Wireless Sensor Monitor Using the eZ430-RF2500

Application Report
SLAA378D – December 2007 – Revised April 2011
Wireless Sensor Monitor Using the eZ430-RF2500
Miguel Morales, Zachery Shivers ............................................................................ MSP430 Applications
ABSTRACT
This application report documents the wireless temperature-sensor network demonstration application
provided with the eZ430-RF2500 development tool. The application uses Texas Instruments SimpliciTI™
wireless communication protocol to set up a simple network in which end devices communicate sampled
temperature and voltage data to a network access point. The access point communicates all collected
data through an available UART to a PC COM port. This port is then used with an accompanying
graphical user interface (GUI) to display the data in a user-friendly manner. This document is intended to
act as a guide for the eZ430-RF2500 firmware only. It does not focus on the use of the accompanying
Network Visualizer GUI or on the SimpliciTI network protocol. For more information on the Network
Visualizer and SimpliciTI network protocol, see Appendix B and www.ti.com/simpliciti, respectively.
NOTE: Due to a change in IAR compiler calling conventions, the Wireless Sensor Monitor v1.03
runs only with version 5.12 and later of the IAR Embedded Workbench KickStart™, available
online at http://focus.ti.com/docs/toolsw/folders/print/iar-kickstart.html. The code does not
compile with KickStart v3.42 and returns a linker error.
Project collateral and source code discussed in this application report can be downloaded from
http://www.ti.com/lit/zip/slac139.
The eZ430-RF2500 is available for order from the TI eStore (http://ti.com/appnote-ad-slaa378-rf2500).
Contents
1
Wireless Sensor Monitor Network Overview ............................................................................ 2
2
Application Firmware ....................................................................................................... 5
3
Performance Overview ................................................................................................... 10
4
References ................................................................................................................. 18
Appendix A
Wireless Sensor Monitor FAQs ................................................................................. 19
Appendix B
Network Visualizer GUI .......................................................................................... 20
List of Figures
1
eZ430-RF2500 Development Kit Components.......................................................................... 2
2
Application Splash Screen ................................................................................................. 3
3
Flowchart for demo_AP.c .................................................................................................. 5
4
Flowchart for Peer Frame Handling ...................................................................................... 8
5
Flowchart for demo_ED.c .................................................................................................. 9
6
End Device Test Platform ................................................................................................ 11
7
End Device Five-Second Current Profile ............................................................................... 12
8
End Device Transmission Current Profile (AP ACK Disabled) ...................................................... 13
9
End Device Transmission Current Profile (AP ACK Enabled) ....................................................... 14
10
Years of Operation vs Transmission Interval .......................................................................... 18
11
Network Visualizer Screen ............................................................................................... 20
12
Network Visualizer Display Settings
13
....................................................................................
Network Visualizer Console Window ...................................................................................
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List of Tables
1
Memory Requirements for Wireless Sensor Monitor v1.04 .......................................................... 10
2
Significant Current and Timing Contributions .......................................................................... 15
3
Expected Current Consumption ......................................................................................... 17
1
Wireless Sensor Monitor Network Overview
1.1
System Component Overview
This wireless temperature sensor network application was created as an example of a wireless application
using the combination of MSP430 microcontroller, a CC2500 low-power wireless radio from Texas
Instruments, and the SimpliciTI network protocol. Specifically, the Wireless Sensor Monitor leverages two
existing solutions to implement the application:
1. eZ430-RF2500 design kit
2. SimpliciTI network protocol
1.1.1
eZ430-RF2500
The eZ430-RF2500 is a complete USB-based MSP430 wireless development tool that provides all of the
hardware and software to evaluate the MSP430F2274 microcontroller and CC2500 2.4-GHz wireless
transceiver. The eZ430-RF2500T target board is an out-of-the-box wireless system that may be used with
the USB debugging interface, as a stand-alone system with or without external sensors, or incorporated
into an existing design. The new USB debugging interface enables eZ430-RF2500 to remotely send and
receive data from a PC using the MSP430 application UART, referred to as the application backchannel.
eZ430-RF2500 features include:
• USB debugging and programming interface featuring a driverless installation and application
backchannel
• 21 available development pins
• Highly integrated, ultra-low-power MSP430 MCU with 16-MHz performance
• Two general-purpose input/output (GPIO) digital I/O pins connected to green and red LEDs for visual
feedback
• Interruptible push button for user feedback [1]
The battery pack with the expansion board is used to remotely run firmware on an eZ430-RF2500T target
board (see Figure 1). For more specific information on the eZ430-RF2500, visit the Texas Instruments
website www.ti.com/ez430-rf or see the eZ430-RF2500 User's Guide (SLAU227). [1]
The eZ430-RF2500 is available for order from the TI eStore (http://ti.com/appnote-ad-slaa378-rf2500).
