Silicon Labs Solutions for Active
Infrared Proximity Sensing
In this training module we are going to take a look at active infrared proximity
detection and how Silicon Labs solves important design challenges when
integrating this technology into end applications.
1
Agenda
 Proximity sensing basic concepts
 Why use proximity sensing
 Background information
 Design considerations
 The Si1120 and its operation
 Development kits
 Summary
2
We are going to first review some basic concepts associated with active proximity
sensing and the system level advantages of using this technology. Next we will
cover a brief overview of the theory behind the measurements and how these are
applied to real world designs. Finally, we will take a look at the Silicon Labs
proximity sensors and their operation including the MCU requirements to interface
to these types of sensors and the tools available to ease the development process.
2
Active Optical Proximity Detection Basic Concept




Emitter generates energy (infrared light)
An object in proximity reflects the energy
An optical sensor detects the energy
Logic or a controller makes a decision regarding the proximity or movement of
the object
Ambient Light Sources
Object in proximity
Transmit
window
Receive window
Emitter
LED
Signal
Processing
Detectors
Visible Light
Photodiode
IR Photodiode
Silicon Labs
Proximity/Ambient Light Sensor
3
Silicon Labs provides solutions for active proximity sensing. A proximity sensing
system is comprised of two elements: the IR emitter and the IR detector. The
emitter generates the energy using an IR LED and control circuitry. The IR energy
is transmitted through the mechanical assembly and is reflected back to the
measurement system by an object in proximity. Within the mechanical assembly
there is also an IR detector or photodiode. The photodiode generates a current
output proportional to the incident light when a bias voltage is applied. The
intensity of the light received by the photodiode is then communicated to an MCU
such as the C8051 family from Silicon Labs. The MCU can then process the data
and generate a measurement of the distance of the reflective object. There are
several additional aspects that may be associated with proximity detection. Some
Silicon Labs proximity sensors utilize a second photodiode to compensate for
ambient light sources (Si1120). These diodes compensate for additional IR energy
present in the environment due to light sources not associated with the measurement
system. Examples of such light sources are incandescent and fluorescent lighting.
Adding the second photodiode enhances the IR proximity detection by providing a
method to compensate for the additional energy present.
3
Why Use Proximity Sensing?
 Reduced power consumption
 Consumers expect
 Longer battery life from portable units
 Lower power from wall powered devices when not in use
 Devices can go into low power modes when reflective objects
are in proximity (cell phone held up to the ear so the unit can
turn off the backlight and display)
 Devices can go into low power modes when not in use to
conserve power and conform to green initiatives
 Value added features to enhance products




