ISR should clear the Receive FIFO Time-out Interrupt just after reading out the RX FIFO by writing
a 1 to the RTIC bit in the SSI Interrupt Clear (SSIICR) register. The interrupt should not be cleared
so late that the ISR returns before the interrupt is actually cleared, or the ISR may be re-activated
unnecessarily.
The End-of-Transmission (EOT) interrupt indicates that the data has been transmitted completely.
This interrupt can be used to indicate when it is safe to turn off the SSI module clock or enter sleep
mode. In addition, because transmitted data and received data complete at exactly the same time,
the interrupt can also indicate that read data is ready immediately, without waiting for the receive
FIFO time-out period to complete.
14.3.4 Frame Formats
Each data frame is between 4 and 16 bits long, depending on the size of data programmed, and is
transmitted starting with the MSB. There are three basic frame types that can be selected:
■ Texas Instruments synchronous serial
■ Freescale SPI
■ MICROWIRE
For all three formats, the serial clock (SSIClk) is held inactive while the SSI is idle, and SSIClk
transitions at the programmed frequency only during active transmission or reception of data. The
idle state of SSIClk is utilized to provide a receive timeout indication that occurs when the receive
FIFO still contains data after a timeout period.
For Freescale SPI and MICROWIRE frame formats, the serial frame (SSIFss) pin is active Low,
and is asserted (pulled down) during the entire transmission of the frame.
For Texas Instruments synchronous serial frame format, the SSIFss pin is pulsed for one serial
clock period starting at its rising edge, prior to the transmission of each frame. For this frame format,
both the SSI and the off-chip slave device drive their output data on the rising edge of SSIClk and
latch data from the other device on the falling edge.
Unlike the full-duplex transmission of the other two frame formats, the MICROWIRE format uses a
special master-slave messaging technique which operates at half-duplex. In this mode, when a
frame begins, an 8-bit control message is transmitted to the off-chip slave. During this transmit, 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 requested data. The returned data can be 4 to 16 bits in length, making the total
frame length anywhere from 13 to 25 bits.
14.3.4.1 Texas Instruments Synchronous Serial Frame Format
Figure 14-2 on page 682 shows the Texas Instruments synchronous serial frame format for a single
transmitted frame.
681July 24, 2012
Texas Instruments-Production Data
Stellaris
®
LM3S6G65 Microcontroller
OBSOLETE: TI has discontinued production of this device.
