30.4.3.2 Preloading of the Client Shift Register

When starting a transaction, the client will first transmit the contents of the shift register before loading new data from DATA. The first character sent can be either the reset value of the shift register, if this is the first transmission since the last reset, or the last received character in the previous transaction.

Preloading can be used to preload data into the shift register while SS is high: this eliminates sending a dummy character when starting a transaction.

Only one data character can be preloaded into the shift register while the synchronized SS signal is high. If the next character is written to DATA before SS is pulled low, the second character will be stored in DATA until transfer begins.

For proper preloading, sufficient time must elapse between SS going low and the first SCK sampling edge, as shown in Figure 30-4. See also the Electrical Characteristics section for timing details.

Preloading is enabled by writing ’1’ to the Client Data Preload Enable bit in the CTRLB register (CTRLB.PLOADEN).

The Preload function must not be used when the Host cannot maintain the SS in a low state until transmission is complete.