New user interface features
Motion detection
Touchless gestures
Devices can change operating state based on proximity
detection
 Example: context changes when a person walks up to a
terminal
4
Power consumption is a growing concern for all electronic devices whether it is
battery powered or wall or aux powered devices (like Power over Ethernet (PoE)).
IR proximity sensing is becoming popular in applications as the sensing mechanism
used reduce the overall power consumption by enabling certain subsystem to be
shut down when not in use.
The Electromagnetic Spectrum
 Visible spectrum = 380 to 760 nm
 Near infrared = 750 to 2,500 nm
 Typical infrared LEDs: 850, 880, 940, & 950 nm
 Emitter wavelength based on silicon sensitivity in the
electromagnetic spectrum
 850 nm (Silicon Labs solutions) or 940 nm are common
for the IR emitter
Silicon response to light
5
The electromagnetic spectrum is a continuum of all electromagnetic waves arranged
according to frequency and wavelength. The sun, earth, and other bodies radiate
electromagnetic energy of varying wavelengths. The electromagnetic spectrum is
more familiar to you than you might think. The microwave you use to heat your
food and the cell phones you use emit and receive energy that is part of the
Electromagnetic Spectrum. The light that our eyes can see is also part of the
electromagnetic spectrum. This visible part of the electromagnetic spectrum
consists of the colors that we see in a rainbow - from reds and oranges, through
blues and purples. Infrared light lies between the visible and microwave portions
of the electromagnetic spectrum. Infrared light has a range of wavelengths, just like
visible light has wavelengths that range from red light to violet. "Near infrared"
light is closest in wavelength to visible light and is what is used as the energy source
in proximity sensing applications.
Ambient Light (Background Lighting)
 Ambient light sensors measure light levels in the visible spectrum (photopic response)
between 380 and 760nm
 Different light sources provide different spectral outputs
 Fluorescent: high visible spectrum, ultra-low infrared, & 100/120Hz flicker
 Incandescent: high visible, high infrared
 Sunlight: ultra-high visible, medium infrared (relative)
 Understanding the background light source can improve the infrared measurement
 Ambient lighting levels in an environment include both visible and infrared levels – these levels
being interferers
 For reflective proximity the ambient light sources are subtracted from the IR measurement
as a way to enhance the IR results
 ALS benefits
 Display backlight power reduction
 Improved user visual experience
6
Ambient light is all of the light (visible and infrared) in the environment that is not
generated by the measurement system. Ambient light is generated by many sources
including incandescent and fluorescent lighting, LED lighting and sunlight among
others. Ambient light can be important for accurate measurements because each
different source generates varying levels of infrared energy. It can reduce the
overall dynamic range of the system and if not compensated will cause
measurement errors in the final results.
Relating Illuminance to Real-world Ambient Light
Light meter
7
Illuminance is the total luminous flux (the flux density corrected for human vision)
in a given area. The table above is provided in order to get an idea of how ambient
light relates to real world environments by comparing the illuminance values for
each. It can be seen from this table that proximity sensing applications are required
to compensate for a wide range of ambient light conditions. And as mentioned,
these values represent the visible spectrum. There are IR components that are part
of the entire radiant flux in the environment that can interfere with the proximity
measurements.
Design Considerations (1 of 3)
 Proximity distance to be measured
 Light intensity is inversely proportional to the distance
 Distance is doubled due to the return trip from the reflective object
 Light must be able to reach the detector
 Determines IR emitter optical output characteristics (thus the current
requirements as well)
 Filters can increase the overall range
 Reflective object characteristics
 Size and shape of object
 Surface of object
 Specular/scatter reflections
 Angle of reflection
 Absorption
8
Designers of devices that implement IR proximity detection need to be concerned
with several items. First and foremost the detection range must be determined. In
order to detect the reflected energy the object must be within range of the IR energy
source. When determining the proximity range the IR energy output must be set in
order to accommodate the decrease in light which is proportional to the inverse
square of the distance. Taking into account that the light must travel in the transmit
and receive path, the energy that the detector sees is inversely proportional to the
fourth power of the distance from the emitter.
The reflective object characteristics are also important factors to be considered and
ultimately the goal is to maximize the amount of reflected energy received by the
detector. The size and shape of the object determines how much of the emitter’s
beam width is reflected and the shape determines the angle of reflection. In
addition, surfaces have small irregularities that may reflect or scatter the IR energy.
For example, a polished surface reflects better because the amount of irregularities
on the surface is small compared to a surface that may be a rougher. In either case
there are typically two types of reflections, specular and scatter. The specular
reflection is the reflection that provides the highest intensity of the emitted energy
due to the angle of incidence of the object. Another consideration is the absorption
of the object. In many applications the human hand is the reflective object which
can absorb some of the IR energy.
Design Considerations (2 of 3)
 Environment
 Reflected light from other objects (multipath)
 IR emitter
 Electrical to optical conversion efficiency
 Emission angle
 Narrow angle provides greater measurement distance
 Wavelength
 Age
 Ambient Light
 Fluorescent Lamps (ballast flicker)
 Sunlight (large background light level, high levels of IR energy)
 Incandescent Lamps (high levels of IR energy)
 Optical Isolation between the emitter and the detector
9
When the IR energy is emitted from the source it can bounce off other objects and make its way back
into the detector. This phenomena is called multipath and can be added to the detected signal. The
IR emitter is an important consideration in proximity designs. There are several parameters
associated with the IR emitter that need to be evaluated for these systems. Many proximity
measurement systems are found in battery powered applications so power consumption is important.
Choosing an IR emitter (like an LED) with a high electrical to optical efficiency would provide
better battery performance by allowing lower currents to be used while hitting the range
requirements of the application. The emission angle of the LED is also important and also
contributes to the range of the system, but also the size of the target and the area around the device
that can be monitored. All emitters have a certain range of wavelengths that they output. The peak
