Datasheet

Data Sheet AD7291
Rev. C | 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 must 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
read from the T
SENSE
conversion result register (0x02) and the T
SENSE
average result register (0x03). For details,
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
the I
2
C bus, thus ensuring improved dc performance of the AD7291. When this feature is enabled, the
conversion time can 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 must 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 must 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.