Ultra-Low-Power Motion Detection Using the

Application Report
SLAA283B – December 2005 – Revised May 2015
Ultra-Low-Power Motion Detection Using the
MSP430F2013
Zack Albus .......................................................................................................... MSP430 Applications
ABSTRACT
Motion detection using pyroelectric passive infrared (PIR) sensor elements is a common method used for
such applications. An implementation of such a system using the 16-bit Sigma-Delta ADC integrated into
the MSP430F2013 to detect motion is presented in this application report.
Design information and related software can be downloaded from http://www.ti.com/lit/zip/slaa283.
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Contents
Hardware Design ............................................................................................................
Software Design .............................................................................................................
Summary ......................................................................................................................
References ...................................................................................................................
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4
5
5
List of Figures
................................................................................
1
MSP430F2013 Motion Detection System
2
PIR Sensor Output and Signal Conditioning ............................................................................. 3
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3
Motion Detection Software Flowchart ..................................................................................... 4
List of Tables
1
Typical System Power Budget (Over 1 second) ......................................................................... 5
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SLAA283B – December 2005 – Revised May 2015
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1
Hardware Design
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Hardware Design
A system capable of detecting motion using a dual element PIR sensor is shown in Figure 1 using the
MSP430F2013 microcontroller. Using the integrated 16-bit Sigma-Delta analog-to-digital converter (ADC)
and built-in front-end PGA (SD16_A), the MSP430F2013 provides all the required elements for interfacing
to the PIR sensor in a small footprint. With integrated analog and a 16-MHz 16-bit RISC CPU, the
MSP430F2013 offers a great deal of processing performance in a small package and at a low cost.
MSP430F2013
VCC
3V
CR2032
R1
CPU
DCO
1.2 V
VLO
AX+
D
PIR
R2
C1
S
RB C2
SD16_A
Px.y
AX-
RLED
VSS
Figure 1. MSP430F2013 Motion Detection System
Figure 1 shows a simplified circuit that is used to process the PIR sensor output signal. The external
components consist of the bias resistor, RB, required for the sensor and two RC filters formed by R1/C1
and R2/C2.
The two filters serve two different purposes. Because the input to the SD16_A is differential, both a
positive and negative input must be provided. R1/C1 serves as an antialiasing filter on the AX+ input.
The second RC filter made up of R2/C2 serves to create a DC bias for the AX- input of the SD16_A. This is
required due to the large offset of the PIR source output with respect to VSS with relation to the input
range specification for the SD16_A. Figure 2 shows the respective signals in the circuit during detection of
a motion event.
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Hardware Design
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Direct PIR Source Output
(S, DC-coupled)
Motion-Triggered
PIR Output
AX+ Input Signal
AX- Input Signal
SD16_A Differential
Input (CH2-CH1)
Figure 2. PIR Sensor Output and Signal Conditioning
In Figure 2, channel 1 is the direct output of the sensor. With a sensor drain voltage of 3 V, the output
offset is approximately 500 mV. Connecting AX- directly to VSS and the sensor source output to AX+ would
be valid only if the internal SD16_A PGA gain setting is 1. With such a small peak-to-peak sensor output,
as seen on channel 2, a higher gain setting is required, which eliminates the possibility that AX- can be tied
directly to VSS.
Alternatively, a DC bias voltage can be generated to drive the AX- input. This is created from the R2/C2
low-pass filter. This signal is shown on channel 3. The sensor output signal after the antialiasing filter
connected to AX+ is shown on channel 2. By heavily low-pass filtering the sensor output before connecting
to AX- as well, a simple DC bias is established, maintaining the input range requirements of the SD16_A.
The mathematical difference, CH2-CH3, is shown on M1. This is the differential voltage seen at the
differential input pair, AX, of the SD16_A.
A PGA gain of 4x with an oversampling rate (OSR) of 256 has been used in this implementation.
Additional gains and OSRs up to 32 and 1024, respectively, are possible for systems requiring additional
sensitivity. See the MSP430F2013 data sheet (SLAS491) for possible SD16_A PGA settings and
appropriate analog input ranges.
In addition to the PIR sensor and the analog signal conditioning, a port pin is used to drive an LED. The
LED is illuminated to indicate to the user that motion has been detected. This signal could also be used to
drive an analog switch or relay to turn on a lamp or otherwise indicate motion in a real-world system.
As a final aspect of the hardware design, use of a Fresnel lens is critical to establishing good directionality
of the sensor detection field. The internal architecture of the dual element sensor provides good noise
immunity and false trigger rejection but also creates a limited directionality of the sensor’s sensitivity. Use
of the lens widens this field, making the final solution more robust.
