AD7787 Data Sheet
Rev. A | Page 10 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 noted.
COMMUNICATIONS REGISTER (RS1, RS0 = 0, 0)
The communications register is an 8-bit write-only register. All communications to the part must start with a write operation to the
communications register. The data written to the communications 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 part.
Table 5 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 the parenthesis 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 5. 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 part 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 to 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’s registers are being selected during
this serial interface communication (see Table 6).
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, i.e., the contents of the data register are placed on
the DOUT 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 00111100 (Channel AIN1) or 00111101
(Channel AIN2) 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 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.
CR1 to
CR0
CH1 to
CH0
These bits are used to select the analog input channel. Channel AIN1 or AIN2 can be selected or an internal short
(AIN1(−)/AIN1(−)) can be selected. Alternatively, the power supply can be selected, i.e., the ADC can measure the
voltage on the power supply, which is useful for monitoring power supply variation. To perform this measurement,
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 when this channel is selected. Any change in channel resets the
filter and a new conversion is started.
