There are boards (e.g. AD4630-24) which take the SCLK and echo back to
the FPGA through a level shifter - doing this removes the effect of
round-trip timing delays from the level shifter. This is commonly done
whenever isolators are used since they are very slow.
By setting the ECHO_SCLK parameter to 1, the IP will use the incoming
echoed SCLK clock to latch the SDI line(s). The sdi_data_valid is still
synchronous to the SPI clock, and it's generated after the last valid
SDI latch.
The designer's responsibility is to time the SDI shift registers in order to
respect the design requirements.
Istvan Csomortani
Sergiu Arpadi