Pulse Oximetry on the MSP430
Overview and Introduction
•What is Pulse Oximetry?
• Signal Characteristics
• Filtering Requirements
• MSP430 Features
• MSP430 Timing
• SHARC comparison
Pulse Oximetry
Signal Characteristics
• Will detect a maximum heart rate of 200 BPM, which is 3.3 BPS
• Needs to sample at least double that (Nyquist), but 5-10 times ideally
• Will sample the signal at 32 Hz
Filtering Requirements
- LPF at 3.3 Hz sampled at 32 Hz (16 tap, 32 tap, 1000 tap)
Magnitude Response (dB)
0
0
-10
-10
-20
-20
Magnitude (dB)
Magnitude (dB)
Magnitude Response (dB)
-30
-40
-30
-40
-50
-50
-60
-60
-70
-70
0
5
10
Frequency (Hz)
15
0
5
10
Frequency (Hz)
15
Filtering Requirements
Magnitude Response (dB)
0
Magnitude (dB)
-20
-40
-60
-80
-100
0
10
5
Frequency (Hz)
15
MSP430 Features
• Higher End MSP430s have certain features
that can make FIR filters significantly faster
MSP430 Timing Analysis
• With DMA setup it’s about 7 cycles / tap
•With no DMA setup it’s about 28 cycles / tap
Tap Size
32
32
32
50
50
50
100
100
100
1000
1000
1000
CPU Speed (Mhz)
4
8
16
4
8
16
4
8
16
4
8
16
CPU Usage
0.248%
0.124%
0.062%
0.328%
0.164%
0.082%
0.584%
0.292%
0.146%
5.84%
2.92%
1.46%
Tap Size
32
32
32
50
50
50
100
100
100
1000
1000
1000
CPU Speed (Mhz)
4
8
16
4
8
16
4
8
16
4
8
16
CPU Usage
0.7168%
0.3584%
0.1792%
1.12%
0.56%
0.28%
2.24%
1.12%
0.56%
22.4%
11.2%
5.6%
MSP430 Timing Analysis
• We can slow down the clock to save power
Tap Size
DMA
CPU Speed for 50% usage
32
YES
19.84kHz
32
NO
57.34kHz
50
YES
26.24kHz
50
NO
89.60kHz
100
YES
46.72kHz
100
NO
179.2kHz
1000
YES
467.2kHz
1000
NO
1.792Mhz
SHARC Comparison
• SHARC has a lot more features to make FIR quick (SIMD, parallel instructions,
dm/pm, etc)
• When optimized the SHARC can do 0.5 cycles / FIR tap
• This is around 14 times faster than the fastest MSP430 setup
• In class we sample at 48 kHz. If we were to use an MSP430 with a 16 Mhz clock
speed with 50% utilization, we could only use a 23 tap FIR.
• Obviously the MSP430 would not be suitable for our labs, but works well in this
aplication
Conclusion
• MSP430 is an adequate solution for pulse oximetry due to it’s low sampling rate
• The clock speed can be lowered quite a bit to save power
• May be able to get by with a lower end MSP430 Chip, but there will be power
trade offs
• Questions?
References
Jeff Bachiochi, “Light to Frequency Conversion – TSL230R-Based Pulse Oximeter”, Circuit Cellar, Issue 173 & 174,
2004.
Kripasagar Venkat, Efficient Multiplication and Division Using MSP430, (Texas Instruments, 2006),
http://www.ti.com/lit/an/slaa329/slaa329.pdf.
Kripasagar Venkat, Efficient MSP430 Code Synthesis for an FIR filter, (Texas Instruments, 2007),
http://www.ti.com/lit/an/slaa357/slaa357.pdf.
Murugavel Raju, Digital FIR Design Using the MSP430F16x (Texas Instruments, 2004),
http://www.ti.com/lit/an/slaa228/slaa228.pdf.
“Pulse Oximetry”, http://www.oximetry.org/, last modified September 10th 2002,
http://www.oximetry.org/pulseox/principles.htm.
Texas Instruments, The MSP430 Hardware Multiplier, (Texas Instruments, 1999),
http://www.ti.com/lit/an/slaa042/slaa042.pdf.
Vincent Chan, Steve Underwood, A Single-Chip Pulseoximeter Design Using the MSP430, (Texas Instruments,
2012), http://www.ti.com/lit/an/slaa274b/slaa274b.pdf.