AD9912ASLASHPCBZ datasheet(27/38 Pages) AD

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Data Sheet

AD9912

SERIAL CONTROL PORT

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Rev. G | 27 of 38

Read

If the instruction word is for a read operation (I15 = 1), the next

N × 8 SCLK cycles clock out the data from the address specified

in the instruction word, where N is 1, 2, 3, or 4, as determined by

[W1:W0]. In this case, 4 is used for streaming mode where four or

more words are transferred per read. The data readback is valid on

the falling edge of SCLK.

The default mode of the AD9912 serial control port is bidirectional

mode, and the data readback appears on the SDIO pin. It is

possible to set the AD9912 to unidirectional mode by writing to the

SDO active bit (Register 0x0000, Bit 0 = 1), and in that mode, the

requested data appears on the SDO pin.

By default, a read request reads the register value that is currently

in use by the AD9912. However, setting Register 0x0004, Bit 0 =

1 causes the buffered registers to be read instead. The buffered

registers are the ones that take effect during the next I/O update.

Figure 50. Relationship Between Serial Control Port Register Buffers and

Control Registers of the AD9912

The AD9912 uses Register 0x0000 to Register 0x0509. Although

the AD9912 serial control port allows both 8-bit and 16-bit instruc-

tions, the 8-bit instruction mode provides access to five address

bits (A4 to A0) only, which restricts its use to Address Space 0x00

to Address Space 0x31. The AD9912 defaults to 16‑bit instruction

mode on power-up, and the 8-bit instruction mode is not supported.

THE INSTRUCTION WORD (16 BITS)

The MSB of the instruction word is R/ W, which indicates whether

the instruction is a read or a write. The next two bits, [W1:W0],

are the transfer length in bytes. The final 13 bits are the address

([A12:A0]) at which to begin the read or write operation.

For a write, the instruction word is followed by the number of bytes

of data indicated by Bits[W1:W0], which is interpreted according to

Table 9.

Bits[A12:A0] select the address within the register map that is

written to or read from during the data transfer portion of the

communications cycle. The AD9912 uses all of the 13-bit address

space. For multibyte transfers, this address is the starting byte

address.

Table 9. Byte Transfer Count

Bytes to Transfer (Excluding the 2-Byte Instruction)

Streaming mode

MSB/LSB FIRST TRANSFERS

The AD9912 instruction word and byte data can be MSB first or

LSB first. The default for the AD9912 is MSB first. The LSB first

mode can be enabled by writing a 1 to the LSB first bit in the serial

configuration register and then issuing an I/O update. Immediately

after the LSB first bit is set, all serial control port operations are

changed to LSB first order.

When MSB first mode is active, the instruction and data bytes

must be written from MSB to LSB. Multibyte data transfers in MSB

first format start with an instruction byte that includes the register

address of the most significant data byte. Subsequent data bytes

must follow in order from high address to low address. In MSB first

mode, the serial control port internal address generator decrements

for each data byte of the multibyte transfer cycle.

When LSB first = 1 (LSB first), the instruction and data bytes

must be written from LSB to MSB. Multibyte data transfers in LSB

first format start with an instruction byte that includes the register

address of the least significant data byte followed by multiple

data bytes. The serial control port internal byte address generator

increments for each byte of the multibyte transfer cycle.

The AD9912 serial control port register address decrements from

the register address just written toward 0x0000 for multibyte I/O

operations if the MSB first mode is active (default). If the LSB first

mode is active, the serial control port register address increments

from the address just written toward 0x1FFF for multibyte I/O

operations.

Unused addresses are not skipped during multibyte I/O operations.

The user should write the default value to a reserved register

and should write only zeros to unmapped registers. Note that it

is more efficient to issue a new write command than to write the

default value to more than two consecutive reserved (or unmapped)

registers.

Table 10. Serial Control Port, 16-Bit Instruction Word, MSB First

MSB

LSB

I15

I14

I13

I12

I11

I10

R/W

A12

A11

A10

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