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 character in the previous transmission.

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. If the shift register is not preloaded, the current contents of the shift register will be shifted out.

Only one data character will 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 in Timing Using Preloading. See also the Electrical Characteristics chapters for timing details.

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

Figure 1. Timing Using Preloading