![Intel IXP45X Developer'S Manual Download Page 860](http://html1.mh-extra.com/html/intel/ixp45x/ixp45x_developers-manual_2073092860.webp)
Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors—Synchronous Serial Port
Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
Developer’s Manual
August 2006
860
Order Number: 306262-004US
For SPI format, SSP_SCLK and SSP_TXD are low, and SSP_SFRM is high, in idle
mode or when the SSP is disabled. When transmit (outgoing) data is ready, SSP_SFRM
goes low and stays low for the remainder of the frame. The MSB of serial data is driven
onto SSP_TXD a half-cycle later, and halfway into the first bit period SSP_SCLK
asserts high and continues toggling for the remaining data bits. Data transitions on the
falling edge of SSP_SCLK. From 4 to 16 bits may be transferred per frame.
With assertion of SSP_SFRM, receive data is simultaneously driven from the peripheral
on SSP_RXD, MSB first. Data transitions on SSP_SCLK falling edges and is sampled
by the controller on rising edges.
At the end of the frame, SSP_SFRM is de-asserted high one clock period after the last
bit has been latched at its destination, and the completed incoming word is shifted into
the “incoming” FIFO. The peripheral can tri-state SSP_RXD after sending the last
(LSB) bit of the frame. SSP_TXD retains the last value transmitted when the controller
goes into idle mode, unless the SSP port is disabled or reset (which forces SSP_TXD to
zero).
For back-to-back transfers, start and completion are like those of a single transfer, but
SSP_SFRM does not deassert between words. Both transmitter and receiver know the
word length, and keep track internally of the start and end of words (frames). There
are no “dead” bits; the least significant bit of one frame is followed immediately by the
most significant bit of the next.
shows one of the four possible configurations for the Motorola SPI frame
format for a single transmitted frame and when back-to-back frames are transmitted.