output should correspond to the peak sensitivity of the detector. By selecting components that have
the peak wavelengths the same provides the greatest efficiency of the system. For example, having
an emitter with a peak output of 880nm would not be as good a choice as having an emitter that has a
peak output of 850nm when using the Silicon Labs Si1120 that has its highest sensitivity at 850nm.
In addition to the emitter’s peak wavelength the effects of aging of the emitter should also be
considered as intensity and other parameters may changed over time.
Ambient light will be covered in another section so we won’t cover it right now other than to say it
can affect the overall measurement since background light sources contain wavelengths in the IR
spectrum.
The mechanical design of the complete assembly plays an important role in proximity sensing
applications. The more isolation that can be provided by the mechanical assembly the more dynamic
range that will be available to the sensor. For example, if we completely isolate the emitter and the
receiver then all of the signal received is from the reflected object and there is minimal offset in the
reading. If we allow too much light to enter the cavity where the detector is then the emitter energy
can be received without be reflected by any objects. This will increase the measurement offset and
reduce the dynamic range available for the reflected energy.
Design Considerations (3 of 3)
 Single port
 Internal reflection over 100X larger than signal
 100% reflectance variation prevents dumb threshold setting
 OK for short range (< 2.4 cm) motion triggered application
 Dual port has maximum sensitivity
 Allows for > 1 meter range either with lenses or by integrating pulses
 As long as fixed coupling is less than reflectance tolerates 100% variance in reflection
 May be “portless” by using existing products holes (but must have high optical
isolation)
 As distance increases the dual port appears as a single port due to background
IR
Object in proximity
Not recommended
Transparent window
Detector
10
Transmit LED
internal reflection
Object in proximity
Receive window
Detector
Transmit window
Transmit LED
Optical block
In the last section we discussed elements of the mechanical assembly for IR proximity measurements. One of the
key issues to consider when implementing the mechanical design is to reduce the optical coupling. In systems
where there is a lot of reflection or optical coupling from everything else other than the object it is difficult to
detect the actual signal of interest. These other reflections create very large offsets much bigger than the objects
reflection.
Here we see two different topologies: single port and dual port. The single port has a single transparent window
for both the transmit and receive energy. From the purple lines you can tell that the single port has high internal
reflectance which will use up much of the dynamic range of the sensor. For this reason single port designs are
not recommended. The single port design may be useful for short range, simple detection where the object’s
reflection provides enough signal to be resolved. However, the sensing system requires higher resolution since
the signal of interest is small compared to the unwanted internal reflections.
The dual port design has separate windows for the transmit and receive energy and provides a barrier between
the two paths thus reducing the optical reflections. For greater detection range, we can either use lenses or
integrating pulses. Lenses can either be done on the transmitter or the receiver. The easiest option is to use an
LED with an integrated lens. Integrating pulses is also called “averaging.” This can remove the sample-tosample variation to lower the noise floor, and therefore, allow better range. Portless designs use no enclosure
and allow the light to pass unobstructed. In portless designs there should still be high optical isolation to provide
for increased dynamic range.
In any of the cases outlined, the signal detection capability is dependent on the dynamic range of the system: the
background IR vs. the reflected energy from the object. In the single port case the dynamic range is consumed
by the internally reflected energy. In the dual port case the distance and the ambient IR energy eventually take
up the dynamic range. As the object moves farther away from the sensor the lower the reflected energy and the
more influence the ambient IR has on the measurement.
Proximity/ALS Product Snapshot
Si1102-A-GM Proximity Detector
Si1120-A-GM PWM Proximity/ALS Sensor
Logic 0/1 output
PWM output
Standalone detector
(no host required)
Designed to work with
host processor
No ambient light sensor
Integrated ambient light sensor
Single LED driver
Single LED driver
Dedicated digital logic
Dedicated digital logic
3x3 ODFN
3x3 ODFN
Samples and EVB available
Samples and EVB available
Market
Segment / Application
Module manufacturers The module manufacturers pre-package sensors for the markets below
Consumer products
Industrial
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White goods such as refrigerators, microwaves, washers, dryers, and
dishwashers; office equipment such as phones, switchboards, printers,
copiers, scanners; consumer products such as set-top boxes and health
products
Instrumentation panels, touch panels, automatic plumbing valves, paper
dispensers, fluid dispensers, machine safety, cash machines, security
systems, handheld scanners and computers, ticketing and voting machines,
in-flight entertainment, casino/gaming machines and industrial controllers
There are two devices currently for IR proximity detection: the Si1102 and the
Si1120. The features for each are listed in the table. The key difference is that the
Si1120 provides ambient light detection and provides a PWM output directly
proportional to the reflected energy. The Si1102 does not include an ALS
photodiode and is used as a standalone device to provide simple binary logic
detection without ambient light detection. The Si1120 ALS photodiode helps
identify the background ambient light characteristics for an improved infrared
result. Each of these sensors can be used in a wide variety of applications as shown
in the figure.
11
Si1120—PWM Proximity/ALS Sensor
 Low power, high performance proximity and ALS sensor
 PWM output
 High proximity detection range
 Up to 50 cm with single pulse
 Low power
 Typical 10uA average current consumption
3x3 ODFN
Integrated differential photodiode,
signal processor and LED driver
 Low noise ambient cancelling for maximum sensitivity
 Seven precision optical measurement modes
 3 proximity
 3 DC ambient
 1 calibration
 High EMI immunity without shielded packaging
 Operating temperature: -40 to 85 ºC
12
Here we see the key features of the Si1120 which integrates all of the analog
features required to implement robust IR proximity detection as well as ambient
light sensing in a single package. The Si1120 provides a single pulse PWM output
that is easily measured with an MCU and provides low power operation. We will
see in upcoming slides how to configure the Si1120 and take proximity
measurements with a sample application.
12
Benefits of the Implementation
 Single — pulse operation
 Reduces power consumption

