Datasheet
AD9954
Rev. B | Page 21 of 40
SYNCHRONIZATION—REGISTER UPDATES (I/O
UPDATE)
Functionality of the SYNC_CLK and I/O UPDATE
Data into the AD9954 is synchronous to the SYNC_CLK
signal (supplied externally to the user on the SYNC_CLK pin).
The I/O UPDATE pin is sampled on the rising edge of the
SYNC_CLK.
Internally, SYSCLK is fed to a divide-by-four frequency divider
to produce the SYNC_CLK signal. The SYNC_CLK signal is
made available to the system on the SYNC_CLK pin. This
enables synchronization of external hardware with the device’s
internal clocks. This is accomplished by providing the SYNC_CLK
signal as an output that external hardware can then use to
synchronize against.
The I/O update signal coupled with SYNC_CLK is used to
transfer internal buffer contents into the control registers. The
combination of the SYNC_CLK pin and the I/O UPDATE pin
provides the user with constant latency relative to SYSCLK and
ensures phase continuity of the analog output signal when a
new tuning word or phase offset value is asserted.
Figure 23 and Figure 24 demonstrate an I/O update timing
cycle and synchronization.
Synchronization logic notes include the following:
The I/O update signal is edge detected to generate a single-
cycle clock signal that drives the register bank flops. The I/O
update signal has no constraints on duty cycle. The minimum
low time on I/O update is one SYNC_CLK clock cycle.
The I/O UPDATE pin is set up and held around the rising
edge of SYNC_CLK. Setup and hold time specifications can
be found in Table 2.
03374-006
SYSCLK
SDIO
SYNC_CLK
DISABLE
10
0
SCLK
TO CORE LOGIC
CS
OSK
D
Q
PS<1:0>
D
Q
I/O UPDATE
D
Q
÷4
SYNC_CLK
GATING
EDGE
DETECTION
LOGIC
REGISTER
MEMORY
I/O BUFFER
LATCHES
Figure 23. I/O Synchronization Block Diagram
SYNC_CLK
SYSCLK
AB
DATA IN
REGISTERS
DATA IN
I/O BUFFERS
I/O UPDATE
THE DEVICE REGISTERS AN I/O UPDATE AT POINT A. THE DATA IS TRANSFERRED FROM
THE ASYNCHRONOUSLY LOADED I/O BUFFERS AT POINT B.
0
3374-007
N – 1
N
N + 1N
N + 1
N + 2
Figure 24. I/O Synchronization Timing Diagram