Duty Cycle and Power Optimization

APPLICATION NOTE AND9310/D
Duty Cycle and
Power Optimization
Revison 2
2
Table of Contents
1.
Duty Cycle .................................................................................................................................... 3
1.1. General............................................................................................................................................... 3
1.2. Hardware description ...................................................................................................................... 3
1.3. Software description ........................................................................................................................ 4
Dynamic RSSI threshold level calculation .................................................................................... 5
1.4. Duty cycle with AX5051 ................................................................................................................... 6
Scenario with 100kbit/s .................................................................................................................... 7
Other scenarios ................................................................................................................................. 8
2.
Conclusion .................................................................................................................................... 9
3.
Appendix .................................................................................................................................... 10
3.1. References ....................................................................................................................................... 10
AX5051 downloads ......................................................................................................................... 10
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Duty Cycle
1.
Duty Cycle
1.1. General
Consider a battery powered device (target), which should receive data from a second
device (initiator) from time to time. To reduce power consumption, the target switches its
receiver on for only a short while, checking if there is any RF activity, and returns to sleep if
there is no data to receive.
target
initiator
Figure 1
definition of initiator and target
Current consumption
The ratio of the time ton during which the target is powered on, to the time toff during which
the target is powered off is called duty cycle. If for example the target is powered off for 1
second, powered on for 1 millisecond, powered off for 1 second and so on, its duty cycle is
1:1000.
ton
toff
device
is on
device is in SLEEP mode
Figure 2
time
illustration of duty cycle
1.2. Hardware description
The hardware used in this document consists of an AX5051 evaluation board that is plugged
into a suitable controller board, containing a PIC16F886 microcontroller from Microchip. The
microcontroller is required to configure and control the AX5051 transmitter.
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Duty Cycle
AXSEM
AX5051
Microchip
16F886
Figure 3
hardware used for duty cycle measurements
The PIC16F886 has a feature called ‘Ultra low power wake up’ (ULPWU). With this feature, an
external capacitor is charged when the system is running, and discharged when the system is
in sleep mode. When the condensator voltage reaches a defined level, the processor is
woken up by an interrupt. This feature is very suitable for duty cycle applications.
1.3. Software description
The software demonstrates how duty cycle can be implemented in software. The software
can be compiled for the transmitter or for the receiver. When defining CONST_TX, the source
code will be compiled for the transmitter, if CONST_RX is defined, the source code will be
compiled for the receiver. Make sure, only CONST_TX or CONST_RX is defined at one time. The
transmitter sends a 1.2s long preamble every time a button is pressed. After the data is
transmitted, it enters sleep mode and consumes only 650 nA. The receiver is in sleep mode
too, but it wakes up every second. First the crystal oscillator needs to be switched on. This can
take 1.7ms. After the crystal oscillator is running, the receiver is switched on and the program
checks if the RSSI is above the defined threshold. If this is the case, 5 bytes of data are
received, to check if there is a valid preamble present. If the transmitter is sending data at
that time, the receiver will be switched on, until the data is received. This will take max. 1.2
seconds. After this the receiver is switched off, and the controller enters sleep mode again.
See Figure 4 for an illustration of the flow during the wake up phase.
RSSI above threshold?
wake up
yes
valid preamble?
yes
no
receive data
no
sleep
power down
145 us
500us @ 100kbit/s
Figure 4
max 1.2s
flow during duty cycle wake up
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Duty Cycle
The software was compiled and tested with sdcc. To compile the code, please install the
gputils (http://gputils.sourceforge.net), and modify the makefile according to your system
settings. For downloading and debugging, the MPLAB IDE from Microchip was used together
with the MPLAB ICD2 programmer/debugger.
Important

Make sure, that when porting the software to another platform, calling delay(x) will
keep the processor busy for x ms. The delay must be exact, otherwise some parts of
the program may not work correct (e.g. starting the crystal oscillator).

As mentioned above, CONST_RX and CONST_TX must be defined before compiling
the code.