Figure 1. eZ430-RF2500 Development Kit Components
SimpliciTI is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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1.1.2
SimpliciTI Network Protocol
SimpliciTI is a proprietary, low-power radio-frequency (RF) protocol targeting simple, small RF networks.
The SimpliciTI network protocol was designed for easy implementation with minimal microcontroller
resource requirements. The protocol runs out of the box on TI's MSP430 ultra-low-power microcontrollers
and multiple RF transceivers.
Small, low-power RF networks typically contain battery-operated devices, which require long battery life,
low data rate, and low duty cycle, and have a limited number of nodes communicating directly with each
other. With the SimpliciTI network protocol, MCU resource requirements are minimal, resulting in lower
system cost for low-power RF networks.
Despite the modest resources required, the SimpliciTI network protocol supports end devices in a
peer-to-peer network topology, the option to use an access point to store and forward messages, and
range extenders to extend the range of the network.
The SimpliciTI network protocol can be used in a wide range of low-power applications including alarm
and security (smoke detectors, glass breakage detectors, carbon monoxide sensors, and light sensors),
automated meter reading (gas meters and water meters), home automation (appliances, garage door
openers, and environmental devices), and active RFID.
The SimpliciTI network protocol is provided as source code under a free license without royalties. For
information on compatibility, updates, and the latest version of the SimpliciTI protocol, visit
www.ti.com/simpliciti. [2] Note that this application report has been written using SimpliciTI v1.1.1.
1.2
Network Overview
Although identical, the two target boards that come with the eZ430-RF2500 kit come preprogrammed with
distinct firmware to exist as members of the Wireless Sensor Monitor network. A SimpliciTI Access Point
(AP) manages the network and is always on, receiving sampled data from one or more SimpliciTI End
Devices (ED) once per second. The EDs of the Wireless Sensor Network contain the sensors that
implement the end-application for the network and spend most of their time in low power mode 3 (LPM3),
waking up once a second to sample their ambient temperature and battery voltage and send the results to
the network's AP. Upon receiving the data from any ED on the network, the AP sends it through its
application UART to a COM port on the PC for presentation by the Network Visualizer GUI. To learn more
about setting up the serial communications interface using the Network Visualizer GUI, see Appendix B.
1.2.1
Access Point (AP)
The first task an AP executes is the transmission of a startup splash to the COM port.
Figure 2. Application Splash Screen
The network AP is then initialized as the network hub. Upon completion of the initialization procedure, the
AP transmits text notifying success:
Using the ADC10's internal temperature sensor, the AP begins to measure the ambient temperature once
per second for transmission to the PC. In addition, the AP continuously listens for new EDs joining the
network and for packages from EDs that have already joined. Using two indicator LEDs, an AP notifies the
user of the two transactions in the network: a red LED indicates the transmission of the AP's
measurements to the PC; a green LED indicates the receipt of a packet from one of the network EDs.
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1.2.2
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End Device (ED)
On startup, the ED immediately begins searching for an AP to which to connect. While searching, both the
green and red LEDs toggle on/off. Upon discovery of an AP, the ED attempts a network link, flashing the
red LED to indicate the link attempt. If it cannot link to the AP, it continues to flash the red LED. Once
connected to the network AP, all LEDs are turned off, and the ED defaults to LPM3, toggling the green
LED only when active.
1.3
Modes of Operation
There are two display options specified for the AP firmware's communication to the PC COM port, each of
which has two possible modes of operation. The user must select one of the two modes per display
option. Each mode can be entered by sending a character to the AP through the COM port connection.
Characters are not case sensitive. The four characters are:
Temperature Display Option 1
C - Output all temperatures in degrees Celsius
F - Output all temperatures in degrees Fahrenheit
Data Format Display Option 2
V - Output all data in extended Verbose mode
M - Output all data in shortened Minimal mode
1.3.1
Verbose Mode
The following is an example of the output from an AP in Verbose mode:
Node:
Temp:
Battery:
Strength:
This is the device identifier for the hub; it is an assigned integer based on the order in
which an ED has joined the network. The AP is given the identifier "HUB0".
This is the temperature measured by the node. This can be in either degrees Celsius or
Fahrenheit, as specified by the Temperature Display Option.
This is the voltage of the power supply as measured by the ADC10 on the MSP430.
This is the measured received signal strength indicator (RSSI) given by the CC2500
radio. It is output as a percentage for readability.
NOTE: AP RSSI is always output as 000%.
RE:
This indicates whether a received packet has gone through a range extender (RE).
NOTE: The AP firmware does not check for this, so its value is always No.
