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AD9511

Rev. A | Page 41 of 60

SERIAL CONTROL PORT

The AD9511 serial control port is a flexible, synchronous, serial

communications port that allows an easy interface with many

industry-standard microcontrollers and microprocessors. The

AD9511 serial control port is compatible with most

synchronous transfer formats, including both the Motorola SPI®

and Intel® SSR protocols. The serial control port allows

read/write access to all registers that configure the AD9511.

Single or multiple byte transfers are supported, as well as MSB

first or LSB first transfer formats. The AD9511 serial control

port can be configured for a single bidirectional I/O pin (SDIO

only) or for two unidirectional I/O pins (SDIO/SDO).

SERIAL CONTROL PORT 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

as either an input only or as both an input/output. The AD9511

defaults to two unidirectional pins for I/O, with SDIO used as

an input and SDO as an output. Alternatively, SDIO can be used

as a bidirectional I/O pin by writing to the SDO enable register

at 00h<7> = 1b.

SDO

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

mode (00h<7> = 0b, default) as a separate output pin for

reading back data. The AD9511 defaults to this I/O mode.

Bidirectional I/O mode (using SDIO as both input and output)

may be enabled by writing to the SDO enable register at

00h<7> = 1b.

CSB

(chip select bar) is an active low control that gates the read

and write cycles. When CSB is high, SDO and SDIO are in a

high impedance state. This pin is internally pulled down by a

30 kΩ resistor to ground. It should not be left NC or tied low.

See the Framing a Communication Cycle with CSB section on

the use of the CSB in a communication cycle.

SCLK (PIN 14)

SDIO (PIN 15)

SDO (PIN 16)

CSB (PIN 17)

AD9511

SERIAL

CONTROL

PORT

Figure 44. Serial Control Port

GENERAL OPERATION OF SERIAL CONTROL PORT

Framing a Communication Cycle with CSB

Each communication cycle (a write or a read operation) is gated

by the CSB line. CSB must be brought low to initiate a

communication cycle. CSB must be brought high at the

completion of a communication cycle (see Figure 52). If CSB is

not brought high at the end of each write or read cycle (on a

byte boundary), the last byte is not loaded into the register

buffer.

CSB stall high is supported in modes where three or fewer bytes

of data (plus instruction data) are transferred (W1:W0 must be

set to 00, 01, or 10, see Table 20). In these modes, CSB can

temporarily return high on any byte boundary, allowing time

for the system controller to process the next byte. CSB 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

has been 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 transfer or by

returning the CSB low for at least one complete SCLK cycle (but

less than eight SCLK cycles). Raising the CSB on a nonbyte

boundary terminates the serial transfer and flushes the buffer.

In the streaming mode (W1:W0 = 11b), any number of data

bytes can be transferred in a continuous stream. The register

address is automatically incremented or decremented (see the

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

end of the last byte to be transferred, thereby ending the

stream mode.

Communication Cycle—Instruction Plus Data

There are two parts to a communication cycle with the AD9511.

The first writes a 16-bit instruction word into the AD9511,

coincident with the first 16 SCLK rising edges. The instruction

word provides the AD9511 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 transferred, and the starting register address

for the first byte of the data transfer.

Write

If the instruction word is for a write operation (I15 = 0b), the

second part is the transfer of data into the serial control port

buffer of the AD9511. The length of the transfer (1, 2, 3 bytes or

streaming mode) is indicated by two bits (W1:W0) in the

instruction byte. CSB 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

CSB is lowered. Stalling on nonbyte boundaries resets the serial

control port.

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

directly into the AD9511’s actual control registers, an additional

operation is needed to transfer the serial control port buffer

contents to the actual control registers of the AD9511, thereby

causing them to take effect. This update command consists of

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