Increases battery life
 Fast detection response


Reduces susceptibility to flicker noise
Low-frequency noise reduction
 DC ambient auto-calibrated before each measurement
 Programmable periodic wake-up with power-down state between strobes
Multi-burst integration
[to compensate for flicker]
Longer detection response
Higher power consumption
Shorter battery life
Competitor
IAVG≈ 50mA
25 us
(up to 2ms during high reflectivity)
IAVG≈ 0.5mA
* Average current estimated with 10ms scan rates including LED current (Si1120)
13
One important aspect of many systems that implement IR proximity is overall
power consumption. Each component in the system contributes to the overall
power budget and the Si1120 is no exception. However, when discussing active
proximity measurements the infrared LED is the primary consumer of energy. The
Si1120 provides lower power operation by enabling single pulse operation thus
limiting the amount of LED “on” time. This is contrasted to traditional approaches
where multiple cycles are required to provide an integration of the light over a
specified time period, thus consuming more current. In upcoming slides we will see
how the Si1120 operates and highlight the PWM output as a representation of the
light intensity. To summarize here, the pulse width increases with increased light
intensity which provides a direct relationship to the current consumption. As light
intensity increases so does the power consumption due to the wider pulse width,
however, the average power consumption is reduced.
Using the Si1120
Let’s take a look at how to use the Silicon Labs Si1120.
14
Si1120 Block Diagram
Signal processing
IR Photodiode
ALS Photodiode
LED driver
15
Here is a block diagram of the Si1120. The analog front end consists of two
photodiodes that measure the incoming electromagnetic energy. One of the
detectors is used to measure the IR energy and the other is used to measure the
energy in the visible spectrum. The complete analog signal chain is integrated and
provides the low going PRX output which is a pulse proportional to the light
intensity. The TXO output is a constant current sink that can be set to 50mA or
400mA (with no current limiting) in order to directly drive an IR LED for the
emitter. STX, SC and MD are control inputs to set the operating mode and STX
also starts the conversion process.
Si1120 Sample Schematic (Single LED)
 PWM output on PRX
 PRX output directly proportional to measured light
 Ends the measurement cycle by turning OFF the LED
 TXO sinks a constant current
 Programmable to 50 or 400 mA
 Can connect directly to the LED cathode
 Can connect to the source of a FET driver
 STX starts the measurement cycle
 Enables the current source to turn ON the LED
 Enables the PRX output
16
During device operation, the PRX, STX and TXO pins control the operation of the
proximity detection system. The pulse width modulated PRX signal is output by the
Si1120 and is a measure of the reflected IR energy. The TXO signal is a constant
current sink for the IR LED which generates the energy that will be reflected off of
the object in proximity. The STX signal is an input to the Si1120 and is used to
start the measurement. As will be seen on a later slide the STX signal can also be
used to abort the measurement as well. Shown in the figure is a sample schematic
using a single IR LED driven directly from the Si1120. There are no external gates
required to drive the LED as the Si1120 can sink the constant current required. The
STX and PRX signal can be connected directly to the MCU for a glue-less interface
for proximity measurements.
Si1120 Measurement Modes

Shutdown mode



Three ambient modes

0.1 uA typical shutdown current



Three proximity modes

PRX400

PRX50




IR LED current = 400 mA




IR LED current = 50 mA
High internal reflectance
Used for single-port optical window to
compensate for high reflectance of the cover
Measures both visible and infrared spectra
Peak response at 530 nm: closest to human eye peak
response (550 nm)
3 kLux maximum (assuming blackbody radiators)
VIRH




Measures both visible and infrared spectra
Peak response at 830 nm
500 Lux maximum (assuming black-body radiators)
VAMB
IR LED current = 50 mA
PRX50H



