AN52491 Implementing Ambient Light Sensing Using PSoC 1.pdf

AN52491
Implementing Ambient Light Sensing Using PSoC® 1
Author: Jaya Kathuria
Associated Part Family: CY8C23X33, CY8C24x23, CY8C24X33, CY8C21X23
CY8C21X34,CY8C24X94, CY8C27X43,CY8C29X66
Related Application Notes: AN75320, AN2397
To get the latest version of this application note, or the associated proje ct file, please
visit http://www.cypress.com/go/AN52491.
®
AN52491 describes how to implement ambient light sensing using PSoC 1. Two example projects interfacing an
external analog ambient light sensor are also presented.
Contents
1
2
3
4
5
6
Introduction ...............................................................1
What are Ambient Light Sensors? ............................2
Features of Ambient Light Sensors ..........................2
Applications ..............................................................3
LX1972A Description ................................................3
Example Project: ALS_ADC Example ......................4
6.1
Device Configuration .......................................4
6.2
Hardware Requisites .......................................8
6.3
Test Procedure ................................................8
6.4
Expected Results .............................................9
7
Example Project: ALS_Comparator Example ......... 11
7.1
Device Configuration ..................................... 11
7.2
Hardware Requisites ..................................... 12
1
7.3
Test Procedure .............................................. 12
7.4
Expected Results ........................................... 13
8
Summary ................................................................ 13
9
Appendix A ............................................................. 14
9.1
Power Calculations ........................................ 14
10 Document History ................................................... 15
Worldwide Sales and Design Support ............................. 16
Products .......................................................................... 16
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PSoC Solutions ............................................................. 16
Cypress Developer Community....................................... 16
Technical Support ........................................................... 16
Introduction
Ambient light sensors are included in mobile phones, laptops, and handheld devices. They sense the environment
lighting conditions and adjust the screen's backlight to comfortable levels. Studies have shown that backlighting is
required only for 40 percent of the usage time. Therefore, an automatic adjustment (auto dimming) of the backlight
offers considerable power savings.
This application note describes the firmware and hardware to interface ambient light sensors, and the signal
processing of analog signals using the PSoC device. It also discusses the advantages of using PSoC in this solution.
®
Note This application note discusses the implementation with PSoC 1 devices and any reference to PSoC in the
®
document will refer to PSoC 1. To learn more about PSoC 1, refer to AN75320 - Getting Started with PSoC 1.
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
2
What are Ambient Light Sensors?
Ambient light sensors are photo detectors designed to perceive brightness in the same way as human eyes. They are
used where the settings of a system must be adjusted to the ambient light conditions as perceived by humans. Two
common photo detectors used in ambient light sensing are phototransistor and photodiode. Both phototransistors and
photodiodes generate an output signal in response to a light input from the electromagnetic spectrum. This is shown
in Figure 1. Table 1 shows their comparison.
3
Features of Ambient Light Sensors
Ambient light sensors must have the following features:




Provide an output value that reflects the frequency sensitivity (V-lambda characteristics) of the human eye.
High accuracy over a wide illumination range
Low temperature coefficient
Compact, surface-mountable package, particularly for handheld applications
Figure 1. Phototransistor, Photodiode, and Visible Light Region of the Electromagnetic Spectrum
Table 1. Phototransistor versus Photodiode Comparison
Phototransistor
Photodiode
Small devices with good performance
High performance but at a larger size
Slower response time
Faster response time
Higher sensitivity (electrical output per optical input)
Lower sensitivity (electrical output per optical input)
Lower stability with temperature
High stability with temperature
Suitable for mobile applications
Suitable for automotive applications
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Implementing Ambient Light Sensing Using PSoC® 1
4
Applications

Detection of ambient light to control display backlighting in:






