TM4C1294NCPDT Datasheet, PDF (1442/1890 Pages) Texas Instruments – Tiva Microcontroller

Ethernet Controller
MAC. This timing information is captured and returned to the software for the proper implementation
of PTP with high accuracy.
20.3.5.1
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 20-9 on page 1443. 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.
1442
Texas Instruments-Production Data
June 18, 2014