TrueTime VME-SG2 560-5608, Manual

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A battery is provided on the board to supply power to the GPS Engine when the main power is
off. The battery power drives RAM memory and a real time clock located on the GPS Engine
card. The RAM is used to store position and almanac information. If this information is not
available at power up the GPS Engine will perform a cold start and enter a Search The Sky
mode to locate satellites. When a satellite is tracked, new almanac data is downloaded. This
process can take anywhere from 12 to 25 minutes. This process is necessary to compute
position and determine accurate time. See Position Mode in section 2.4 for more information.
After the VME-SG-GPS has attained phase lock and computed position, these parameters are
maintained in the battery backed GPS Engine even if power is lost. When power is restored
these parameters will be transferred into the time and position registers in the main processor
board. This occurs a few seconds after the card has initialized itself. If UTC time is selected
for output (config register #2, bit 7) the time information will indicate a difference of several
seconds after power up since GPS time rather than UTC time is maintained in the GPS Engine
during the power down time.
A jumper is installed on JP2 between pins 2 and 3 to connect the battery at the factory prior to
shipment. If the card is to be stored for extended periods (months) the jumper should be
installed between pins 1 and 2 which disconnects the battery. When power is ON there is no
battery current drain.
2.4 OPERATION, DETAILED INFORMATION
2.4.1 GENERATOR
The Generator accumulates time using an internal or external time base and provides
time information to the VME bus consisting of microseconds through thousands of
years. The VME-SG-GPS contains two time capture register sets. Each capture
register consists of five 16 bit word locations where the current time is "frozen". Time is
captured in register number 1 when a time requests via the VME bus occurs. Time is
captured in register number 2 by an External Event input. This allows the user to
request time on demand as needed and also record when an external event has
occurred even if both occur simultaneously.
The generator is started and stopped using Configuration Register #1. The user may
preset the time from milliseconds through years while the Generator is stopped or
running. The generator can be synchronously started using an externally generated 1
PPS signal. The start strobe, when enabled, is input via the External Event input
connector. Either the rising or falling transition of the pulse is selected.
2.4.2 GPS SYCHRONIZED GENERATOR
The GPS Synchronized Generator mode operates as a Generator that is synchronized
to the GPS Engine. The GPS Engine receives transmissions from the Global
Positioning Satellite system and derives time that is traceable to the National Institute of
Standards and Technology (NIST). If the GPS Engine indicates that good time is
available, the card will phase lock the generator time registers. The VME-SG-GPS card
contains a Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO).
The VCTCXO is disciplined to remove any frequency errors. The phase errors are
removed by the Phase Correction logic which advances or retards the generator time
using 100 nanoseconds steps.
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