Data Formats; Serial Data Formats For Transfer To/From Peripherals - Intel IXP45X Developer's Manual

Intel
20.2

Data Formats

20.2.1

Serial Data Formats for Transfer to/from Peripherals

Four pins are used to transfer data between the CPU and external codecs or modems.
Although there are several formats for serial data, they have the same basic structure:
• SSP_SCLK — Defines the bit rate at which serial data is driven onto, and sampled
from, the bus;
• SSP_SFRM — Defines the boundaries of a basic data "unit", comprised of multiple
serial bits;
• SSP_TXD — Is the serial data path for outbound data, from system to peripheral;
• SSP_RXD — Is the serial data path for inbound data, from peripheral to system.
A data frame may contain from 4 to 16 bits, depending on the format selected. Serial
data is transmitted MSB first. Three formats are supported: Motorola* SPI, Texas
Instruments* synchronous serial protocol (SSP), and National Microwire*.
For SPI and Microwire* formats, SSP_SFRM functions as a chip select to enable the
external device (target of the transfer), and is held active-low during the data transfer.
For SSP format, SSP_SFRM is pulsed high for one (serial) data period at the start of
each frame.
Function and use of the serial clock SSP_SCLK is different for each format:
• For Microwire, both data sources switch (change to the next bit) outgoing data on
the falling edge of SSP_SCLK, and sample incoming data on the rising edge.
• For SSP, data sources switch outgoing data on the rising edge of SSP_SCLK, and
sample incoming data on the falling edge
• SPI gives the user the choice of which edge of SSP_SCLK to use for switching
outgoing data, and for sampling incoming data. In addition, the user can move the
phase of SSP_SCLK, shifting its active state one-half period earlier or later at the
start and end of a frame.
While SSP and SPI are full-duplex protocols, Microwire uses a half-duplex master-slave
messaging protocol. At the start of a frame, a single-byte control message is
transmitted from the controller to the peripheral; no data is sent by the peripheral. The
peripheral interprets the message and, if it is a READ request, responds with requested
data, one clock after the last bit of the requesting message. Return data - part of the
same frame - can be from 4 to 16 bits in length. Total frame length is 13 to 25 bits.
Note that the serial clock (SSP_SCLK), if driven by the SSP Port, only toggles while
active data transfer is underway. At other times it will be held in an inactive or idle
state, as defined by the specified protocol under which it operates.
® ®
Intel IXP45X and Intel IXP46X Product Line of Network Processors
Developer's Manual
858
® ®
IXP45X and Intel IXP46X Product Line of Network Processors—Synchronous Serial Port
August 2006
Order Number: 306262-004US