1.3.2
Minimal Mode
The following is an example of the output from an AP in minimal mode:
This mode transmits the data with minimal formatting to reduce bandwidth usage. Its primary purpose is
for parsing by the included PC Network Visualizer Application. The output in minimal mode contains the
same information as in Verbose mode, in the same order. Data is comma separated and has a start
character ($) and an end character (#).
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2.1
Access Point (AP)
BSP_Init()
Initialize MSP430
Transmit splash
screen and
"Initializing Network"
Initialize network
SMPL_Init(sCB)
sJoinSem,
sSelfMeasureSem,
or sPeerFrameSem?
sJoinSem
sPeerFrameSem
sSelfMeasureSem
Define input
message buffer
Read temperature
(ADC10)
Listen for a link
SMPL_LinkListen()
Read voltage
(ADC10)
sNumCurrentPeers++;
Format ADC10 samples
into a message
for transmission
Decrement join
semaphore
Process all waiting
frames
Transmit message to PC
Figure 3. Flowchart for demo_AP.c
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The BSP_Init() SimpliciTI API call initializes the board-specific hardware interfaces such as the
LEDs/switches on the board that are to be used in the application.
The MCU_Init() function performs further MSP430-specific initializations for the application. These include:
• The DCO and MCLK are set to run at 8 MHz.
• Timer_A is set to trigger interrupts at 1-second intervals.
• The universal serial communication interface (USCI) UART is initialized to communicate with the PC
COM port to 9600 Baud and RX/TX; interrupts enabled.
Once the hardware initialization is complete, the TI splash screen is transmitted to the COM port on the
PC and the program calls the SMPL_Init(sCB) function to initialize the radio and the SimpliciTI network
protocol. The sCB parameter is a function pointer to a callback function that is executed within the
interrupt service routine (ISR) upon packet reception by the AP (see Example 1).
Example 1. sCB Callback Function
/*----------------------------------------------------------------------------*/
* Runs in ISR context. Reading the frame should be done in the
* application thread not in the ISR thread.
/*------------------------------------------------------------------------------*/
static uint8_t sCB(linkID_t lid)
{
if (lid)
{
sPeerFrameSem++;
}
else
{
sJoinSem++;
}
// leave frame to be read by application.
return 0;
}
The sCB callback function filters the received packet according to its link ID to identify the source of the
transmission and distinguish an ED join request from a data packet transmission from an ED that has
already established a connection to the network. A link ID of 0 identifies a join request. Upon
acceptance of an ED join request, the AP and the ED must then create a link for communication. Each
link to a new device is enumerated so the AP assigns incremental link IDs as devices join the network.
Due to application considerations, the Wireless Sensor Monitor allows a maximum number of eight
devices to be linked to the AP.
According to the link ID, the sCB callback function identifies and increments either the sPeerFrameSem
or sJoinSem semaphores. The AP code also defines a sSelfMeasureSem semaphore, set by the Timer
A interrupt once a second, so that the AP knows to sample its own temperature and battery voltages for
display. These three semaphores control the program flow after network initialization. Take particular
note of the return value of the sCB function. A return value greater than zero indicates to the SimpliciTI
protocol that the callback function has handled the received frame and releases the frames memory for
reuse. The sCB callback function in demo_AP.c returns zero because the firmware leaves the received
frame in the input buffer to be handled by the application to reduce the amount of time spent in the ISR
context of the SimpliciTI protocol. When a device is expected to maintain a multitude of links to other
applications/nodes in the network, it is especially important to keep code in ISRs small to minimize the
risk of losing packet reception and notification.
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2.1.1
sJoinSem Branch
The sJoinSem semaphore in demo_AP.c is set in the sCB callback function every time that an ED calls
its SMPL_Init() function and, as a result, attempts to join the network. A join to the network is really a
side-effect of initialization; there is not a SimpliciTI API call to request a network join. When the ED
application calls SMPL_Init(), SimpliciTI sends a Join frame with a link ID of zero, informing the AP to
increment the sJoinSem semaphore in the callback function.
When the AP callback function then returns to the main loop, and as long as the AP has made less
than its maximum number of links (as specified in smpl_config.dat), the AP then initiates a
SMPL_LinkListen(). The SMPL_LinkListen() function takes a pointer to the link ID that is be used to
identify all messages from the node requesting the link. SMPL_LinkListen() is a blocking call with a
timeout parameter (introduced in SimpliciTI 1.0.5), meaning it does not return until a successful link has
been created or time has expired, so it is important that the SMPL_LinkListen() call be made only when
the user knows that another device has made the link request using the SMPL_Link() function.
On a successful link creation, the sJoinSem branch increases the number of devices that the AP
recognizes as part of the network and unlocks the sJoinSem semaphore for another device to set.
