Novatech Bitronics M87X Series, Manual

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ML0021 August 15, 2018
75
Copyright 2018 Bitronics, LLC
If the M87x is connected to an IRIG master that is not IEEE-1344 compatible and the year
reported by the M87x’s clock is incorrect, the IRIG Driver may also set the M87x’s day
incorrectly (due to leap year) when it tries to synchronize the device time to the IRIG time.
The time, however, will still synchronize correctly. As a result, if the M87x’s backup fails
(or the year was not set correctly), any data time-stamped by the M87x or any waveform
captures stored may have the wrong year and day but will have the correct time accurate
to several microseconds. This data can still be synchronized to other events from other
devices by simply adding the correct day and year offsets to the time.
5.7.5 Methods of Automatic Clock Adjustments
The automated clock adjustments controlled by the IRIG interface include “jamming” the
clock and “slewing” the clock. Depending on the magnitude of the M87x’s absolute clock
error the clock adjustment algorithms will either jam the clock by directly writing a new
value into the clock registers or slew the clock smoothly by adding or subtracting small
adjustments to the clock registers over a period of time.
5.7.6 Types of M87x Clock Synchronization
There are various degrees (or states) of time synchronization. Upon power up, the device
relies on the value stored in the capacitor backed-up clock to set the correct time, and the
crystal frequency correction constant stored in non-volatile memory to correct the crystal’s
frequency. The M87x will keep time starting from the values read from the clock. There
will be an accumulated time error based upon the frequency error of the Real Time Clock
crystal. The crystal frequency correction constant provides a means for correcting for this
error. If the M87x was never synchronized to an external source (i.e. IRIG-B or network
synchronization protocol), the M87x will not have a value for the crystal frequency
correction constant and the crystal error will be the M87x’s clock error.
5.7.6a Frequency Adjustments and Free Wheeling
The M87x has the capability to add a correction factor to compensate for the crystal’s
effective frequency error rate. This frequency adjustment is accomplished by first
determining the crystal’s error rate and then correcting the clock to reflect that error. The
IRIG-B interface serves as an external accurate time source to determine the crystal’s
typical error rate. The frequency error is calculated and stored in non-volatile memory on
the M87x’s Host CPU board.
When an M87x is connected to an IRIG-B source, it will automatically calculate and store
the crystal’s error in non-volatile memory on the Host CPU board. M87x transducers
utilize this constant to maintain a more accurate clock. If the IRIG-B source is removed the
M87x will no longer receive time corrections from the IRIG-B source, but the device clock
will keep much better time due to the frequency correction constant. This mode of
operation is referred to as “Free Wheeling.”
Although “Free Wheeling” with constant frequency compensation provides a more
accurate M87x clock, it will still drift and is less accurate than having a constant IRIG-B
source connected to the M87x. The frequency error of the crystal will change with time
and temperature. Having a permanent real time IRIG-B clock source allows for constant
minute adjustments to the M87x clock.