VIRL
Measures both visible and infrared spectra
Peak response at 830 nm
100 kLux maximum (assuming blackbody radiators)
Offset calibration mode (OFC)


IR LED not enabled
Measure ambient IR
17
The Si1120 has seven operating modes that range from measuring the ambient light
to the reflected IR as well as determining the output LED drive current from the
TXO pin. PRX400 is best for range, PRX50 is best for power saving (PRX50H is
not generally used). OFC mode is typically used to figure out the minimum PRX
pulse width without an LED which is representative of the PRX pulse width during
dark ambient conditions since the LED isn’t turned on. OFC is used more rarely
than either PRX400 or PRX50 as it is more like a calibration step and probably is
done once every time the system boots and the value is stored in memory. OFC is
used to represent what the PRX50 or PRX400 would have reported if there was no
reflected IR energy. The PRX400 minus the OFC value, for example, is a more
accurate representation of the reflectance. In applications implementing ambient
light sensing, VAMB, VIRL and VIRH will all be used equally. However, not all
applications will require ambient light sensing.
Si1120 Operation: Setting Modes
 MCU controls setting the mode of the Si1120
 SC, MD and STX should be GPIO during this phase
 MD (latch) and STX (latch) are sampled when SC is driven low
 The first two bits of the three bit mode setting is determined from these latched values
 MD set to final bit of the mode setting
 Changing modes typically requires putting the device in the low power mode first
(SC driven high)
 Exception if mode can be selected by just changing MD (static)
SC transitions low to latch the state of MD and STX
SC
MD
Don’t care
STX
Don’t care
MD (latch)
STX (latch)
1
MD (static)
0
0
Once latched, drive MD to the state
to complete the mode setting
While SC high…set the MD and
STX bits to the initial mode state
PRX50 Mode Example: 010
18
As was mentioned previously, the MCU connected to the Si1120 is responsible for
setting the operating mode of the device by generating three control bits. Setting
the mode is controlled by the SC, MD and STX pins and is a two step process since
the MD pin is used twice. The SC pin is the shutdown pin and when high (logic 1)
puts the Si1120 in a low power mode. To set the operating mode, the MCU starts
by putting the Si1120 into the low power mode. Next, the MCU has to set the value
of the first two configuration bits by setting the value using the MD and STX inputs.
Once these bits are set the SC pin is driven low latching the first two control bits
(MD latch and STX latch in the figure) and bringing the Si1120 out of the low
power mode. To complete the configuration the MCU then drives the MD pin to
either a logic 1 or 0 depending on the desired configuration (MD static in the
figure). The MCU then holds this pin static for the time the Si1120 is to remain in
the set mode. Since the MD static value is not latched it can be used to change the
mode of the Si1120 dynamically. For example, in the figure we see some values for
the PRX50 mode which uses MD (static) as a logic zero. The MCU can change the
value of the GPIO used for MD and make it high at any time. This changes the
Si1120 mode to PRX50H without having to go into the low power mode first.
Si1120 Operation: Starting a Measurement
 STX can be set by the MCU as a periodic signal or can be toggled via firmware
 TXO line is driven low by the Si1120 to enable current to flow through the IR LED and IR energy is
emitted
 Once current flows the PRX signal is driven low by the Si1120 to represent the amount of light
reflected back and received by the photodiode
 STX can be driven low prior to the Si1120 completing the measurement
 Reduces current consumption if accurate measurements are not required
 Good for simple detection applications
Measure
ambient IR
IR LED
Turns ON
Abort a
measurement
IR LED
Turns OFF
STX
TXO
PRX
Photodiode starts
measuring reflected
energy (minus ambient IR)
4 us to 2 ms
19
Now that the mode is set we can start using the Si1120 to take IR proximity
measurements. The STX pin is now used as a source to start the measurement and
can be controlled by the MCU hardware (by using timers) or by firmware. When
the STX line is driven high the measurement cycle begins. At the time the STX
goes high, the Si1120 takes the ambient IR reading and stores this for later. After
the ambient measurement the TXO signal is driven low which then starts the
constant current flow through the LED. During this time when TXO is asserted, the
Si1120 actually measures a combination of ambient IR plus the reflected IR. To get
the reflectance, the Si1120 subtracts the ambient IR measured at the beginning of
the measurement cycle from the overall measurement when the LED is emitting the
IR energy. If the accuracy of the measurement is not required, for example when
doing a simple threshold detection, then a measurement cycle can be aborted by
bringing the STX line low again thus raising PRX and TXO. This can reduce
overall power consumption by minimizing the pulse width outputs.
Si1120 Operation: Capturing the Photodiode Output
 The PRX low time is directly proportional to the amount of
light received by the diode
 Use the Programmable Counter Array (PCA) to measure the low
time
 Set to rising and falling edge captures
Input capture is used to latch the value of the counter from PRX
Capture Events
Counter = 0045
0048
0047
0046
0040
0044
0043
0042
0041
Rising edge ISR:
Store final count value
PRX
0044
0048
0040
Capture/Compare
register
Falling edge ISR:
Store initial count value
PRX low time =
Final count – initial count
20
At this point the MCU has controlled the I/O to the Si1120 in order to set the modes
and start a measurement and as a result the PRX signal is driven low to represent the
IR energy detected. The amount of time the PRX signal is low is a representation of
the amount of energy that is reflected. The longer the pulse the more reflected
energy has been detected, or in other words, the closer an object is to the sensor.
The MCU used to control the I/O is also responsible for measuring the PRX pulse
width. In the Silicon Labs C8051F microcontrollers the PCA timing unit can be
used. These modules are capable of performing rising and falling edge triggered
captures of the PRX signal. When the PRX signal is de-asserted the PCA module
captures the time value of the counter. After the measurement completes the PCA
can then capture the rising edge of the PRX signal. There are now two time values
that represent the complete low pulse interval. The subtraction of the falling edge
count from the rising edge counts provides the time for the PRX pulse and can be
passed on to the proximity algorithms.
3 LED Example Schematic

