Datasheet

Data Sheet AD9557
Rev. B | Page 45 of 92
Communication Cycle—Instruction Plus Data
The SPI protocol consists of a two-part communication cycle.
The first part is a 16-bit instruction word that is coincident with
the first 16 SCLK rising edges and a payload. The instruction
word provides the AD9557
serial control port with information
regarding the payload. The instruction word includes the R/
AA
W
EE
AA
bit that indicates the direction of the payload transfer (that is, a
read or write operation). The instruction word also indicates
the number of bytes in the payload and the starting register
address of the first payload byte.
Write
If the instruction word indicates a write operation, the payload
is written into the serial control port buffer of the AD9557. Data
bits are registered on the rising edge of SCLK. The length of the
transfer (1, 2, or 3 bytes or streaming mode) depends on the W0
and W1 bits (see Table 25
) in the instruction byte. When not
streaming,
AA
CS
EE
AA can be deasserted 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
AA
CS
EE
AA is asserted. Deasserting the AA
CS
EE
AA pin on a nonbyte boundary
resets the serial control port. Reserved or blank registers are not
skipped over automatically during a write sequence. Therefore,
the user must know what bit pattern to write to the reserved
registers to preserve proper operation of the part. Generally, it
does not matter what data is written to blank registers, but it is
customary to write 0s.
Most of the serial port registers are buffered (refer to the
Buffered/Active Registers section for details on the difference
between buffered and active registers). Therefore, data written
into buffered registers does not take effect immediately. An
additional operation is required to transfer buffered serial control
port contents to the registers that actually control the device.
This is accomplished with an I/O update operation, which is
performed in one of two ways. One is by writing a Logic 1 to
Register 0x0005, Bit 0 (this bit is autoclearing). The other is to use
an external signal via an appropriately programmed multifunction
pin. The user can change as many register bits as desired before
executing an I/O update. The I/O update operation transfers the
buffer register contents to their active register counterparts.
Read
The AD9557 supports the long instruction mode only. If the
instruction word indicates a read operation, the next N × 8
SCLK cycles clock out the data from the address specified in
the instruction word. N is the number of data bytes read and
depends on the W0 and W1 bits of the instruction word. The
readback data is valid on the falling edge of SCLK. Blank registers
are not skipped over during readback.
A readback operation takes data from either the serial control
port buffer registers or the active registers, as determined by
Register 0x0004, Bit 0.
SPI Instruction Word (16 Bits)
The MSB of the 16-bit instruction word is R/AA
W
EE
AA, which indicates
whether the instruction is a read or a write. The next two bits,
W1 and W0, indicate the number of bytes in the transfer (see
Table 25). The final 13 bits are the register address (A12 to A0),
which indicates the starting register address of the read/write
operation (see Table 27).
SPI MSB-/LSB-First Transfers
The AD9557 instruction word and payload can be MSB first
or LSB first. The default for the AD9557 is MSB first. The LSB-
first mode can be set by writing a 1 to Register 0x0000, Bit 6.
Immediately after the LSB-first bit is set, subsequent serial control
port operations are LSB first.
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 payload 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 Register 0x0000, Bit 6 = 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 payload byte,
followed by multiple data bytes. The serial control port internal
byte address generator increments for each byte of the multibyte
transfer cycle.
For multibyte MSB-first (default) I/O operations, the serial control
port register address decrements from the specified starting address
toward Address 0x0000. For multibyte LSB-first I/O operations,
the serial control port register address increments from the starting
address toward Address 0x1FFF. Reserved addresses are not
skipped during multibyte I/O operations; therefore, the user
should write the default value to a reserved register and 0s 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 26. Streaming Mode (No Addresses Are Skipped)
Write Mode Address Direction Stop Sequence
LSB First
Increment 0x0000 ... 0x1FFF
MSB First
Decrement 0x1FFF ... 0x0000
Table 27. Serial Control Port, 16-Bit Instruction Word, MSB First
MSB
LSB
I15 I14 I13 I12 I11 I10 I9 I8 I7 I6 I5 I4 I3 I2 I1 I0
R/AA
W
EE
W1 W0 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0