External Shaped On-Off Keying Mode Operation
The external shaped on-off keying mode is enabled by writing
CFR1<25> to a Logic 1 and writing CFR1<24> to a Logic 0.
When configured for external shaped on-off keying, the
content of the ASFR becomes the scale factor for the data path.
The scale factors are synchronized to SYNC_CLK via the
I/O UPDATE functionality.
Synchronization; Register Updates (I/O UPDATE)
Functionality of the SYNC_CLK and I/O UPDATE
Data into the AD9952 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
Internally, SYSCLK is fed to a divide-by-4 frequency divider to
produce the SYNC_CLK signal. The SYNC_CLK signal is pro-
vided to the user on the SYNC_CLK pin. This enables synchro-
nization of external hardware with the device’s internal clocks.
This is accomplished by forcing any external hardware to obtain
its timing from SYNC_CLK. The I/O UPDATE signal coupled
with SYNC_CLK is used to transfer internal buffer contents
into the control registers of the device. The combination of the
SYNC_CLK and I/O UPDATE pins provides the user with
constant latency relative to SYSCLK, and also ensures phase
continuity of the analog output signal when a new tuning word
or phase offset value is asserted. Figure 21 demonstrates an I/O
UPDATE timing cycle and synchronization.
Notes to synchronization logic:
1) The I/O UPDATE signal is edge detected to generate a
single rising edge 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.
2) The I/O UPDATE pin is set up and held around the rising
edge of SYNC_CLK and has zero hold time and 4 ns setup
TO CORE LOGIC
Figure 21. I/O Synchronization Block Diagram
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