Each LED is enabled by GPIO from the MCU and controlled by the TXO output from the Si1120

Only one LED is active at any given time
One LED per FET enabled at a time
GPIO
Input capture from PRX
Output frequency to STX
Output SC control
Output MD control
Px.x  MCU port pin
21
An earlier slide showed a single LED implementation where the TXO signal was
used to directly control a single LED. Here is another application example where
the Si1120 is used, in conjunction with an MCU, to control three IR LEDs which
may be useful for motion, position sensing (XYZ) or other similar functions (IR
sliders). Each LED is gated through an external gate that is enabled on a time
multiplexed fashion by the MCU. For example, in the figure the P2.1 signal, when
high, enables the TXO signal to sink current from the D1 LED through Q1. The
other two GPIOs shown (P2.2 and P2.3) would leave those gates disabled while the
D1 LED is used. On the next cycle the P2.1 I/O is de-asserted and the P2.2 I/O is
asserted causing the TXO signal to sink current through the LED connected to Q2.
This cycle repeats for the remaining gates and diodes in the system. The Si1120EK
includes a board that implements this 3 LED scheme and this schematic is a
representation of that system.
MCU Firmware: I/O Port Initialization
 Step 1: Configure the PCA
channel 1 output pin to push-pull
(P1.4)
 Step 2: Set the GPIO pins for
LED enables, MD and SC to
push-pull
 Step 3: Skip the I/O to push the
CEX0 and CEX1 outputs to pins
P1.2 and P1.4
 Step 4: Enable the PCA in the
crossbar
2
1
3
 Step 5: Enable the crossbar
4
Sample code derived from
the Si1120EK 3 LED board
5
22
The previous slides covered how the MCU controls the hardware interface to the
Si1120. In this section, we will illustrate the structure of the firmware used to
control the Si1120 (using a C8051F930). The firmware implementation uses the
hardware outlined for the three LED system discussed prior. Refer to the three LED
schematic on the previous slides to correlate the special function register settings
with the pin designations and Si1120 input/output functions. The specific device
data sheet should also be consulted for definitions of the specific registers used in
these examples.
The I/O of the MCU is connected directly to the Si1120 so we have to configure
them to be either input or output depending on the function. For example, the
Si1120 MD pin is input so the MCU GPIO pin must be set as a push-pull output.
The example shown here provides the I/O settings for all of the control inputs as
well as the PCA pin settings for the application. There is a priority encoded
crossbar in Silicon Labs MCUs that enable all of the digital peripherals in the device
to be placed on the I/O instead of the traditional multiplexed I/O. As a result, there
are skip registers and an enable for the crossbar that needs to be set in order to make
the PCA signals available to specific pins after they have been configured for the
input or output direction. For more information check out any Silicon Labs MCU
data sheet for port input/output more information on the crossbar and its operation.
MCU Firmware: Initializing the PCA
 Step 1: Set the clock source to the
PCA
 Step 2: PCA channel 1 used for a
square wave output
 Derives the sample time for the
Si1120 STX input
 Step 3: PCA channel 0 used for
input capture mode
 Receives the PWM output PRX
from the Si1120
 Rising and falling edge capture to
measure the complete low time of
PRX
 Step 4: Enable the PCA interrupts
 Step 5: The frequency mode uses a
16 bit compare so the values gets
added to the latest compare value
1
2
3
4
5
6
7
 Step 6: Set the LED enables to the
default output
 Step 7: Start the counter
23
As mentioned earlier the PCA is responsible for capturing the timing events for the
PRX going low and again when it goes high. In this application the PCA is also
responsible for setting the STX time base for initiating measurements in which the
fastest rate recommended for the Si1120 is 500 Hz. In this example we will set the
rate to 400 Hz. With that in mind we have to set the clock source to the PCA; and
let’s assume we have a system clock of 24.5 MHz which is very common among
Silicon Labs devices. If we were to use the 16 bit counter without scaling the clock
input we could get a minimum overflow rate of 387 Hz (24.5 MHz/65536). This
hits our target of 400 Hz, however, it does not leave a lot of room to modify the
system level performance, for example, if we wanted to run below 100 Hz to reduce
power. The flip side is we want to provide as much resolution for the measurement
as possible. Using the SYSCLK/4 as the counter input we can get as low as 96 Hz
and provide a resolution of 160 ns. After setting the clock the PCA channels are
then configured. Channel 1 is set to drive the STX output for the 400 Hz output and
channel 0 is set to measure the PRX output. You will notice a section in the code
that sets the PCA0CPL1/CPH1 registers and adds a divide ratio. This is allows a
constant frequency output from the PCA module similar to the hardware
functionality similar to some other timers in the C8051F family.
MCU Firmware: Setting the Si1120 Mode

