AD9257 Analog Devices, AD9257 Datasheet - Page 28

• Manufacturer Part Number: AD9257
• Description: Octal, 14-Bit, 40/65 MSPS Serial LVDS 1.8 V A/D Converter
• Manufacturer: Analog Devices

AD9257
SERIAL PORT INTERFACE (SPI)
The
the converter for specific functions or operations through a
structured register space provided inside the ADC. The SPI
gives the user added flexibility and customization, depending on
the application. Addresses are accessed via the serial port and
can be written to or read from via the port. Memory is organized
into bytes that can be further divided into fields, which are docu-
mented in the Memory Map section. For detailed operational
information, see the
High Speed ADCs via SPI.
CONFIGURATION USING THE SPI
Three pins define the SPI of this ADC: the SCLK/DTP pin, the
SDIO/DFS pin, and the CSB pin (see Table 15). The SCLK
(a serial clock) is used to synchronize the read and write data
presented from and to the ADC. The SDIO (serial data input/
output) is a dual-purpose pin that allows data to be sent to and
read from the internal ADC memory map registers. The CSB
(chip select bar) is an active low control that enables or disables
the read and write cycles.
Table 15. Serial Port Interface Pins
Pin
SCLK
SDIO
CSB
AD9257
SCLK
SDIO
CSB
Function
Serial clock. The serial shift clock input, which is used to
synchronize serial interface reads and writes.
Serial data input/output. A dual-purpose pin that
typically serves as an input or an output, depending on
the instruction being sent and the relative position in the
timing frame.
Chip select bar. An active low control that gates the read
and write cycles.
DON’T CARE
DON’T CARE
serial port interface (SPI) allows the user to configure
AN-877
t
S
R/W
t
DS
Application Note, Interfacing to
W1
W0
t
DH
t
HIGH
A12
t
LOW
A11
Figure 61. Serial Port Interface Timing Diagram
A10
A9
Rev. 0 | Page 28 of 40
t
CLK
A8
A7
The falling edge of the CSB, in conjunction with the rising edge
of the SCLK, determines the start of the framing. An example of
the serial timing and its definitions can be found in Figure 61
and Table 5.
Other modes involving the CSB are available. The CSB can be
held low indefinitely, which permanently enables the device;
this is called streaming. The CSB can stall high between bytes to
allow for additional external timing. When CSB is tied high, SPI
functions are placed in high impedance mode. This mode turns
on any SPI pin secondary functions.
During an instruction phase, a 16-bit instruction is transmitted.
Data follows the instruction phase, and its length is determined
by the W0 and W1 bits.
In addition to word length, the instruction phase determines
whether the serial frame is a read or write operation, allowing
the serial port to be used both to program the chip and to read
the contents of the on-chip memory. The first bit of the first byte in
a multibyte serial data transfer frame indicates whether a read
command or a write command is issued. If the instruction is a
readback operation, performing a readback causes the serial
data input/output (SDIO) pin to change direction from an input to
an output at the appropriate point in the serial frame.
All data is composed of 8-bit words. Data can be sent in MSB-
first mode or in LSB-first mode. MSB first is the default on
power-up and can be changed via the SPI port configuration
register. For more information about this and other features,
see the
ADCs via SPI.
AN-877
D5
Application Note, Interfacing to High Speed
D4
D3
D2
D1
D0
t
H
DON’T CARE
Data Sheet
DON’T CARE