Time Representation - 1.3 English

Versal Devices Integrated 100G Multirate Ethernet MAC Subsystem Product Guide (PG314)

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1.3 English

The IEEE 1588 uses two types of time field formats:

Time-of-Day (ToD) Format
IEEE 1588-2008 format consisting of an unsigned 48-bit second field and a 32-bit nanosecond field. This format is compatible with version 1 of the standard.
Correction Field Format
Introduced in version 2 to handle Transparent clocks and better than 1 ns precision. It is a signed 64-bit integer in units of 2-16 ns. For example, 2.5 ns is represented as 64’h0000_0000_0002_8000.

The system_timer is implemented as an internal, unsigned linear counter which counts time in units of 2-40 ns. Each clock cycle system_timer is incremented by the quantity increment_value, which represents the period of the system timer clock in the same units. For example, for a nominal 644.0625 MHz clock, the increment value would be 42'h18D_3018_D302 (1,705,908,949,762 × 2-40 ns). This increment value is configured by the MRMAC at start-up. If KR4-FEC is in use, a different initial value is configured: 42’h181_8181_8182.

The MRMAC exposes 55 bits of the system_timer through the various timestamping interfaces (that is, TX and RX packet timestamps). This is equivalent to a timer which counts in units of 2‑8 ns (that is, ~3.9 ps). User logic can map these 55 bits directly into Bits[62:8] of a Correction Field format timestamp. The bottom eight correction field bits should be set to 8’h0 as this degree of precision is not supported by the architecture. Note that this same mapping is used in 1-step timestamp insertion.

The least-significant 32 bits of the MRMAC (that is, < 2-8 ns portion of the counter) is not available through the timestamping interfaces. This allows the system_timer to be adjusted at sub-nanosecond levels to track an external master clock as described in the next section.