Synchronous Serial Interface (SSI)
282 October 8, 2006
Preliminary
Regist er 3: SSI Data (SSIDR), offset 0x008
SSIDR is the data register and is 16-bits wide. When SSIDR is read, the entry in the receive FIFO
(pointed to by the current FIFO read pointer) is accessed. As data values are removed by the SSI
receive logic from the incoming data frame, they are placed into the entry in the receive FIFO
(pointed to by the current FIFO write pointer).
When SSIDR is written to, the entry in the transmit FIFO (poin te d to by th e w ri t e p oi n te r ) is wr it te n
to. Data values are removed from the transmit FIFO one value at a time by the transmit logic. It is
loaded into the transmit serial shifter, then serially shifted out onto the SSITx pin at the
programmed bit rate.
When a data size of less than 16 bits is selected, the user must right-justify data written to the
transmit FIFO. The transmit logic ignores the unused bits. Received data less than 16 bits is
automatically right-justified in the receive buffer.
When the SSI is programmed for MICROWIRE frame format, the default size for transmit data is
eight bits (the most significant byte is ignored). The receive data size is controlled by the
programmer . The transmit FIFO and the receive FIFO are not cleared even when the SSE bit in the
SSICR1 register is set to zero. This allows the software to fill the transmit FIFO before enabling the
SSI.
Bit/Field Name Type Reset Description
31:16 reserved RO 0 Reserved bits return an indeterminate value, and should
never be change d.
15:0 DATA R/W 0 SSI Re ceive/Transmit Data
A read operation reads the receive FIFO. A write operation
writes the transmit FIFO.
Software must right-justify data when the SSI is programmed
for a data size that is less than 16 bits. Unused bits at the top
are ignored by the transmit logic. The receive logic
automatically right-justifies the data.
DATA
RO
0
SSI Data (SSIDR)
Offset 0x008
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Reset
Type
000000000000000
RO RO RO RO RO RO RO RO RO RO RO RO RO RO RO
R/W
0
Reset
Type
000000000000000
R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W
reserved
