D13

D12

D11

D10

D9

D8

Function

CH1

CH2

CH3

CH4

CH5

CH6

CH7

CH8

Setting

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

Enable = 1

Disable = 0

LSB

Channel

Bit

D7

D6

D5

D4

D3

D2

D1

D0

Function

TSENSE

DONTC Noise-delayed

bit trial

sampling.

EXT_REF

Polarity of Alert

pin (active

high/active

low)

Clear Alert

RESET

Autocycle

Mode

Default

Setting

Enable = 1

Disable = 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 9. Bit Function Descriptions

Bit

Mnemonic

Comment

D15 to D8

CH1 to CH8

These 8-channel address bits select the analog input channel(s) to be converted. A 1 in any of Bits D15 to D8

selects a channel for conversion. If more than one channel bit is set to 1, the AD7291 will sequence 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 channel selects the temperature sensor channel for conversion. If other analog input channels are also

selected for conversion, the AD7291 sequences through the selected analog voltage input channels first and

then converts the temperature sensor channel after the conversion of last selected voltage channel is

completed.

D5

Noise-

delayed bit

trial sampling

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

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

adversely affected. On power up, the default configuration will have 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 the Alert Status register is cleared,

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

D1

RESET

Setting this bit in the command register resets the content of all internal registers in the AD7291 to their

default state including the command register itself. This bit is returned to a 0 once the reset is completed to

enable the internal registers to be reprogrammed.

D0

Autocycle

Mode

Writing a 1 to this bit in the command registers enables the auto-cycle mode of operation. In this mode, the

channels selected in bit D15 to D7 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.