Data Sheet AD7788/AD7789
Rev. C | Page 11 of 20
ON-CHIP REGISTERS
The ADC is controlled and configured via a number of on-chip
registers, which are described on the following pages. In the
following descriptions, set implies a Logic 1 state and cleared
implies a Logic 0 state, unless otherwise stated.
COMMUNICATIONS REGISTER
(RS1, RS0 = 0, 0)
The communications register is an 8-bit, write only register. All
communications to the device must start with a write operation
to the communications register. The data written to the commun-
ications register determines whether the next operation is a read
or write operation, and to which register this operation takes
place.
For read or write operations, once the subsequent read or write
operation to the selected register is complete, the interface returns
to where it expects a write operation to the communications
register. This is the default state of the interface and, on power-up
or after a reset, the ADC is in this default state waiting for a write
operation to the communications register. In situations where the
interface sequence is lost, a write operation of at least 32 serial
clock cycles with DIN high returns the ADC to this default state
by resetting the entire device. Table 7 outlines the bit designations
for the communications register. CR0 through CR7 indicate the
bit location, CR denoting the bits are in the communications
register. CR7 denotes the first bit of the data stream. The number in
parentheses indicates the power-on/reset default status of that bit.
CR7 CR6 CR5 CR4 CR3 CR2 CR1 CR0
WEN(0)
0(0) RS1(0) RS0(0)
R/W(0)
CREAD(0) CH1(0) CH0(0)
Table 7. Communications Register Bit Designations
Bit Location Bit Name Description
CR7
WEN Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually
occurs. If a 1 is the first bit written, the device does not clock on to subsequent bits in the register. It stays at
this bit location until a 0 is written to this bit. Once a 0 is written to the
WEN bit, the next seven bits are
loaded to the communications register.
CR6 0 This bit must be programmed with a Logic 0 for correct operation.
CR5 to CR4 RS1 to RS0
Register Address Bits. These address bits are used to select which of the ADC registers are being selected
during this serial interface communication (see Table 8).
CR3
R/
W A 0 in this bit location indicates that the next operation is a write to a specified register. A 1 in this position
indicates that the next operation is a read from the designated register.
CR2 CREAD
Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the serial
interface is configured so that the data register can be continuously read, that is, the contents of the data
register are placed on the DOUT/
RDY pin automatically when the SCLK pulses are applied. The communications
register does not have to be written to for data reads. To enable continuous read mode, the instruction
001111XX must be written to the communications register. To exit the continuous read mode, the
instruction 001110XX must be written to the communications register while the DOUT/
RDY pin is low.
While in continuous read mode, the ADC monitors activity on the DIN line so that it can receive the
instruction to exit continuous read mode. Additionally, a reset occurs if 32 consecutive 1s are seen on DIN.
Therefore, DIN should be held low in continuous read mode until an instruction is to be written to the device.
These bits are used to select the analog input channel. The differential channel can be selected
AIN(+)/AIN(−) or an internal short AIN(−)/AIN(−) can be selected. Alternatively, the power supply can be
selected, that is, the ADC can measure the voltage on the power supply, which is useful for monitoring
power supply variation. The power supply voltage is divided by 5 and then applied to the modulator for
conversion. The ADC uses a 1.17 V ± 5% on-chip reference as the reference source for the analog-to-digital
conversion. Any change in channel resets the filter and a new conversion is started.
