SPI Operation
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SPRUI07–March 2020
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Serial Peripheral Interface (SPI)
9.3.4 Data Format
The four-bit SPICHAR register field specifies the number of bits in the data character (1 to 16). This
information directs the state control logic to count the number of bits received or transmitted to determine
when a complete character has been processed.
The following statements apply to characters with fewer than 16 bits:
• Data must be left-justified when written to SPIDAT and SPITXBUF.
• Data read back from SPIRXBUF is right-justified.
• SPIRXBUF contains the most recently received character, right-justified, plus any bits that remain from
previous transmission(s) that have been shifted to the left (shown in Example 9-1).
Example 9-1. Transmission of Bit From SPIRXBUF
Conditions:
1. Transmission character length = 1 bit (specified in bits SPICHAR)
2. The current value of SPIDAT = 737Bh
(1)
x = 1 if SPISOMI data is high; x = 0 if SPISOMI data is low; master mode is assumed.
SPIDAT (before transmission)
0 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1
SPIDAT (after transmission)
(TXed) 0 ← 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 x
(1)
← (RXed)
SPIRXBUF (after transmission)
1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 x
(1)
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