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MRF89XA
3.13
Battery Power Management
Configuration Values
Note:
Many host microcontrollers cannot be
Battery life can be greatly extended in MRF89XA appli-
cations where transmissions from field nodes are infre-
quent, or network communications can be
concentrated in periodic time slots. For example, field
nodes in many wireless alarm systems report opera-
tional status a few times a day, and can otherwise sleep
unless an alarm condition occurs. Sensor networks that
monitor parameters that change relatively slowly, such
as air and soil temperature in agricultural settings,
switching lights ON/OFF, only need to transmit
updates a few times per an hour.
At room temperature, the MRF89XA draws a maximum
of 1 μ A in Sleep mode, with a typical value of 100 nA.
To achieve minimum Sleep mode current, the CSCON
pin (pin 14), SDI pin (pin 17) and SCK pin (pin 18) must
be held logic low, while the CSDAT pin (pin 15) and
SDO pin (pin 16) must be held logic high.
The MRF89XA can go from Sleep mode through
Stand-by mode and Synthesizer mode to Transmit (or
Receive) mode in less than 6 ms. For configuring and
driving the device different operating modes refer to
Table 2-3 . At a data rate of 33.33 kbps, a 32-byte
packet with a 4-byte preamble and a 4-byte start pat-
tern takes about 10 ms to transmit. Assume that the
MRF89XA then switches to Receive mode for one sec-
ond to listen for a response and returns to Sleep mode.
On the basis of reporting every six hours, the ON to
Sleep duty cycle is about 1:21,259, greatly extending
battery life over continuous transmit-receive or even
stand-by operation.
The required timing accuracy for the microcontrollers in
a sleep-cycled application depends on several factors:
? The required “time-stamp” accuracy of data
reported by sleeping field nodes. R-C Sleep mode
timers built into many microcontrollers have a tol-
erance of ±20% or more. For applications that
require more accurate time-stamping, many
microcontrollers can run on a watch crystal during
Sleep mode and achieve time-stamp accuracies
better than one second per 24 hours.
? If the base station and any routing nodes present
in a network must sleep cycle in addition to the
field nodes. Watch crystal control will usually be
needed to keep all nodes accurately synchronized
to the active time slots.
? If the base station and any routing nodes present
in a network can operate continuously (AC pow-
ered, solar charged batteries), and a loose time
stamp accuracy is OK, the microcontrollers in
sleeping field nodes can usually operate from
internal low-accuracy R-C timers
operated from the MRF89XA buffered
clock output if sleep cycling is planned. In
Sleep mode, the MRF89XA buffered clock
output is disabled, which will disable the
microcontroller unless it is capable of
automatically switching to an internal
clock source when external clocking is
lost.
Therefore, as previously mentioned, Sleep mode is the
lowest power consumption mode in which the clock
and all functional blocks of the device are disabled. In
case of an interrupt, the device wakes up, switches to
Active mode and an interrupt signal generated on the
IRQ pin indicates the change in state to the host micro-
controller. The source of the interrupt can be
determined by reading the status word of the device.
To reduce current consumption, the MRF89XA
should be placed in the low-power consuming Sleep
mode. In Sleep mode, the 12.8 MHz main oscillator
is turned OFF, disabling the RF and baseband cir-
cuitry. Data is retained in the control and FIFO regis-
ters and the transceiver is accessible through the
SPI port. The MRF89XA will not enter Sleep mode if
any interrupt remains active, regardless of the state
of the CLKOCNTRL bit (CLKOUTREG<7>). This
way, the microcontroller can always have a clock sig-
nal to process the interrupt. To prevent high-current
consumption, which results in shorter battery life, it is
highly recommended to process and clear interrupts
before entering Sleep mode. The functions which are
not necessary should be turned off to avoid
unwanted interrupts. To minimize current consump-
tion, the MRF89XA supports different power-saving
modes, along with an integrated wake-up timer.
When switching from Sleep mode to Stand-by mode,
the crystal oscillator will be active for no more than 5
ms. Switching from Stand-by mode to Synthesizer
mode, the PLL will lock in less than 0.5 ms. PLL lock
can be monitored on the PLOCK pin (pin 23) of the
MRF89XA. The radio can then be switched to either
Transmit or Receive mode. When switching from any
other mode back to Sleep mode, the device will drop to
its Sleep mode current in less than 1 ms.
? 2010–2011 Microchip Technology Inc.
Preliminary
DS70622C-page 91
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