October 2014
16
E
MBEDDED
W
IRELESS
such as the modulation scheme, the speed at
which data is transmitted, and the RF output
power into the antenna, will all affect the over-
all power consumption. As a general guideline,
shorter periods of active operation will result
in lower average power consumption. This
can include ensuring that all the devices used
in the design, from an LED to a microcontrol-
ler or transmitter, spend the maximum time
in low-power mode.
Using a higher data rate will mean that the
design uses more power, but the trade-off is
that shorter packet lengths will reduce energy
consumption. The modulation scheme will
also have a role to play in managing energy
consumption. ASK or OOK modulation uses
less energy because there are periods when the
RF power is reduced with ASK modulation, or
zero with OOK. The average current draw will
also be lower with ASK. Despite this, the pre-
ferredmodulation scheme is FSK because it can
achieve significantly higher data rates. If basic
one-way communication is all that is needed,
the design could use a simple RF transmit-
ter; but if the goal is certification to a wireless
standard such as IEEE 802.15.4 then a special-
ized controller may be preferred. For example,
Microchip PIC12LF1840T48Amicrocontroller
has an integrated transmitter which supports a
data rate of 10 Kbps in OOK mode and 100
Kbps in FSK mode. This data is therefore sent
ten times faster using FSK modulation than
with OOK. At higher data rates, an RF receiver
can also receive and decode FSK signals much
more efficiently than with ASK modulation.
The low-power shutdown modes of the micro-
controller can also be used tominimize energy
consumption. The frequency with which the
sensor needs to transmit data will depend on
the response time of each application. Extend-
ing the time between active periods will mean
that the controller will spend more time in
low-power mode and therefore reduce the
average power consumption. The current
draw will also be determined by the type of
data that the sensor captures between trans-
missions. Receiving data from op-amps and a
bridge load cell, for example, will demand a
relatively large current compared to the cur-
rent used during the transmission of RF data.
An
example
design
based
on
PIC12LF1840T48A microcontroller shows
the calculations for energy consumption: the
integrated transmitter of the microcontroller
has a maximum frequency deviation of up to
200 kHz which allows a maximum bit-rate
of 100 Kbps. Using a small data packet with
a 16-bit preamble, a 16-bit synchronization
pattern and a 32-bit payload, one complete
data packet can be transmitted in just 640 μs.
With the energy measured in joules (J) this
provides:
1J = 1W * 1s = 1V * 1A * 1s
The energy consumption used for sending one
data package is calculated by:
E = 10.5mA * 640μS = 10.5mA * 3.0v * 640μS
= 31.5mW * 640uS = 20.16μJ
The start-up time for the crystal oscillator is
typically 650 μs, with an energy draw of 5 mA
during start-up. The start-up power consump-
tion is therefore calculated by:
E1 = 5mA * 3.0v * 650μs = 9.75μJ
The data transmission used in the example
design contains 16 bits of preamble, 16 bits
of synchronization pattern and 32 bits of data.
For the selected bit-rate of 100 Kbps, this gives
a transmission time of 640 μs. For a RF trans-
mission of +0 dB at 868 MHz, using FSK mod-
ulation, the power consumption is 12 mA.
E2 = 12mA * 3v * 640μs = 23.04μJ
Using a simple transmission at 10 kbps the
energy used would be:
E2 = 7.5mA * 3v * 6.40ms = 144μJ
This comparison shows the difference in the
energy used and reinforces the importance of
using a higher data rate.
The PIC12LF1840T48A transmitter will auto-
matically time-out and revert to a low-power
shutdown mode after sending the last data bit.
With a minimum timeout period of 2ms the
additional energy consumption will be:
E3 = 12mA * 3v * 2ms = 72μJ
These calculations provide a total power draw
for the transmission of a single data packet of:
E = E1 + E2 + E3 = 9.75μJ + 23.04μJ + 72μJ
= 104.79μJ
A miniature solar cell that generates a current
output of 4.5μA at 3V will need to be active
for the number of seconds that are required to
get enough energy for a single data transmis-
sion. Using a low-cost solar cell as an example,
a best-case scenario of 3V at 40μA, only gen-
erates 120μW of power:
3V * 40μA = 120μW
The calculation for the amount of time
required to collect sufficient energy to send a
single data transmission is:
T = 104.79μJ / 120μW = 0.87s
This shows that the sensor unit has to wait
for 0.87 seconds between two sequential data
transmissions assuming that the solar cell has
a constant light source. In real-world applica-
tions the natural light, which is the primary
source of energy, is available only during
the day. The calculation, therefore, must be
extended to take into consideration the fact
that the harvesting system must store the
energy harvested during the day so that it can
be used during the night. Another factor to
consider is that the energy required to carry
out the actual sensor measurement in not
included in the example calculations. There
are a number of options which can be imple-
mented to store the energy harvested during
daylight hours. These options include using
a supercapacitor as the storage element, or
trickle-charging low-cost NiMH rechargeable
Figure 1. Block diagram of the PIC12LF1840T48A microcontroller
