System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
3-25
The RTI can generate a “time tick” that is useful for a run time software time tick or to wake up the MCU
from a low power (Sleep) Mode. Long battery life in many ZigBee and/or 802.15.4 applications is based
on long periods of “sleep” or low power modes. The MCU can put the modem in the Off or Hibernate
Mode and then go to a low power mode such as a Stop2 or Stop3 Mode. Although the application may
require the sleep period to be much longer than 1 second, the MCU can wake up every second, increment
a software counter, and then return to sleep if the software counter has not reached the target count.
3.11.4
MCU Computer Operating Properly (COP) Watchdog
Another independent MCU timer is the COP timer which is intended to force a system reset when the
application fails to execute as expected. To prevent the reset, the software must reset the COP timer (if
enabled) periodically before the timer times out. The COP timer use is optional and can be programmed
to have a period of 2
18
or 2
13
cycles of the bus rate clock. The COP is covered in
.
3.12
Low Power Considerations
Many ZigBee and/or 802.15.4 applications such as sensor End Devices are required to be battery operated.
As expected, long battery life is highly desirable and is very dependent on application parameters.
Over-the-air operation uses modes, i.e., RX, TX, and CCA, where power is highest. As a result, the time
between radio operations should be kept at the longest possible period that the application will allow.
When designing low power operation of the MC1321x SiP, consider:
•
The modem and the MCU each have several low power options
•
The modem is entirely controlled by the MCU; the low power options/combinations will be
determined by MCU programming
•
The power down control of the modem must be maintained by the MCU in its power down
configuration
•
All unused port GPIO (including those not pinned-out) must be configured to a known state
(preferably output low) to prevent unwanted leakage current.
Lowest power in a system is more than just putting the modem and/or the MCU in a low power mode. The
relationship between the functions, the timing between them, and clock management must all be
considered. The duty cycle between active operations is also very important as it can impact whether sleep
operation or active operation will have the biggest impact over an extended time period.
3.12.1
Modem Low Power States
lists the modem low power states and the modes are covered in detail in
. There are three low power modes available:
•
Off - Requires the modem reset M_RST input to stay asserted low. Lowest possible power and all
functions are disabled. Digital I/O are tristated
•
Hibernate - Has next lowest power, all hardware blocks deactivated, RAM and SPI register data
are retained. Digital I/O retain their state
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