2.1.2
sPeerFrameSem Branch
The sPeerFrameSem semaphore is incremented in the sCB callback function every time that the AP
receives a frame from a peer application. In the case that the sPeerFrameSem semaphore has been
set or incremented, the AP first defines a message buffer in which to store the current frame being
analyzed and then loops through its input queue searching for messages until it has processed all of
the waiting frames.
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The flowchart in Figure 4 shows how the application handles messages from its peers.
Create a message buffer
to strore incoming frame
i == sNumCurrentPeers
NO
for (i = 0; i < sNumCurrentPeers; ++i)
Received a message from
activeLinkIDs[i]?
Main program loop
YES
Retrieve RSSI
information from sender
Transmist RSSI data and
message to the PC
Toggle LED to indicate message
receipt and decrement
sPeerFrameSem[aphore]
Figure 4. Flowchart for Peer Frame Handling
2.1.3
sSelfMeasureSem Branch
The sSelfMeasureSem semaphore is specific to this demo application in that it is set at a user-specified
time interval to execute an application-layer routine. It is a prime example of how easy it is to combine
an eZ430-RF2500 user application like temperature and voltage sampling with the SimpliciTI RF
protocol by threading the requirements of both applications - ADC10 sampling at one-second intervals
(application) and network management/peer-frame handling (SimpliciTI protocol) - into a very low level
task manager.
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2.2
End Device (ED)
Timer ISR wake-up
BSP_Init()
Read temperature
(ADC10)
Yes
Address already
written into flash?
Enter LPM3 until timer interrupt
No
Create random address
(TI_getRandomIntegerFromVLO)
and write address to flash
Retrieve device address from flash
Read battery voltage
(ADC10)
Overwrite device address
and
Initialize MSP430
Format ADC samples
for transmission
Initialize/join network
SMPL_Init((uint8_t(*)(linkID_t))0)
Send data to AP
SMPL_Send()
Link to Access Point
SMPL_Link()
Figure 5. Flowchart for demo_ED.c
The initialization of the ED device is similar to the AP in that both devices use the SMPL_Init() call to
initialize onto the network. The initialization procedure is different for the ED, however, in two important
ways: the generation of the device address at runtime and the parameter passed to the SMPL_Init()
function.
As part of the initialization procedure, EDs in this application create a random 4-byte address, write that
address into flash memory for reuse on system reset, and then write over their default build-time device
address using the SMPL_Ioctl() API. Because a SimpliciTI AP identifies new devices on the network by
their device addresses, storing this randomly-generated address in flash and checking this predefined
location at device initialization assures that an ED that has lost power or is reset is always recognized
as the same device (i.e., is given the same link ID) by the AP.
The random address is created using the results from the TI_getRandomIntegerFromVLO function
inside the vlo_rand.s43 library file provided with the project. This library uses the rising edges of the
very low frequency oscillator clock found in 2xx devices to trigger samples of a system clock that are
then interpreted into a 4-byte device address. By changing the frequency of the system clock between
triggers, the randomization of resulting device addresses is increased, and the user can be confident
that two devices will not create the same network address. For more information on the random number
generation library, see Random Number Generation Using the MSP430 (SLAA338). [6]
After writing over the build-time device address, the SMPL_Init() call is then made, initializing the ED
onto the AP’s network by checking for matching join tokens and exchanging network parameters such
as the link token and whether or not the ED is a polling device. The ED joining the AP-managed
network is, therefore, a side effect of the SMPL_Init( ) call; there is no such thing as a "join" API call.
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The SMPL_Init( ) function does not take a callback function pointer – as in the case of the AP –
because it does not need to be notified when a packet has arrived from the AP. The AP does not send
packets to the ED. A callback function is used when the ED firmware wants to be informed that a
packet has been received to immediately retrieve/process the message. It is also possible to handle
messages from the AP without a callback function by using SMPL_Receive( ) to poll the input frame
queue until the call returns a successful value. In both cases, SMPL_Receive( ) is used only to poll the
input frame queue for the received message, but the latter solution is less reactive than the former, as
the code spends its time polling the input frame queue rather than executing other application tasks.
Therefore, the recommended program flow for a non-polling ED to receive messages from the AP is to
use a callback function that sets a semaphore, informing the code that a message has been received
and that it needs to call the SMPL_Receive( ) API to retrieve/process the message.
