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Ethernet AVB Endpoint User Guide www.xilinx.com 75
UG492 July 23, 2010
Chapter 8
Real Time Clock and Time Stamping
This chapter considers two of the logical components that are partially responsible for the
AVB timing synchronization protocol.
• “Real Time Clock”
• “Time Stamping Logic”
These are both described in this chapter as they are closely related.
Real Time Clock
A significant component of the PTP network wide timing synchronization mechanism is
the Real Time Counter (RTC), which provides the common time of the network. Every
device on the network will maintain its own local version.
The RTC is effectively a large counter which consists of a 32-bit nanoseconds field (the unit
of this field is 1 nanosecond and this field will count the duration of exactly one second,
then reset back to zero) and a 48-bit seconds field (the unit of this field is one second: this
field will increment when the nanosecond field saturates at 1 second). The seconds field
only wraps around when its count fully saturates. The entire RTC is therefore designed
never to wrap around in our lifetime. The RTC is summarized in Figure 8-1.
X-Ref Target - Figure 8-1
Figure 8-1: Real Time Counter (RTC)
Seconds field (48 bits unsigned) Nano Seconds field (32 bits unsigned)
counts from 0 until fully saturated,
then wraps around to 0
counts from 0 to 1 x 10
9
-1,
then resets to 0
IEEE802.1AS Real Time Counter (RTC)