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Software Design
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Software Design
With low power as an essential goal in this application, analog sampling and data processing is kept to a
minimum required to reliably detect motion. Figure 3 shows the software flow of the software
implementation described.
Main
Initialization:
VLO, WDT+,
P1 and P2,
SD16_A
Enter LPM3 with
Interrupts Enabled
WDT+ ISR
SD16_A ISR
Enter WDT+ ISR
(~340-ms interval from
ACLK/8 = VLO/8 ≈ 1.5 kHz)
Enter SD16_A ISR
Turn off VREF
LED ON?
NO
|ResultNEW –
ResultOLD| >
Threshold?
YES
Turn on VREF
Start Conversion
Exit in LPM0 on RETI
(DOC and SMCLK active)
Turn LED
OFF
YES
Turn on LED
NO
ResultOLD = ResultNEW
Exit in LPM3 on RETI
(DCO and SMCLK disabled)
RETI
RETI
Figure 3. Motion Detection Software Flowchart
The software consists of three main elements: main routine, watchdog timer interrupt service routine and
analog-to-digital converter interrupt service routine. The entire flow is interrupt driven using the internal
very low frequency, very low power VLO oscillator. The VLO is approximately 12 kHz and provided
internally on the ACLK clock line. This signal is then divided by 8 and drives the WDT+ to give the CPU an
interval wakeup. With an interval divider of 512, this equates to a wake-up time of 512 clocks / (12 kHz / 8)
= 341 ms. After initialization of all peripherals. the CPU enters into LPM3 via the VLO waiting for a WDT+
interrupt trigger.
After 341 ms, the WDT+ ISR is entered and serves two basic functions: first, to start a new SD16_A
conversion and, second, to control the LED indicating motion. If no motion was detected in the last
measurement (meaning the LED is off), the SD16_A internal reference is enabled and a new conversion is
started. Before exiting the WDT+ ISR, the status register value to be popped upon RETI is modified so
that the DCO and SMCLK used to clock the SD16_A will remain active. This causes the CPU to switch
from LPM3 to LPM0 after RETI.
4
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Summary
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During this time, the SD16_A is completing the conversion process. This takes 256 clocks / 1 MHz DCO ×
4 = 1.024 ms. The factor of 4 comes from the INTDLYx setting of the SD16_A. This setting allows the
SD16_A to take up to four conversions before interrupting the CPU to allow for any potential analog input
changes that might impact the SD16_A decimation filter, causing an invalid result. This is important,
because the SD16_A is used in a single conversion mode in this application. See the MSP430x2xx Family
User's Guide (SLAU144) for more information concerning this setting.
After the conversion is complete, the SD16_A ISR is entered and the internal reference is disabled. The
absolute difference between the new result and the prior result is calculated, and compared against a
preset threshold. When this threshold is exceeded, motion has been detected and the LED is enabled.
The CPU exits the ISR back into LPM3 (DCO and SMCLK are disabled) and the next WDT+ interrupt is
awaited.
3
Summary
Using this flow, the average current consumption is maintained at a low level, low enough that the entire
system can be powered from a standard CR2032 3-V battery at approximately 9.4 µA average ICC. Table 1
shows the breakdown of operation versus current consumption.
Table 1. Typical System Power Budget (Over 1 second)
Function
Duration
Active Current
Average Current
PIR325 sensor
1s
6 µA
6 µA
SD16_A and VREF +
DCO
1.024 ms, ~2.94 times per second
810 µA + 85 µA
2.69 µA
CPU Active
(1 MHz at 3 V)
262 MCLKs per second: 262 µs
300 µA
0.08 µA
MSP430 Standby
(LPM3 with VLO)
996.7 ms
0.6 µA
.0598 µA
Total
9.37 µA
The method shown here is quite simple in terms of the measurement and algorithm applied to detect
motion. With up to 2KB flash and up to 16 MIPs of processing power, the MSP430F2013 can be used to
implement a much higher level of signal processing to add sensitivity as well as selectivity to a given PIR
profile. The integrated analog and processing power of the MSP430F2013 family provides a low cost yet
powerful MCU solution which can be used to differentiate custom motion detection applications.
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References
1. MSP430x2xx Family User’s Guide (SLAU144)
2. MSP430F20x1, MSP430F20x2, MSP430F20x3 Mixed-Signal Microcontrollers data sheet (SLAS491)
3. "Infrared Parts Manual: PIR325 & FL65", GLOLAB Corporation, www.glolab.com, 2003
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Revision History
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Revision History
Changes from March 5, 2009 to May 21, 2015 ................................................................................................................. Page
•
Added link to download related zip file ................................................................................................. 1
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
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Revision History
SLAA283B – December 2005 – Revised May 2015
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