Image Processing Reference
In-Depth Information
One aspect that is often overlooked is the time the elements need to turn on and off. For example,
the transceiver will need some minimum time for its oscillators to stabilize. While they wait, both
thetransceiverandtheCPUburnpower.histhereforeneedstobeminimized.hesameobviously
holdstrueforboththeCPUandthesensor.
Wireless Vibration Monitoring Case Example
The sensor node uses analog electronics components to filter the “raw” vibration signal into a
desired format. he CPU has control over the activation/deactivation of this analog part, and only
activates it for the duration of the measurement. The analog part has a fairly long settling time,
∼
s, which forces the node to wait until a usable signal is available for processing (fortunately,
the CPU can spend this time in the standby low-power mode).
Another very important settling time is the time it takes for the CPU to become fully opera-
tional when waking up from a power save mode. For the MSP variant used, this time is only
µs from both the of mode and the standby mode. But, the of mode has the added benefit of
allowing us to further decrease power consumption by shutting off the oscillator signal from the
RTC to the CPU (which is used to generate the CPU clock). However, without the oscillator sig-
nal, the CPU will default to an inaccurate, internal, RC-generated, lower frequency clock. hus,
when waking up from the of mode, the RTC oscillator signal has to be activated before the CPU
will stabilize, which adds to the µs wake-up time.
Timersareanintegralpartofanysleep-wake-upscheme.Generally,ultralowpowerCPUsonlyhave
or bit resolution timers, which, when used to generate interrupts at scheduled timeouts, have
fairly “short” maximum intervals (in the order of a few seconds). If a longer sleep interval is desired,
the timer is forced to wake up (i.e., the CPU becomes active), reset its counter, and go to sleep several
times during that interval. Furthermore, in some power-saving modes (as described in the previous
example), the CPU shuts down its internal clock, which disallows the use of the internal timers.
Thus, if internal timers are the choice for the wake-up scheme, some power-saving modes will not be
usable.
Wireless Vibration Monitoring Case Example
The duty cycle of the vibration monitoring device is weeks. The MSP CPU used is driven
by a kHz crystal, and only provides bit timers, which equates to a maximum interval of
onlys.hiswouldrequirethetimerinterruptto(a)activate,(b)resetthetimercounter,and
(c) decrement the total sleeping time, a total of , times ( days
, , s and a s
timeout) during the week interval! In combination with the desire to use the lowest power-
saving mode (of mode) of the CPU, the solution was to choose an external timer, i.e., an RTC,
which supports very long timeout intervals (actually years), and allows the CPU to stay in the
“off power-saving mode during the sleep time.
=
27.6.3 HMI
Asnotedinprevioussections,alocalHMIiscommonlyrequiredtocheckthestatus,calibrate,and
maybe even instruct the device to perform a measurement. From a low-power design point of view,
this represents additional components that consume power. Depending on the requirements on
packaging or the available power budget, several alternatives are used in the industry ranging from
simple LEDs and buttons to LCDs. When designing the HMI functionality, great care must be taken
how the HMI is used. As battery-powered systems cannot afford to constantly have the HMI active, it
