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The SSI only provides an SSIClk while transmitting data. When receiving data in master mode, a
dummy write to the
SSIDR
register must be performed before any read so that the SSIClk can be
properly received by the slave and allow data to be sent to the receive FIFO of the master.
When configured as a master or slave, serial data received through the
SSInDAT1/SSInRX
pin is
registered prior to parallel loading into the attached slave or master receive FIFO, respectively.
Note:
When operating in Legacy Mode, the QSSI's
SSInXDAT1
signal functions as
SSInRX
.
17.3.3
Advanced, Bi- and Quad- SSI Function
Bi-SSI uses two data pins,
SSInXDAT0
and
SSInXDAT1
, that can be configured to receive or
transmit data. In Quad-SSI mode,
SSInXDAT0
,
SSInXDAT1
,
SSInXDAT2
and
SSInXDAT3
allow
four bits of data to be received or transmitted at once. Note that in bi- and quad-SSI data transfers
are only half-duplex.
By programming the
MODE
bits in the
SSICR1
register, Advanced, Bi- or Quad- SSI can be enabled.
A direction bit,
DIR
, is provided to program the direction of operation during a Bi- or Quad SSI-
transaction. Since Bi- and Quad-SSI cannot be full duplex, the
DIR
bit defines whether or not the
RX FIFO is disabled. In Advanced operation, if the QSSI module TX (write) mode is enabled, the
RX FIFO is automatically prevented from receiving any data. When Advanced SSI is in RX (read)
mode, it operates as a full-duplex interface.
In Bi- and Quad-SSI mode, because only 8-bit data is allowed, the
DSS
bit field must be programmed
to 0x7 in the
SSICR0
register before transferring data to the Rx and TX FIFOs. For a data transmit,
the 8-bit data packet is placed in a TX FIFO entry bits [7:0] and the mode of operation is inserted
in the three most significant bits of the TX FIFO entry. The mode of operation bits [15:13] in the TX
FIFO are used by the QSSI module for configuring the data on the proper pins. The following modes
that may be placed on bits [15:13] of the FIFO entry are:
■ Bi-SSI mode (0x1)
■ Quad-SSI mode (0x2)
■ Advanced SSI mode (0x3)
When data is first written to the TX FIFO, a
SSInFss
is asserted low indicating the start of a frame.
At the end of transmission, bit 12 of the last data entry in the TX FIFO signifies whether a a frame
is ending. When the
EOM
bit is 1 it indicates a End of Message (EOM or STOP frame) and
SSInFss
is subsequently forced high. The
EOM
bit is cleared in the
SSICR1
register on the same clock that
the write to TXFIFO is completed. An
EOM
bit value of 0 indicates no change in transmission. If TX
FIFO is emptied and
SSInFSS
is still asserted low, it remains low but
SSInCLK
is not pulsed.
Likewise, if
SSInFss
is high when the TX FIFO is empty, it remains high.
During a Bi-SSI transmit frame, data is shifted out by two bits and placed on the corresponding two
SSInDATn
pins. For a Quad-SSI transmit frame data is shifted out by four bits and placed on the
corresponding four
SSInDATn
pins.
In Bi-, Quad- and Advanced SSI, the lower byte of the Rx FIFO contains received data. The upper
byte contains no valid information.
Note:
While the master is in Bi- or Quad-SSI mode, if the
DSS
bit in the
SSICR0
register is not
set to 0x7, the QSSI module reverts to Legacy mode and behavior is not guaranteed.
The
SSICRI1
register bits
DIR
and
MODE
are used to program what operation is needed for the next
data bytes that are being loaded into the FIFO. Table 17-2 on page 1231 shows available modes of
operation:
June 18, 2014
1230
Texas Instruments-Production Data
Quad Synchronous Serial Interface (QSSI)