This brings to light an important difference between calling the SMPL_Receive( ) API when an ED is a
polling versus non-polling device. An ED can be configured as a polling ED by defining the macro,
RX_POLLS, in the smpl_config.dat file. Defining the RX_POLLS macro changes the behavior of the
SMPL_Receive( ) call such that a call to the SMPL_Receive( ) transmits a query to the AP for pending
messages – changing the state of the radio. Upon receiving the query from a polling ED, the AP
responds with the pending message or, if no message is being held, a frame with no payload. This is
called store-and-forward messaging and is not used in this example. A call to SMPL_Receive( ) when
the device is non-polling does not change the state of the radio, but rather polls the local input buffer for
a previously-received packet. The ED nodes in this application report are non-polling. For further
information on device configurations, see the SimpliciTI documentation. [2]
Once the SMPL_Init( ) call returns successfully – meaning the ED has verified its join token and is now
allowed onto the AP’s network – the ED calls the SMPL_Link( ) API to create the explicit link required to
exchange packets of information. Only after the ED has both joined the network [SMPL_Init()] and
created a link to the AP [SMP_Link( )] does it enter the main while loop, waking up once per second to
sample and communicate its ambient temperature/battery voltage.
3
Performance Overview
The memory requirements of the built code are shown in Table 1. The build with IAR Embedded
Workbench for MSP430 KickStart v5.10.1 used optimization settings set to Size → High. The build with
Code Composer Studio v4.1.2.00027 used optimization: 3 (highest) and speed/size tradeoff: 0 (size).
Table 1. Memory Requirements for Wireless Sensor
Monitor v1.04
Compiler
Version
Code
Composer
Studio
Access Point
IAR
10
ROM (bytes)
RAM (bytes)
10554
875
End Device
8086
293
Access Point
11816
790
End Device
9100
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To analyze the current profile of the application, the hardware setup in Figure 6 was used:
Tektroniz DPO7104
Oscilloscope
IN_1 – IN_2
IN_1
Access Point
demo_AP.c
(eZ430-RF2500)
IN_2
–
3V
Data
Power supply
Agilent E3640A
+
5-W Resistor
End Device
demo_ED.c
(eZ430-RF2500)
Figure 6. End Device Test Platform
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The oscilloscope shot in Figure 7 shows the current profile of an ED over five seconds. EDs send data
to the AP once per second. By decreasing the duty cycle for data transmission as much as possible,
the radio and MCU are active for minute amounts of time, allowing an ED to run for long periods of time
on the same batteries.
Figure 7. End Device Five-Second Current Profile
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Figure 8 shows one of the spikes after decreasing the time step and sampling in high-resolution mode.
At this time scale, the waveform can be more closely analyzed for specific events in hardware or
software and their respective power contributions.
Figure 8. End Device Transmission Current Profile (AP ACK Disabled)
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Figure 9 is the current profile when automatic access-point acknowledgments and frequency agility are
enabled. With automatic acknowledgment enabled, the end device waits for an acknowledgment packet
to be sent from the AP to confirm the data was successfully transferred. With frequency agility enabled,
the end device automatically switches RF channels when data is not acknowledged. These features
make data transfer more robust, especially when packet loss is critical, i.e., at very low transmit periods
or in noisy environments. However, the current consumption is more than doubled due to increased
time in RX mode on the CC2500.
Figure 9. End Device Transmission Current Profile (AP ACK Enabled)
NOTE: Both APP_AUTO_ACK and FREQUENCY_AGILITY defines are disabled (commented
out) by default. To enable them, remove the comment in smpl_nwk_config.dat. To enable
FREQUENCY_AGILITY, you must also enable APP_AUTO_ACK.
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Table 2 and the following power consumption discussion describe the most significant contributors to
power and program flow in both hardware and software. Other sources of current consumption, such as
the USCI current consumption, do exist in the application but are not highlighted in its analysis due to
their relatively small significance in the overall current consumption.
NOTE: The code to toggle the LED has been commented out for the power consumption
calculation and that the typical value for the ADC10OSC of 5 MHz is used for calculation.
