Altera 10-Gbps Ethernet MAC MegaCore Function - Manual (Page 161)

Appendix B: Time-of-Day (ToD) Clock B–5
February 2014 Altera Corporation 10-Gbps Ethernet MAC MegaCore Function User Guide
B.6.1. Adjusting ToD’s Drift
You can use the DriftAdjust and DriftAdjustRate registers to correct any drift in the
ToD clock.
For example, in the case of a ToD for 10G with period of 6.4ns, the nanosecond field is
converted directly to PERIOD_NS while the fractional nanosecond need to be
multiplied with 2
16 or 65536 in order to convert to PERIOD_FNS. This results in 0x6
PERIOD_NS and 0x6666.4 PERIOD_FNS.
PERIOD_NS only accepts 0x6666 and ignores 0x0000.4, which in turn would cause some
inaccuracy in the configured period. This inaccuracy will cause the ToD to drift from
the actual time as much as 953.67ns after a period of 1 second. You would notice that
after every 5 cycles, 0x0000.4 accumulates to 0x0002. If the TOD is able to add 0x0002
of fractional nanosecond into the ToD once after every period of 5 cycles, then it will
correct the drift.
Therefore, for the 10G case, DRIFT_ADJUST_NS is now configured to 0x0,
DRIFT_ADJUST_FNS
is configured to 0x0002 and ADJUST_RATE is configured to 0x5.
0x1C DriftAdjust RW
The drift of ToD adjusted periodically by adding a
correction value as configured in this register space.
■ Bits 0 to 15: Adjustment value in fractional
nanosecond (DRIFT_ADJUST_FNS). This value is
added into the current ToD during the adjustment.
The default value is 0.
■ Bits 16 to 19: Adjustment value in nanosecond
(DRIFT_ADJUST_NS). This value is added into the
current ToD during the adjustment. The default value
is 0.
■ Bits 20 to 32: Not used.
0x0
0x20 DriftAdjustRate RW
The count of clock cycles for each ToD’s drift adjustment
to take effect.
■ Bits 0 to 15: The number of clock cycles
(ADJUST_RATE). The ToD adjustment happens once
after every period in number of clock cycles as
indicated by this register space.
■ Bits 16 to 32: Not used.
0x0
Table B–6. ToD Clock Registers (Part 2 of 2)
Byte
Offset
Name R/W Description
HW
Reset