AD5590
Rev. A | Page 37 of 44
ADC Control Register
The control register on the ADC is a 12-bit, write-only register.
Data is loaded from the ADIN pin of the ADC on the falling
edge of ASCLK. The data is transferred on the ADIN line at the
same time as the conversion result is read from the ADC. The
data transferred on the ADIN line corresponds to the ADC
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 ASYNC falling edge) is
loaded to the ADC control register. MSB denotes the first bit
in the data stream. The bit functions are outlined in
Table 21.Writing of information to the ADC control register takes place
on the first 12 falling edges of ASCLK in a data transfer, assuming
the MSB, that is, the write bit, has been set to 1. If the ADC
control register is programmed to use the shadow register,
writing of information to the shadow register takes place on
all 16 ASCLK falling edges in the next serial transfer (see
Figure 72). The shadow register is updated on the rising edge
of ASYNC and the track-and-hold begins to track the first
channel selected in the sequence.
If the weak/TRI bit in the ADC control register is set to 1, rather
than returning to true three-state upon the 16th ASCLK falling
edge, the ADOUT line is instead pulled weakly to the logic level
corresponding to ADD3 of the next serial transfer. This is done
to ensure that the MSB of the next serial transfer is set up in
time for the first ASCLK falling edge after the ASYNC falling
edge. If the weak/TRI bit is set to 0 and the ADOUT line has
been in true three-state between conversions, then depending
on the particular DSP or microcontroller interfacing to the
ADC, the ADD3 address bit may not be set up in time for the
DSP/microcontroller to clock it in successfully. In this case,
ADD3 is only driven from the falling edge of ASYNC and must
then be clocked in by the DSP on the following falling edge of
ASCLK. However, if the weak/TRI bit had been set to 1, then
although ADOUT is driven with the ADD3 address bit from
the last conversion, it is nevertheless so weakly driven that
another device may still take control of the bus. It does not lead
to a bus contention (for example, a 10 k pull-up or pull-down
resistor would be sufficient to overdrive the logic level of ADD3
between conversions), and all 16 channels may be identified.
However, if this does happen and another device takes control
of the bus, it is not guaranteed that ADOUT becomes fully
driven to ADD3 again in time for the read operation when
control of the bus is taken back.
This is especially useful if using an automatic sequence mode
to identify to which channel each result corresponds. Obviously,
if only the first eight channels are in use, the ADD3 address bit
does not need to be decoded, and whether it is successfully clocked
in as a 1 or 0 does not matter as long as it is still counted by the
DSP/microcontroller as the MSB of the 16-bit serial transfer.
Table 21. ADC Control Register
MSB
LSB
DB11
DB10
DB9
DB8
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
Write
SEQ
ADD3
ADD2
ADD1
ADD0
PM1
PM0
Shadow
Weak/TRI
Range
Coding
Table 22. ADC Control Register Bit Functions
Bit
Name
Description
11
Write
The value written to this bit of the control register determines whether the following 11 bits are loaded to the control
register or not. 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, therefore 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
9:6
ADD3:ADD0
These four address bits are loaded at the end of the current conversion sequence and select which analog input
channel is to be converted on in the next serial transfer, or can select the final channel in a consecutive sequence, as
described i
n Table 25. The selected input channel is decoded as shown i
n Table 23. The address bits corresponding to
the conversion result are also output on ADOUT prior to the 12 bits of data (see th
e Serial Interface section). The next
channel to be converted on is selected by the mux on the 14th ASCLK falling edge.
5, 4
PM1, PM0
These two power management bits decode the mode of operation of the ADC, as shown i
n Table 24.3
Shadow
The shadow bit in the control register is used in conjunction with the SEQ bit to control the use of the sequencer
2
Weak/TRI
This bit selects the state of the ADOUT line at the end of the current serial transfer. If it is set to 1, the ADOUT line is
weakly driven to the ADD3 channel address bit of the ensuing conversion. If this bit is set to 0, ADOUT returns to three-
1
Range
This bit selects the analog input range to be used on the ADC. If it is set to 0, then the analog input range extends from
0 V to 2 × VREFA. If it is set to 1, then the analog input range extends from 0 V to VREFA (for the next conversion). For 0 V to
2 × VREFA, ADCVDD = 4.75 V to 5.25 V.
0
Coding
This bit selects the type of output coding the ADC uses for the conversion result. If this bit is set to 0, the output coding
for the ADC is twos complement. If this bit is set to 1, the output coding from the ADC is straight binary (for the next
conversion).