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Architecture
1321
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Real-Time Clock (RTC)
27.2 Architecture
27.2.1 Clock Source
The clock reference for the RTC is an external 32.768-kHz crystal or an external clock source of the same
frequency. The RTC also has a separate power supply that is isolated from the rest of the system. When
the CPU and other peripherals are without power, the RTC can remain powered to preserve the current
time and calendar information.
The source for the RTC reference clock may be provided by a crystal or by an external clock source. The
RTC has an internal oscillator buffer to support direct operation with a crystal. The crystal is connected
between pins RTC_XI and RTC_XO. RTC_XI is the input to the on-chip oscillator and RTC_XO is the
output from the oscillator back to the crystal. For more information about the RTC crystal connection, see
your device-specific data manual.
An external 32.768-kHz clock source may be used instead of a crystal. In such a case, the clock source is
connected to RTC_XI, and RTC_XO is left unconnected.
If the RTC is not used, the RTC_XI pin should be held low and RTC_XO should be left unconnected. The
RTCDISABLE bit in the control register (CTRL) can be set to save power; however, the RTCDISABLE bit
should not be cleared once it has been set. If the application requires the RTC module to stop and
continue, the RUN bit in CTRL should be used instead.
27.2.2 Signal Descriptions
The RTC signals are listed in
Table 27-1. Real-Time Clock Signals
Signal
I/O
Description
RTC_XI
I
RTC time base input signal. RTC_XI can either be driven with a 32.768-kHz reference clock, or RTC_XI
and RTC_XO can be connected to an external crystal. This signal is the input to the RTC internal
oscillator.
RTC_XO
O
RTC time base output signal. RTC_XO is the output from the RTC internal oscillator. If a crystal is not
used as the time base for RTC_XI, RTC_XO should be left unconnected.
27.2.3 Isolated Power Supply
The RTC has a power supply that is isolated from the rest of the system. This allows the RTC to continue
to run while the rest of the system is not powered. In this state, the RTC time and calendar counters
continue to run, but the powered down CPU is not able to receive RTC interrupts. Separate power supply
pins for the RTC are provided on the device package.
27.2.3.1 Split-Power Circuitry
To decrease power consumption, RTC includes leakage-isolation circuitry that is activated by setting the
SPLITPOWER bit in the control register (CTRL). Because of its isolated power supply, RTC does not have
a power-on hardware reset signal. Therefore, upon initial device power-on, the RTC is in an unknown
state until it has been properly configured. After the RTC module has been configured once, it functions as
programmed as long as its power supply and clock source are provided.
27.2.3.2 Power Considerations
The RTC leakage-isolation circuitry requires that the CPU supply be powered down to VSS when the RTC
is powered on while the rest of the device is powered off. A floating CPU supply creates undesired RTC
leakage current. Also, the RTC power consumption is higher when the CPU is powered on versus the
RTC power consumption when the CPU is powered off. Therefore, if the RTC module is expected to run
from a small-capacity power supply (ex. watch battery) while the rest of the device is powered off, a power
system should be implemented such that the RTC is powered from a high-capacity power supply when the
CPU is powered on.