Datasheet

1. The master broadcasts the PTP Sync messages to all its nodes. The Sync message contains
the master's reference time information. The time at which this message leaves the master's
system is t1. This time is captured at the MII interface.
2. The slave receives the Sync message and also captures the exact time, t2, using its timing
reference.
3. The master sends a Follow_Up message to the slave, which contains t1 information for later
use.
4. The slave sends a Delay_Req message to the master, noting the exact time, t3, at which this
frame leaves the MAC.
5. The master receives the message, capturing the exact time, t4, at which it enters its system.
6. The master sends the t4 information to the slave in the Delay_Resp message.
7. The slave uses the four values of t1, t2, t3, and t4 to synchronize its local timing reference to
the master's timing reference.
Most of the PTP implementation is done in the software above the UDP layer. However, the hardware
support is required to capture the exact time when specific PTP packets enter or leave the Ethernet
MAC. This timing information is captured and returned to the software for the proper implementation
of PTP with high accuracy.
24.3.7.1 System Time Module
The System Time module maintains a 64-bit time and is updated using the MOSC clock source as
the PTP clock reference. This time is the source for taking snapshots (timestamps) of the Ethernet
frames being transmitted or received. Two methods of updating the system time counter are
implemented. The counter can be initialized or corrected using the coarse correction method. In
this method, the initial value or the offset value is written to the MAC System Time - Seconds
Update (EMACTIMSECU) register along with the MAC System Time - Nanoseconds Update
(EMACTIMNANOU) register. For initialization the system time counter is written with the value in
these registers, while for system time correction, the offset value is added to or subtracted from the
system time.
In the fine correction method, the slave clock's frequency drift with respect to the master clock is
corrected over a period of time instead of in one clock, as in coarse correction. This helps maintain
linear time and does not introduce drastic changes (or a large jitter) in the reference time between
PTP Sync message intervals. In this method, an accumulator sums up the contents of the
EMACTIMADD register, as shown in Figure 24-12 on page 1633. The arithmetic carry that the
accumulator generates is used as a pulse to increment the system time counter. The accumulator
and the addend are 32-bit registers. Here, the accumulator acts as a high precision frequency
multiplier or divider.
December 13, 20131632
Texas Instruments-Advance Information
Ethernet Controller