AD9520-2SLASHPCBZ datasheet(54/84 Pages) AD

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AD9520-2

Rev. 0 | Page 54 of 84

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 read data bits are registered on the falling edge. This pin is

internally pulled down by a 30 kΩ resistor to ground.

SDIO (serial data input/output) is a dual-purpose pin and acts

either as an input only (unidirectional mode) or as an input/

output (bidirectional mode). The AD9520 defaults to the

bidirectional I/O mode (0x000[7] = 0b).

SDO (serial data out) is used only in the unidirectional I/O

mode (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 kΩ

resistor to VS.

AD9520

SERIAL

CONTROL

PORT

SCLK/SCL

SDO

SDIO/SDA

Figure 61. 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. The AD9520

serial 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, the AD9520 is in bidirectional mode. Short

instruction mode (8-bit instructions) is not supported. Only

long (16-bit) instruction mode is supported.

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

(see

). In this mode, the

Table 43

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 43), any number of data bytes

can be transferred in a continuous stream. The register address

is automatically incremented or decremented (see the SPI

MSB/LSB First Transfers section). CS must be raised at the end

of the last byte to be transferred, thereby ending streaming mode.

Communication Cycle—Instruction Plus Data

There are two parts to a communication cycle with the AD9520.

The first part writes a 16-bit instruction word into the 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

Because data is written into a serial control port buffer area, not

directly into the actual control registers of the AD9520, an

additional operation is needed to transfer the serial control port

buffer contents to the actual control registers of the AD9520,

thereby causing them to become active. The update registers

operation consists of setting 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 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.

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