AD9520-3
Data Sheet
Rev. A | Page 54 of 80
SPI SERIAL PORT OPERATION
Pin Descriptions
SCLK (serial clock) is the serial shift clock. This pin is an input.
SCLK is used to synchronize serial control port reads and writes.
Write data bits are registered on the rising edge of this clock,
and the read data bits transition on the falling edge of SCLK.
This pin is internally pulled down by a 30 k resistor to
ground.
SDIO (serial data input/output) is a dual-purpose pin and acts
either as an input only (unidirectional mode) or as both an input
and an output (bidirectional mode). The
AD9520 defaults to
the bidirectional I/O mode (Register 0x000[7] = 0b).
SDO (serial data out) is used only in the unidirectional I/O
mode (Register 0x000[7]) as a separate output pin for reading back
data.
CS (chip select bar) is an active low control that gates the read
and write cycles. When CS is high, SDO and SDIO are in a high
impedance state. This pin is internally pulled up by a 30 k
resistor to VS.
AD9520
SERIAL
CONTROL
PORT
SCLK/SCL
16
SDO
18
SDIO/SDA
17
CS
15
07216-
036
Figure 62. Serial Control Port
SPI Mode Operation
In SPI mode, single or multiple byte transfers are supported, as
well as MSB first or LSB first transfer formats. Th
e AD9520serial control port can be configured for a single bidirectional
I/O pin (SDIO only) or for two unidirectional I/O pins (SDIO/
SDO). By default, th
e AD9520 is in bidirectional mode. Short
instruction mode (8-bit instructions) is not supported. Only
long (16-bit) instruction mode is supported. It is possible that
the serial activity on the SDIO/SDO pins may induce jitter on
the PLL while data is being transmitted.
A write or a read operation to the
AD9520 is initiated by pulling
CS low.
The CS stalled high mode is supported in data transfers where
three or fewer bytes of data (plus instruction data) are transferred
(s
ee Table 44). In this mode, the CS pin can temporarily return
high on any byte boundary, allowing time for the system controller
to process the next byte. CS can go high on byte boundaries only
and can go high during either part (instruction or data) of the
transfer.
During this period, the serial control port state machine enters
a wait state until all data is sent. If the system controller decides
to abort the transfer before all of the data is sent, the state machine
must be reset by either completing the remaining transfers or by
returning CS low for at least one complete SCLK cycle (but fewer
than eight SCLK cycles). Raising the CS pin on a nonbyte boundary
terminates the serial transfer and flushes the buffer.
In the streaming mode (see
Table 44), any number of data bytes
can be transferred in a continuous stream. The register address
is automatically incremented or decremented (see th
e SPIof the last byte to be transferred, thereby ending streaming mode.
Communication Cycle—Instruction Plus Data
There are two parts to a communication cycle with th
e AD9520.The first part writes a 16-bit instruction word into th
e AD9520,coincident with the first 16 SCLK rising edges. The instruction
word provides the
AD9520 serial control port with information
regarding the data transfer, which is the second part of the
communication cycle. The instruction word defines whether
the upcoming data transfer is a read or a write, the number of
bytes in the data transfer, and the starting register address for
the first byte of the data transfer.
Write
If the instruction word is for a write operation, the second part
is the transfer of data into the serial control port buffer of the
AD9520. Data bits are registered on the rising edge of SCLK.
The length of the transfer (one, two, or three bytes, or streaming
mode) is indicated by two bits (W1:W0) in the instruction byte.
When the transfer is one, two, or three bytes, but not streaming,
CS can be raised after each sequence of eight bits to stall the bus
(except after the last byte, where it ends the cycle). When the bus
is stalled, the serial transfer resumes when CS is lowered. Raising
the CS pin on a nonbyte boundary resets the serial control port.
During a write, streaming mode does not skip over reserved or
blank registers, and the user can write 0x00 to the reserved
register addresses.
Because data is written into a serial control port buffer area, not
directly into the actual control registers of th
e AD9520, an
additional operation is needed to transfer the serial control port
buffer contents to the actual control registers of th
e AD9520,thereby causing them to become active. The update registers
operation consists of setting Register 0x232[0] = 1b (this bit is
self-clearing). Any number of bytes of data can be changed
before executing an update registers. The update registers
simultaneously actuates all register changes that have been
written to the buffer since any previous update.
Read
The
AD9520 supports only the long instruction mode. If the
instruction word is for a read operation, the next N × 8 SCLK
cycles clock out the data from the address specified in the
instruction word, where N is 1 to 3 as determined by Bits[W1:W0].
If N = 4, the read operation is in streaming mode, continuing
until CS is raised. Streaming mode does not skip over reserved
or blank registers. The readback data is valid on the falling edge
of SCLK.