LM3S1621 Datasheet, PDF (743/947 Pages) Texas Instruments – ARM and Thumb are registered trademarks and Cortex is a trademark

Stellaris^® LM3S1621 Microcontroller
If the SSI is enabled and valid data is in the transmit FIFO, the start of transmission is signified by
the SSIFss master signal being driven Low, causing slave data to be immediately transferred onto
the SSIRx line of the master. The master SSITx output pad is enabled.
One-half period later, valid master data is transferred to the SSITx line. Once both the master and
slave data have been set, the SSIClk master clock pin becomes Low after one additional half
SSIClk period, meaning that data is captured on the falling edges and propagated on the rising
edges of the SSIClk signal.
In the case of a single word transmission, after all bits of the data word are transferred, the SSIFss
line is returned to its idle High state one SSIClk period after the last bit has been captured.
However, in the case of continuous back-to-back transmissions, the SSIFss signal must be pulsed
High between each data word transfer because the slave select pin freezes the data in its serial
peripheral register and does not allow it to be altered if the SPH bit is clear. Therefore, the master
device must raise the SSIFss pin of the slave device between each data transfer to enable the
serial peripheral data write. On completion of the continuous transfer, the SSIFss pin is returned
to its idle state one SSIClk period after the last bit has been captured.
15.3.4.6
Freescale SPI Frame Format with SPO=1 and SPH=1
The transfer signal sequence for Freescale SPI format with SPO=1 and SPH=1 is shown in Figure
15-9 on page 743, which covers both single and continuous transfers.
Figure 15-9. Freescale SPI Frame Format with SPO=1 and SPH=1
SSIClk
SSIFss
SSIRx
SSITx
Q MSB
MSB
4 to 16 bits
LSB Q
LSB
Note: Q is undefined.
In this configuration, during idle periods:
■ SSIClk is forced High
■ SSIFss is forced High
■ The transmit data line SSITx is arbitrarily forced Low
■ When the SSI is configured as a master, it enables the SSIClk pad
■ When the SSI is configured as a slave, it disables the SSIClk pad
If the SSI is enabled and valid data is in the transmit FIFO, the start of transmission is signified by
the SSIFss master signal being driven Low. The master SSITx output pad is enabled. After an
additional one-half SSIClk period, both master and slave data are enabled onto their respective
transmission lines. At the same time, SSIClk is enabled with a falling edge transition. Data is then
captured on the rising edges and propagated on the falling edges of the SSIClk signal.
January 21, 2012
743
Texas Instruments-Production Data