Step 1: Make STX a GPIO pin by disabling
the PCA to the crossbar

Step 2: Drive GPIO pin SC high

Step 3: Set the MD latch and the STX latch
values for the desired mode

Step 4: Drive SC low to latch the data

Step 5: Drive MD to the final value for the
desired mode

Step 6: Return STX to the PCA output
1
2
3
4
5
3
2
4
5
SC
MD
STX
Don’t care
Don’t care
MD (latch)
1
MD (static)
STX (latch)
6
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Setting the mode to the Si1120 requires a two stage process that involves
controlling three output pins of the MCU: MD, STX and SC. The description of
how to set the mode was discussed earlier so the firmware implementation is shown
here. Within the Silicon Labs MCUs there is a crossbar and each peripheral that
passes through the crossbar has to be enabled which provides the connection to the
I/O pins. When setting up the Si1120 mode we are going to reuse the STX pin as it
is used to set the mode as well as initiate measurements. For this reason step one
above disables the PCA through the crossbar allowing the firmware to control the
GPIO pin directly. The remaining steps follow the mode setting procedure outlined
previously by setting the Si1120 to the low power mode by raising SC and changing
the I/O. The numbered lines of code correspond to the steps given and the follow
the timing diagram for setting the modes.
MCU Firmware: PCA Channel 1 ISR
 PCA channel 1 generates a square wave output
 16 bit comparisons determine the sample interval for each LED by
driving STX
 After each comparison the PCA compare register is added to the
Next_Compare_Value
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Once the initialization sequence is completed the firmware has to be set up to
service the operation of the Si1120 using the PCA interrupt service routines (ISR).
PCA channel one was configured to be the square wave output which was generated
by adding our ratio value to the current count to always provide the same overflow
rate. Within the channel one ISR the next count value is determined and loaded into
the compare registers to provide the next overflow. That is all the ISR does for this
PCA output driving the STX signal.
MCU Firmware: PCA Channel 0 ISR
 PCA channel 0 measures the PRX low
time by capturing the falling and rising
edges
 PRX goes low causing a capture event
and the ISR stores the data
 Subsequent PRX rising edge causes a
second capture event once again
storing the data
 The difference between the two
captured values provides PRX low time
 Once PRX low time complete the ISR
changes to the next LED
26
…
The PCA channel 0 ISR is a little more complicated as it is measuring the PRX low
time by capturing the falling and rising edges and is entered twice. The ISR first
determines if the event was from a falling edge by measuring the pin value. If low,
the ISR then stores the captured timer value and exits and waits for the next edge.
On the next trigger event, which would be a rising edge, the ISR once again stores
the captured timer value and then determines the time interval between edges by
subtracting the falling edge capture from the rising edge capture. This calculation
provides the measure of the reflected energy. In the case of the three LED example,
once the value of the reflected energy is measured for the current LED the ISR also
moves the external gate enable to the next LED in the sequence and the cycle
repeats.
Development Kits
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Three – LED Motion Demonstration
 Seven LEDs on board
 Can implement several measurement modes