Table 2. Significant Current and Timing Contributions
Time for
Execution
Current
Hardware
Value
Unit
Sleep Modes
Software
Value
Unit
300
µs
150
µs
809
µs
ADC10 REFON + Delay for Stabilization [4]
30
µs
LPM0 + Sync +
Temp Sample (64 × ADC10CLKs) +
Conversion (13 × ADC10CLKs) [7]
48
µs
Oscillator (D in Figure 8)
(1)
MSP430 low-power mode 0 (LPM0) [4]
1.1
mA
XOSC startup time
MSP430 low-power mode 3 (LPM3) [4]
900
nA
Ripple counter (E in Figure 8)
CC2500 sleep state [5]
400
nA
Timeout before CHP_RDY Hi→Lo [5]
2.7
mA
MSP430 Active Mode
PLL (G in Figure 10)
8 MHz = DCO = SMCLK, 3 V [4]
MSP430 ADC10
Calibration of radio frequency synthesizer [5]
Temperature sample (A in Figure 8)
fADC10CLK = 5.0 MHz, ADC10ON = 1,
REFON = 1, REFOUT = 0,
ADC10DIV = 0x4 (ADC10CLK / 5) (2)
850
µA
1.5
mA
ADC10 2.5-V REFON + delay for stabilization [4]
30
µs
7
µs
140
µs
140
µs
CC2500 Active Modes
VCC sample (B in Figure 8)
Idle [5]
PLL calibration [5]
7.4
mA
LPM0 + Sync + VCC Sample (16 × ADC10CLKs)
+ Conversion (13 × ADC10CLKs) [5]
Receive (RX) (weak input signal,
DEM_DCFILT_FILT_OFF = 0, 250 kbps) [5]
18.8
mA
Message handling overhead (C and F in Figure 8)
Transmit (TX) (250 kbps, 0-dB output power) [5]
21.2
mA
Calculate temperature and VCC from ADC
sample results (MSP430 active and radio idle)
Transmit message to TX FIFO and prepare
message TX (SMPL_Send)
(MSP430 active and radio idle) (1)
(1)
RX/TX modes (H, I, and J in Figure 8)
RX mode (clear channel assessment)
(1)
180
µs
Switch between RX and TX mode
(1)
30
µs
TX mode (message transmission)
(1)
800
µs
Radio and MCU low-power modes (K in Figure 8)
CC2500 prepares for sleep; turns off 32-MHz
crystal (2) (MSP430 active and radio idle)
(1)
(2)
70
µs
Measured using an oscilloscope
See the CC2500 Low-Cost Low-Cost Low-Power 2.4 GHz RF Transceiver Data Sheet (SWRS040): add IADC10 to IREF+.
Notice that the ADC10 sample times are significantly different. This is because the internal temperature
sensor requires 30-µs sampling time, whereas the VCC measurement requires only 1.4 µs. [4]
Therefore, the ADC10 must be initialized differently for each measurement – different divisors and a
different number of sampling cycles. To account for the ADC10’s internal oscillator speed variance over
temperature, these values must be sufficient to provide the required sample time at the ADC10OSC
corner-case operating speed of 6.3 MHz. Hence, the sampling time in typical- and worst-case operating
scenarios is longer than the minimum sampling time required for the measurement, as shown in
Table 2. See the code for the recommended ADC10 initialization procedures.
SLAA378D – December 2007 – Revised April 2011
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Performance Overview
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The MSP430 contributions to program flow and power consumption are straightforward and can be
traced through an analysis of the application-level firmware. The radio events, however, are abstracted
from the user by design and often occur by default, executed by hardware and invisible to the
programmer.
The radio events are denoted using the dotted lines in the chart, whereas MSP430 events are not
explicitly shown. Events A and B occur every time that the radio is awakened from sleep into idle mode
by a SMPL_Ioctl(IOCTL_OBJ_RADIO, IOCTL_ACT_RADIO_AWAKE, 0) call. Events C and D occur
every time that the radio is put into a receive state from IDLE.
1. XOSC startup – D
XOSC is the CC2500 oscillator used to source the chip’s system clock.
2. Ripple Timer Timeout – E
A setting inside the radio’s configuration registers specifies how many times a ripple counter must
timeout after a successful XOSC startup routine before signaling the CC2500 chip ready symbol
(negative edge on CHP_RDY). In this case, the requirement is 64 timeouts, or 150 µs.
3. IDLE → RX Mode + PLL calibration – G
Upon changing states from IDLE to either RX or TX modes, the PLL (the on-chip frequency
synthesizer used for RX and TX (de-)modulation) is automatically calibrated according to a setting
inside the radio’s configuration registers. This frequency synthesizer must be calibrated regularly,
and takes 809 µs to enter RX or TX mode from the IDLE radio state.
4. RX Mode – H
Receive mode is necessary for a successful transmission so that a Clear Channel Assessment
(CCA) can be completed before transmission. A CCA check is done to verify that the radio is not
currently receiving a packet and that another signal on the channel is not registering an RSSI value
over a certain threshold (in effect, it checks whether another radio is already transmitting on the
channel of interest). If the channel is occupied, SimpliciTI backs off for a random time and perform
CCA again. When the CCA check is complete, the radio can then switch to transmit (TX) mode and
transmit its intended application payload. Through this procedure, SimpliciTI maintains a
listen-before-talk, or Carrier Sense Multiple Access (CSMA) algorithm for communication between
nodes.
For more information on CC2500 settings, see the CC2500 Low-Power RF Transceiver data sheet
[5].
Due to these four radio requirements, it is important to understand that the transmission of the data
(time spent in TX mode) is only partly responsible for the majority of total current consumption drawn.
