I am going to use a Cypress Psoc microcontroller as a UART to SPI bridge to load a ADF4118 freq synth. The ADF411x input register length is 21 bit. It would however be convenient to load 3 bytes (24bit) into the reg to simplify microcontroller code. ADF411x data sheet page 24 (Interfacing) says that 24 bit load is OK. Please define if it is the first (MSBs) or the last (LSBs) 21 bits of the 24 bit transfer that is loaded in the input register after the last (24th) SPI clk cycle.