Datasheet

AD7908/AD7918/AD7928
Rev. D | Page 15 of 32
CONTROL REGISTER
The control register on the AD7908/AD7918/AD7928 is a 12-bit, write-only register. Data is loaded from the DIN pin of the
AD7908/AD7918/AD7928 on the falling edge of SCLK. The data is transferred on the DIN line at the same time that the conversion
result is read from the part. The data transferred on the DIN line corresponds to the AD7908/AD7918/AD7928 configuration for the next
conversion. This requires 16 serial clocks for every data transfer. Only the information provided on the first 12 falling clock edges (after
CS
falling edge) is loaded to the control register. MSB denotes the first bit in the data stream. The bit functions are outlined in . Table 7
MSB LSB
WRITE SEQ DON’TCARE ADD2 ADD1 ADD0 PM1 PM0 SHADOW DON’TCARE RANGE CODING
Table 7. Control Register Bit Functions
Bit Mnemonic Comment
11 WRITE
The value written to this bit of the control register determines whether or not the following 11 bits are loaded to the
control register. If this bit is a 1, the following 11 bits are written to the control register; if it is a 0, the remaining 11 bits
are not loaded to the control register, and it remains unchanged.
10 SEQ
The SEQ bit in the control register is used in conjunction with the SHADOW bit to control the use of the sequencer
function and access the SHADOW register (see the SHADOW register bit map).
9 DON’TCARE
8 to 6
ADD2 to
ADD0
These three address bits are loaded at the end of the present conversion sequence and select which analog input
channel is to be converted in the next serial transfer, or they can select the final channel in a consecutive sequence as
described in Table 10. The selected input channel is decoded as shown in Table 8. The address bits corresponding to
the conversion result are also output on DOUT prior to the 12 bits of data, see the Serial Interface section. The next
channel to be converted on is selected by the mux on the 14th SCLK falling edge.
5, 4 PM1, PM0
Power Management Bits. These two bits decode the mode of operation of the AD7908/AD7918/AD7928 as shown in
Table 9.
3 SHADOW
The SHADOW bit in the control register is used in conjunction with the SEQ bit to control the use of the sequencer
function and access the SHADOW register (see Table 10).
2 DON’TCARE
1 RANGE
This bit selects the analog input range to be used on the AD7908/AD7918/AD7928. If it is set to 0, the analog input
range extends from 0 V to 2 × REF
IN
. If it is set to 1, the analog input range extends from 0 V to REF
IN
(for the next
conversion). For 0 V to 2 × REF
IN
, AV
DD
= 4.75 V to 5.25 V.
0 CODING
This bit selects the type of output coding the AD7908/AD7918/AD7928 uses for the conversion result. If this bit is set to
0, the output coding for the part is twos complement. If this bit is set to 1, the output coding from the part is straight
binary (for the next conversion).
Table 8. Channel Selection
ADD2 ADD1 ADD0 Analog Input Channel
0 0 0 V
IN
0
0 0 1 V
IN
1
0 1 0 V
IN
2
0 1 1 V
IN
3
1 0 0 V
IN
4
1 0 1 V
IN
5
1 1 0 V
IN
6
1 1 1 V
IN
7