Much of the total current consumption is tied directly to the overhead incurred from the CC radio and
MSP430 initializing each transmission. It is, therefore, important to maximize the amount of data that
can be sent per transmission and minimize the number of transmissions per unit of time.
16
Wireless Sensor Monitor Using the eZ430-RF2500
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Performance Overview
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Table 3 calculates the expected current consumption for the application. The radio and MSP430
components of the application have been separated.
Table 3. Expected Current Consumption
Radio Event
Current Consumed
Time Executed
Amps * Seconds Consumed
XOSC startup
2.7 mA (1)
Ripple counter timeout
1.75 mA
150 µs
Idle mode
1.5 mA
375 µs
563 nA*s
PLL calibration
7.5 mA
809 µs
6067 nA*s
RX mode
18.8 mA
TX mode
300 µs
21.2 mA
(1)
180 µs
(1)
800 µs
(1)
810 nA*s
262 nA*s
3384 nA*s
16 960 nA*s
Total
28 047 nA*s
Current Consumed
Time Executed
Amps * Seconds Consumed
MSP430 active current
2.7 mA
2.705 ms
7304 nA*s
MSP430 LPM0 current
1.1 mA
55 µs
61 nA*s
ADC10 active
850 µA
115 µs
98 nA*s
Total
7463 nA*s
Transmission Total
35 508 nA*s
MSP430 Event
(1)
Estimated from oscilloscope measurement
The average current calculation must also take into account the sleep current of an ED. The less
frequently that an ED transmits, the more significant that the sleep current's contribution to the overall
average becomes.
sleep_current_contrib = (MSP430 idle current + CC2500 idle current) × (period_of_transmission –
application_execution_time)
(1)
= 1.3 [µA] * (1 [s] – 2.838 [ms])
= 1.296 µA*s
The average current consumption for the application can then be derived using the following equation:
average_current_consumption = (sleep_current_contrib + transmission_total) / period_of_transmission
(2)
average_current_consumptionEXPECTED = (1.296 [µA*s] + 35.508 [µA*s] ) / 1 [s]
= 36.80 µA
For comparison, the second method used to calculate the average current consumption – integrating
the curve using an oscilloscope – resulted in an area under the transmission waveform of 184 µV*s.
Divided by the real resistor value of 5.92 Ω and added to the sleep_current_contrib from equation (1),
the average measured current consumption is derived as:
average_current_consumptionMEASURED = (measured_voltage_reading / 5 Ω) / period_of_transmission
(3)
= (202 [µV*s] / 5.92 Ω) + sleep_current_contrib) / 1 [s]
= (34.12 [µA*s] + 1.296 [µA*s]) / 1 [s]
= 35.42 µA
Given the simplification of the current analysis and the accuracy of various measurement tools and the
accuracy of various measurement tools, the slight difference between the two numbers is acceptable.
To calculate the life expectancy of an ED on the network, and assuming two typical AAA batteries with
a capacity of 1000 mA*hr under the hypothetical condition in which the batteries hold their voltage
ideally until their capacity is exhausted:
hours_of_operation = current_rating / average_current
(4)
Calculated life expectancy:
= 1000 [mA*hrs] / 0.03680 [mA] = 27173 [hrs] / 24 [hrs/day] = 1132 [days] / 365 [days/yr] = 3.10
years
= 3 years, 1 month, and 6 days
SLAA378D – December 2007 – Revised April 2011
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References
www.ti.com
Measured life expectancy:
= 1000 [mA*hrs] / 0.03542 [mA] = 28232 [hrs] / 24 [hrs/day] = 1176 [days] / 365 [days/yr] = 3.22
years
= 3 years, 2 months, and 20 days
Life Expectance (years)
Figure 10 charts the expected years of operation due to power consumption of an ED node transmitting
the current application payload at different time intervals, verifying that to minimize the power
consumption of an application, a programmer should always:
1. Minimize the number of transmissions
2. Fit as many bytes into the transmission packet as is feasible for the application.
Transmission Period (s)
Figure 10. Years of Operation vs Transmission Interval
It should be noted that in the real world, the application life expectancy is lower due to battery leakage,
battery voltage dissipation, and the requirements of the application itself (e.g., 2.5-V VCC for the ADC10
reference). To mitigate these effects, one could select batteries that hold their specified voltage for as
long as possible and implement a function that changes the ADC10 voltage reference down to 1.5 V
once the battery voltage reaches 3 V. This function would extend the usable battery life by decreasing
the required voltage for proper ADC10 operation down to the minimum ADC10 voltage of 2.2 V
specified in the MSP430F2274 data sheet. [4]
Using this document, the supporting documentation, and the Wireless Sensor Monitor firmware, the
reader now has all the tools needed to integrate a low-cost easy-to-use wireless solution into existing or
developing applications. For more information and ideas on how to use the eZ430-RF2500 to evaluate
or implement your wireless solution, see http://www.ti.com/ez430-rf.