Single proximity measurements
Three LED motion measurements
Two LED slider measurements
Ambient light sensing
 Provides interface to C8051F930 low power MCU
 Highlights time division multiplex scheme for monitoring multiple LEDs
 Interfaces to the QuickSense Studio GUI
DK Part #: Si1120EK
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Three IR LED Motion Demo
 Hand moves left to right to left
blue/red lead
pink leads
 Hand moves bottom to top to bottom
red
pink
blue
blue leads
pink/red lead
29
Let’s take a look at a sample acquisition using the Si1120EK. In this example the
LEDs labeled red, pink and blue are enabled using the jumpers on the board. In the
top plot a hand moves from left to right and then back again. From the figure you
can see the red and blue traces start at the same time indicating that the Si1120 is
receiving the reflected energy from the hand from both LEDs at the same time. The
only difference is the magnitude since the blue LED is farther from the Si1120 then
the red. As the hand moves to the right the pink LEDs reflected energy starts to get
detected as shown by the increase in magnitude. The opposite is true for the reverse
direction in the plot. As the hand moves right to left the pink leads the red and blue.
IR Slider Demonstration


An infrared slider can sense as far as the most distant IR LED can be sensed
by the Si1120
Multiple methods of set-point gesture recognition including:



A left to right or right to left motion with a pause indicating set-point
A left to right or right to left swipe
USB connection is all that is required
DK Part #: IrSLIDEREK
30
Another reference kit that highlights the use of the Si1120 is the IR Slider
(IRSLIDEREK). This board provides two LED emitters spaced equally from the
Si1120. As the hand moves across the range of interest it generates the profile
shown in the plot. From these inputs the location of the hand can be detected.
There are also gestures that can be recognized by the MCU such as single taps or
double taps all without ever touching anything. Some additional gestures that can be
interpreted are: a left to right or right to left motion with a ‘press’ and ‘retract’ and
a simple approach and retract of an object without any lateral movement.
Front Panel Evaluation Kit
 Infrared proximity sensing
 Ir proximity sensing is provided
via an Si1120 controlling a single
IR LED
 Capacitive proximity sensing
 Capacitive sensing is provided
via a a single pad surrounding
four touch buttons connected to
an C8051F800
 QuickSense Studio support
 Real-time performance analysis
support provided via USB
connection to QuickSense Studio
 Kit Contents
 FrontPanel2-EB demo board
 USB cable
 FrontPanel2EB quick start guide
31
F340
EK Part #: FrontPanel2EK
Si1102 Evaluation Kit
 Infrared threshold detection
 The Si1102EK demonstrates a simple
infrared proximity threshold—a visible
LED illuminates when an object
crosses an invisible threshold
 Configurable range
 Proximity threshold range and scan
rate is controlled by two thumb-wheel
potentiometers
 Battery operated
 This self-contained demonstration
operates from a single coin-cell
battery
 Kit Contents
 Si1102AB demo board
 Si1102EK Quick Start Guide
32
EK Part #: Si1102EK
Summary of the QuickSense Infrared Sensors
 Si1102/1120 provides industry-leading performance
 High sensitivity infrared photodiode
 Industry-leading range up to 50 cm
 Innovative single pulse proximity sensor
 Immune to fluorescent flicker
 Ultra-low system power
Si1120-A-GM
 Integrated ambient light photodiode (Si1120)
 Reduces backlight power in low-light settings
 Addressing the needs of user interface development
 QuickSense Studio provides rapid deployment
 Patent pending touch-less slider and gestures
 Algorithms including background light type
determination
33
33
Learn More at the Education Resource Center
 Visit the Silicon Labs website to get more information on Silicon Labs
products, technologies and tools
 The Education Resource Center training modules are designed to get
designers up and running quickly on the peripherals and tools needed
for application development
 http://www.silabs.com/ERC
 http://www.silabs.com/mcu





Si1102EK: www.silabs.com/Si1102EK
Si1120EK: www.silabs.com/Si1120EK
IrSliderEK: www.silabs.com/slider
FrontPanel2EK: www.silabs.com/frontpanel
AN442: Designer’s Guide for the Si1102 & Si1120
 To provide feedback on this or any other training go to:
http://www.silabs.com/ERC and click the link for feedback
34
Visit the Silicon Labs Education Resource Center to learn more about the MCU
products.
www.silabs.com/MCU
35