SAM-M10Q - Integration manual
RXM-PMREQ message. System reset with RESET_N signal also terminates the software standby
mode and restarts the receiver. A system reset clears the BBR content.
As V_IO is supplied, the PIOs can be driven by an external host processor. No buffers are required for
isolating the PIOs, which reduces cost.
The "force" flag must be set in UBX-RXM-PMREQ to enter software standby mode.
V_BCKP should be left open if not used.
2.7 Time
Maintaining receiver local time and keeping it synchronized with GNSS time is essential for proper
timing and positioning functionality. This section explains how the receiver maintains local time and
introduces the supported GNSS time bases.
2.7.1 Receiver local time
The receiver is dependent on a local oscillator for both the operation of its radio parts and also for
timing within its signal processing. No matter what nominal frequency the local oscillator has, u-blox
receivers subdivide the oscillator signal to provide a 1-kHz reference clock signal, which is used to
drive many of the receiver's processes. In particular, the measurement of satellite signals is arranged
to be synchronized with the "ticking" of this 1-kHz clock signal.
When the receiver first starts, it has no information about how these clock ticks relate to other time
systems; it can only count time in 1 millisecond steps. However, as the receiver derives information
from the satellites it is tracking or from aiding messages, it estimates the time that each 1-kHz
clock tick takes in the time base of the chosen GNSS system. This estimate of GNSS time based on
the local 1-kHz clock is called
receiver local time
.
As receiver local time is a mapping of the local 1-kHz reference onto a GNSS time base, it
may experience occasional discontinuities, especially when the receiver first starts up and the
information it has about the time base is changing. Indeed, after a cold start, the receiver local
time will initially indicate the length of time that the receiver has been running. However, when the
receiver obtains some credible timing information from a satellite or an aiding message, it will jump
to an estimate of GNSS time.
2.7.2 GNSS time bases
GNSS receivers must handle a variety of different time bases as each GNSS has its own reference
system time. What is more, although each GNSS provides a model for converting their system time
into UTC, they all support a slightly different variant of UTC. So, for example, GPS supports a variant
of UTC as defined by the US National Observatory, while BeiDou uses UTC from the National Time
Service Center, China (NTSC). While the different UTC variants are normally closely aligned, they
can differ by as much as a few hundreds of nanoseconds.
Although u-blox receivers can combine a variety of different GNSS times internally, the user must
choose a single type of GNSS time and, separately, a single type of UTC for input (on EXTINT pins)
and output (via the TIMEPULSE pin) and the parameters reported in corresponding messages.
The CFG-TP-TIMEGRID_TP* configuration item allows the user to choose between any of the
supported GNSS (GPS, GLONASS, BeiDou, etc.) time bases and UTC. Also, the CFG-NAVSPG-
UTCSTANDARD configuration item allows the user to select which variant of UTC the receiver
should use. This includes an "automatic" option which causes the receiver to select an appropriate
UTC version itself, based on the GNSS configuration, using, in order of preference, USNO if GPS is
UBX-22020019 - R01
2 Receiver functionality
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