The PIC16F886 has a hardware stack, so the number of function calls is limited to only
a few levels. Because of this fact, the whole state machine is implemented in the
main loop. Avoid calling functions within other functions whenever possible!
Dynamic RSSI threshold level calculation
There are several problems with a fixed RSSI threshold level. If it is set too high, valid data
arriving with low levels will not wake up the target, even if it could have received the data
correctly. Loss of sensitivity is the result. The other problem arises, when the RSSI level is set too
low. In this case, the RSSI check will always be positive, thus the target will stay in RX mode
longer than required. Higher current consumption is the result. There is a trade-off between
sensitivity and current consumption that must be considered by the developer.
There is no RSSI threshold suitable for every situation. Consider a situation, where the target is
placed in a very noisy environment. This will require a higher RSSI threshold than if the target is
placed in an environment where the background noise is low. For targets, that change the
location regularly (car key for instance) it is required, that the RSSI threshold is adapted
dynamically. This can be done by averaging a certain number of RSSI values that where
calculated in previous wake up phases.
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Duty Cycle
1.4. Duty cycle with AX5051
All results in this document are obtained by using the AX5051 as an RF receiver, which is being
switched on every second. At first the RSSI is checked against a predefined threshold to
determine if there is any RF activity present. If not, the AX5051 is switched off immediately by
the microcontroller. Otherwise 5 bytes have to be received to determine if a valid preamble
is present. To wake up the receiver, the transmitting unit needs to transmit a preamble which
is longer than the time toff.
There are three stages used: Sleep, crystal startup and receive:
Sleep
Microcontroller and receiver are switched off for nearly one second. To
wake up the microcontroller from sleep, a low power oscillator is required.
The power consumption of the microcontroller in sleep mode was
measured to be 0.65uA. The current consumption of the AX5051 in power
down mode was measured to be 0.73uA.
Crystal startup
When the AX5051 comes out of power down mode, it takes some time until
the crystal oscillator settles on its nominal frequency of 16MHz. The time until
the crystal is fully settled, was measured to be 1.7ms. The current drawn by
the controller was measured while it was running at full speed (8MHz).
Receive
After the crystal has settled, the receiver is switched on. At first the RSSI is
calculated to see if it is above the defined threshold. If not, sleep mode is
entered immediately. Otherwise 5 bytes are received, to check if a valid
preamble is present. All diagrams in this document assume that no valid
preamble is present (which is the normal case).
To provide a general impression, the currents drawn by the AX5051 and the microcontroller
where measured separately. Note that the microcontroller’s current consumption is device
specific and must be evaluated individually.
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Duty Cycle
Scenario with 100kbit/s
145 us
100 kbit/s
with RSSI below threshold
750 uA
22.2 mA
average current consumption
PIC16F886: 2.03 uA
AX5051: 5.14 uA
total: 7.17 uA
1.7 ms
998.2 ms
0.65 uA
750 uA
0.65 uA
702 uA
0.73 uA
Sleep
0.73 uA
Crystal startup
Figure 5
Receive
Current consumption with 100kbit/s, RSSI below threshold
500 us
100 kbit/s
with RSSI above threshold
750 uA
22.2 mA
average current consumption
PIC16F886: 2.3 uA
AX5051: 13.02 uA
total: 15.32 uA
1.7 ms
997.8 ms
0.65 uA
750 uA
0.65 uA
702 uA
0.73 uA
0.73 uA
Sleep
Crystal startup
Figure 6
Receive
Current consumption with 100kbit/s, RSSI above threshold
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Duty Cycle
Other scenarios
Table 1 compares the current consumption for different bitrates. Note that the current
consumption for every bitrate choose is equal, if the RSSI is below the threshold. This is due to
the fact that calculating the RSSI is independent of the bitrate.
bitrate [kbit/s]
AX5051 current [uA]
PIC16F886 current [uA]
total current [uA]
RSSI low
RSSI high
RSSI low
RSSI high
RSSI low
RSSI high
1.2 (FSK)
5.14
744.12
2.03
26.98
7.17
771.09
4.8(FSK)
5.14
189.14
2.03
8.24
7.17
197.38
9.6(FSK)
5.14
96.64
2.03
5.12
7.17
101.76
38.4(FSK)
5.14
27.27
2.03
2.78
7.17
30.05
100(FSK)
5.14
13.02
2.03
2.3
7.17
15.32
250(FSK)
5.14
7.69
2.03
2.12
7.17
9.81
400 (ASK)
5.14
6.36
2.03
2.07
7.17
8.43
600 (ASK)
5.14
5.62
2.03
2.05
7.17
7.67
Table 1
Average current consumption for different bitrates
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Conclusion
2.
Conclusion
The current used during receive mode is the main factor impacting the average current
consumption (this is especially true for low bitrates). It is therefore very important to keep the
time when the receiver is switched on, as short as possible. Even if the AX5051 does not have
built in WOR (Wake On Radio) capabilities, the overall current consumption (including the
microprocessors current) is low compared to competitors. This is achieved by using the great
flexibility provided by the AX5051 that makes it possible to reduce the time, the receiver is
switched on, to a minimum. With the flexibility of the AX5051, one is free to optimize for own
requirements, and is not bound to a specific packet format.
145 us
100 kbit/s
with RSSI below threshold
750 uA
22.2 mA
3.219 uAs
1.7 ms
998.2 ms
0.65 uA
750 uA
0.65 uA
702 uA
0.73 uA
0.73 uA
0.7287 uAs
Sleep
Figure 7
Crystal startup
1.193 uAs
Receive
Distribution of current consumption during one cycle
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Appendix
3.
Appendix
3.1. References
AX5051 downloads
Download AX5051 datasheet and programming manual from http://www.onsemi.com
Download duty_cycle source code from http://www.onsemi.com
AX5051 HARDWARE
Find local distributors for AX5051 transceiver at http://www.onsemi.com
Order development kit AX5051-DVK at http://www.onsemi.com
PIC16F886 MICROCONTROLLER
Datasheet for the microcontroller used
http://ww1.microchip.com/downloads/en/DeviceDoc/41291E.pdf
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