What happens in a communication cycle if not enough or too many bits get written in a data cycle for a given register size? If too many, will it keep the existing bits already in the register from a previous data cycle or will it fill the missing bits with a default value?
Typically, sending too few bits will result in no change to the existing register content, while sending too many bits will fill the designated register with the appropriate bits as usual. In the case of extra bits, those are lost.
In either case, the wrong number of bits will confuse the SPI controller. Many of our DDSs offer an "IO Reset" or "IO Sync" pin that you can use to resynchronize the SPI controller in such cases.
Thanks for the information. Can you write anything to the bits that are 'not used' in the registers?
The recommended procedure for "unused" or "reserved" bits is to write the same bit value as indicated by the default value for the register.
If the data sheet does not provide a default value for the register, then the next best option is to read the register first, then write the read value associated with the unused/reserved bit(s).
Thank you for the response