This strategy also uses a periodic synchronization
signal, but the NCO accumulator is not reset. Only the internal frequency registers
inside each NCO are corrected by the synchronization signal. This makes sure the NCO
returns to the proper frequency after an upset occurs, but the phase of the NCO is
not corrected. This strategy is appropriate for non-phased-array systems where an
occasional, arbitrary phase jump can be tolerated.
- Program NCO_AR=0 to configure the accumulator to NOT be reset by a synchronization event.
- Configure the NCO to use SYSREF as the synchronization source (NCO_SYNC_SRC=2). Program SPI_SYNC=1.
- Make sure the SYSREF generator is configured for continuous/periodic output.
- Since the accumulator is not reset periodically,
the user has the freedom to program any NCO frequency regardless of the SYSREF
frequency. The period of SYSREF must still be a multiple of the multiframe/EMB
period.
- While the NCO is operating, any upset to the
internal frequency register is corrected on the next SYSREF pulse (the frequency
word is re-copied from the SEU-immune FREQ register). The NCO operates for a
brief time at an improper frequency before returning to the correct frequency,
but with arbitrary phase.