Figure 14-10. MICROWIRE Frame Format (Single Frame)
SSIClk
SSIFss
SSIRx
0
SSITx
8-bit control
4 to 16 bits
output data
LSB
MSB
MSB
LSB
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
SSITxpin. SSIFss remains Low for the duration of the frame transmission. The SSIRxpin 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.
Note: The off-chip slave device can tristate the receive line either on the falling edge of SSIClk
after the LSB has been latched by the receive shifter or when the SSIFss pin goes High.
For continuous transfers, data transmission begins and ends in the same manner as a single transfer.
However, the SSIFss line is continuously asserted (held Low) and transmission of data occurs
back-to-back. The control byte of the next frame follows directly after the LSB of the received data
from the current frame. Each of the received values is transferred from the receive shifter on the
falling edge of SSIClk, after the LSB of the frame has been latched into the SSI.
687July 24, 2012
Texas Instruments-Production Data
Stellaris
®
LM3S6G65 Microcontroller
OBSOLETE: TI has discontinued production of this device.
