LM3S9997 Datasheet, PDF (607/1218 Pages) Texas Instruments – Stellaris® LM3S9997 Microcontroller

Stellaris® LM3S9997 Microcontroller
After all bits have been transferred, in the case of a single word transmission, the SSIFss line is
returned to its idle high state one SSIClk period after the last bit has been captured.
For continuous back-to-back transmissions, the SSIFss pin remains in its active Low state until the
final bit of the last word has been captured and then returns to its idle state as described above.
For continuous back-to-back transfers, the SSIFss pin is held Low between successive data words
and termination is the same as that of the single word transfer.
15.3.4.7
MICROWIRE Frame Format
Figure 15-10 on page 607 shows the MICROWIRE frame format for a single frame. Figure
15-11 on page 608 shows the same format when back-to-back frames are transmitted.
Figure 15-10. MICROWIRE Frame Format (Single Frame)
SSIClk
SSIFss
SSITx
SSIRx
MSB
8-bit control
LSB
0 _MSB
LSB
4 to 16 bits
output data
MICROWIRE format is very similar to SPI format, except that transmission is half-duplex instead of
full-duplex and uses a master-slave message passing technique. Each serial transmission begins
with an 8-bit control word that is transmitted from the SSI to the off-chip slave device. During this
transmission, no incoming data is received by the SSI. After the message has been sent, the off-chip
slave decodes it and, after waiting one serial clock after the last bit of the 8-bit control message has
been sent, responds with the required data. The returned data is 4 to 16 bits in length, making the
total frame length anywhere from 13 to 25 bits.
In this configuration, during idle periods:
■ SSIClk is forced Low
■ SSIFss is forced High
■ The transmit data line SSITx is arbitrarily forced Low
A transmission is triggered by writing a control byte to the transmit FIFO. The falling edge of SSIFss
causes the value contained in the bottom entry of the transmit FIFO to be transferred to the serial
shift register of the transmit logic and the MSB of the 8-bit control frame to be shifted out onto the
SSITx pin. SSIFss remains Low for the duration of the frame transmission. The SSIRx pin remains
tristated during this transmission.
The off-chip serial slave device latches each control bit into its serial shifter on each rising edge of
SSIClk. After the last bit is latched by the slave device, the control byte is decoded during a one
clock wait-state, and the slave responds by transmitting data back to the SSI. Each bit is driven onto
the SSIRx line on the falling edge of SSIClk. The SSI in turn latches each bit on the rising edge
of SSIClk. At the end of the frame, for single transfers, the SSIFss signal is pulled High one clock
period after the last bit has been latched in the receive serial shifter, causing the data to be transferred
to the receive FIFO.
June 15, 2010
607
Texas Instruments-Advance Information