4
References
1. eZ430-RF2500 User's Guide (SLAU227)
The eZ430-RF2500 is available for order from the TI eStore
(http://ti.com/appnote-ad-slaa378-rf2500).
2. SimpliciTI Developer's Notes located in SimpliciTI-IAR-1.1.1 (SWRC099)
3. Measuring Power Consumption With CC2430 and Z-Stack (SWRA144)
4. MSP430x22x2, MSP430x22x4 Mixed Signal Microcontroller Data Sheet (SLAS504)
5. CC2500 Low-Cost Low-Cost Low-Power 2.4 GHz RF Transceiver Data Sheet (SWRS040)
6. Random Number Generation Using the MSP430 (SLAA338)
7. MSP430x2xx Family User's Guide (SLAU144)
18
Wireless Sensor Monitor Using the eZ430-RF2500
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SLAA378D – December 2007 – Revised April 2011
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Appendix A Wireless Sensor Monitor FAQs
A.1 Which version of IAR is required to launch the eZ430-RF2500 Wireless Sensor Monitor v1.04
project?
IAR KickStart 5.12 or later is required. It includes support for the eZ430-RF2500 emulator and allows
the inclusion of library files that exceed the KickStart 4-KB C-code limitation.
A.2 The project does not debug/run properly.
Before debugging and when switching between End Device and Access Point projects (or vice versa),
clean the project by clicking Project → Clean.
A.3 Why does temperature vary among End Devices?
Calibration of the MSP430 ADC10 for temperature is application specific, and beyond the scope of this
document. For this application, a simple 1 point ambient temperature calibration was deemed
acceptable. Greater accuracy can be achieved using other methods such as 2-point calibrations in
temperature controlled environments.
A.4 Why does my battery measure 3.5 V?
The battery measurement for this application is limited to 1 decimal point and rounds down. Therefore,
if a 3.6-V battery is measured as 3.59 V, the End Device shows it as 3.5 V.
A.5 What is the maximum number of supported end devices?
Each Access Point can link to a maximum of eight end devices using the current project.
A.6 Can the network be extended beyond 30 end devices?
Yes, the device address for a device is 4 bytes long, so the hard-stop for possible devices on a
SimpliciTI network is 232 nodes. However, each node on the network can link to only 30 nodes. The
network limitations are also very much dependent on the amount of RAM that a device contains. Each
additional node on the network requires additional RAM for the input/output buffers and network
management data structures. To allow plenty of room for further application development, this
demonstration application was limited to eight nodes.
SLAA378D – December 2007 – Revised April 2011
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www.ti.com
Appendix B Network Visualizer GUI
The user has the option of running the project from the CD that comes with the eZ430-RF2500 kit or
downloading the project from the web at http://www.ti.com/lit/zip/slac139. The devices in an
eZ430-RF2500 kit come preprogrammed with the Wireless Sensor Monitor v1.04 firmware and may be
reprogrammed at any time. If the user experiences any issues on hardware or software installation, see
the eZ430-RF2500 User's Guide for detailed explanations of project installs and instructions on how to
update the application firmware.
An Access Point running the Wireless Sensor Monitor v1.04 firmware transfers its data to a terminal
window through its backchannel UART at a fixed rate of 9600 bps. The user can open a terminal
window to see the streaming data or can view a graphical representation of the network using the
Network Visualizer GUI.
The Network Visualizer GUI reads the data coming in through the COM port and depicts the network in
an easy-to-understand graphical format according to user settings. Figure 11 is a screenshot of the GUI
using the two devices that come with the eZ430-RF2500 kit.
Figure 11. Network Visualizer Screen
The GUI displays the temperature and voltage readings that are transmit per device and can be run
as-is by opening its executable file. It is possible to configure the GUI display by navigating to the Menu
→ Settings window, shown in Figure 12. In the Settings window, the user sets the port over which the
AP is communicating, values for the color gradient of the EDs, temperature display units, and the
resulting distance from the center node for the interpreted RSSI values.
20
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Appendix B
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Figure 12. Network Visualizer Display Settings
It is also possible to see the data streaming through the selected COM port terminal by navigating to
the Menu → Console window. An example console window is displayed in Figure 13. For further
questions on the Network Visualizer GUI, see the Help menu.
Figure 13. Network Visualizer Console Window
NOTE: Texas Instruments does not provide the source code for the Network Visualizer GUI.
SLAA378D – December 2007 – Revised April 2011
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