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2.6.2 SDP Pins
The Software Defined Pins (SDP) can be used to provide a timing communication path between the
Module and Carrier. A board level signal that communicates time is a key element that facilitates clock
synchronization between elements of a platform. Examples of such elements include, but are not limited
to, CPU, Chipset, FPGA and others.
Modules
should
connect the SDP signal to a module element pin capable of propagating (transmitting)
time, and/or time-stamping (receiving) the signal to extract time information from it. If implemented, the
direction of the signal with respect to the module element
should
be able to be determined by system
software.
Pulse Per Second (PPS):
A PPS signal conveys both frequency and phase and can be used to transfer time information between
elements within a platform. It is commonly used because it encapsulates both frequency and time into a
single signal. It is preferred over other methods that require more complex implementations of hardware
and software. A GPS is probably the most widespread, high-quality, clock source capable of generating
a PPS signal.
Platform-level Synchronization Implementation Examples:
Example1:
The Network Interface Controller (NIC) on the COM Module is Precision Time Protocol (PTP)
capable and the COM designer has connected a software configurable, timing aware, pin on the NIC to
the SDP pin on the module/carrier interface. Software can configure the NIC to output a PPS signal onto
this pin that connects it to one or more elements on the module and/or carrier board.
Example2:
The carrier board has provisions for connecting a PPS output from a GPS to the SDP signal
connection to the module. The module element (i.e. NIC, CPU, Chipset) can receive the timing
information from the carrier board and adjust its time accordingly.
Precision Time Protocol - Background
Standards such as IEEE 1588, 802.1AS, and Time Sensitive Networking (TSN) provide standards for
synchronizing time between nodes on a local area network. Additional benefits of the standards may
include lower latency and improved network traffic Quality of Service (QoS). Systems that commonly
require synchronization include those made up of distributed nodes that perform measurement, control,
and compute functions. These nodes may have clock sources with varying degrees of accuracy and
stability.
System-wide time synchronization with sub-microsecond accuracy is supported, by PTP standards, with
minimal network and compute resource utilization.
It is the merger of the platform-level synchronization and network level synchronization pieces that
enable real-time distributed systems. Additional information regarding the aforementioned standards can
be found in their respective specifications and widely available supporting documents.
Software Implementation:
The software architecture and features required to support platform and network level
synchronization are outside the scope of this specification.