5
Mobile devices - mobile phones, personal digital assistants (PDA), personal media players.
Computing devices - TFT LCD monitor for note book computer.
Consumer devices – TFT LCD TV, Plasma TV, video camera, digital still camera.
Automatic residential and commercial lighting management.
Automatic dimming of instruments in automobiles to ensure reliable visibility.
Headlamp control in cars to improve road safety by automatically turning on the lights in twilight or when entering
a tunnel.
LX1972A Description
The LX1972A sensor from Microsemi is used to demonstrate the example projects accompanying this application
note. The LX1972A is a low cost silicon light sensor with a spectral response that closely emulates the human eye.
LX1972A provides a linear, accurate, and very repeatable current transfer function. High gain current mirrors on the
chip multiply the PIN diode photo-current to a sensitivity level that can be voltage scaled with a standard value
external resistor.
Table 2 summarizes the important parameters of LX1972A.
Table 2. LX1972A Specifications
Parameter
Value
Input irradiance, Ev = 100 lx, VDD – VSS = 2 V (Worst case)
Input irradiance, Ev = 1000 lx, VDD – VSS = 2.7 V (Worst case)
Minimum Operational Voltage
Input irradiance, Ev = 2000 lx, VDD – VSS = 3 V (Worst case)
Output (Photocurrent)
Typical 235 µA at VCC=5.0 V, Ev =1000 lx (Fluorescent light is used as
light source)
Dark Current
50 nA at VCC=5.0 V, Ev=0 lx, TA = 25oC
Wavelength of Maximum Sensitivity (Range)
580 nm (360 to 650 nm)
Figure 2 shows its spectral response versus wavelength.
The LX1972A is ideal for applications in which the measurement of ambient light is used to control display
backlighting such as mobile phones and PDAs.
Figure 2. Relative Spectral Response versus Wavelength
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
6
Example Project: ALS_ADC Example
The example project demonstrates how easy and effective it is to interface and process the signal from ALS sensors
with PSoC. This example uses the PSoC CY8C29x66 family to implement the analog front end required for the ALS
interface. The current output from the ALS is converted to a voltage signal using an external resistor. This voltage
signal is then passed through a Programmable Gain Amplifier (PGA) before converting the analog signal to digital
data using an ADC. An incremental ADC with up to 13-bits of resolution is used for the conversion purpose. The
example project performs the following tasks:
Input

Converts output current from the ALS to voltage signal using a resistor (R = 5.1 K, as shown in Figure 7).
Note Capacitor (C = 10 µf) is placed across the current sample resistor to reduce voltage spikes (Figure 7).
Signal Processing Stage


Amplifies the voltage signal to the desired range using a PGA inside PSoC
Converts the amplified signal to digital data using an incremental ADC implemented completely inside PSoC
Output



Displays the ADC output on the LCD
Controls the brightness of LED based on ambient light intensity (ALS output).
Provides the converted ADC count from ALS and brightness value of the LED (PWM pulse width) to the host
through I2C interface
Note the project uses a periodic sleep-wake up cycle to scan the ALS every 125 ms. Refer to the project code for
details.
6.1
Device Configuration
The block layout of the PSoC project is shown in Figure 6. Table 3 shows the resource requirement for this
application for different PSoC families. Note that PSoC drives the VCC of the ambient light sensor. This allows a zero
sleep current when the sensor is completely powered down by PSoC. The project is tested using CY3210 PSoCEval1
board with CY8C29466-24PXI device. It can also be implemented in any PSoC with continuous time (CT) and switch
capacitor (SC) analog blocks by configuring them to implement ADCs: CY8C21x34, CY8C21x23, CY8C24x33,
CY8C24x94, CY8C27x43, or CY8C29x66.
The project uses a PGA user module, which occupies one continuous time (CT) block to implement the amplifier
functionality. The gain of the PGA is set to 1 to provide a full scale output of 5 V (with 5.1 KΩ resistor) when a mobile
phone LED flash light is used to illuminate the ALS. The gain can be adjusted depending on the max ambient light
luminosity desired from the sensor. In this case, the max luminosity is set to the mobile phone LED flash light
luminosity. The reference of the PGA is set to Vss as the voltage input is referenced to Vss only. The input signal
from P0[5] is routed via AnalogColumn_InputSelect_1 Mux.
Figure 3. PGA UM Configuration
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
The output from the PGA feeds an incremental ADC configured to run at 13-bit resolution. The ADC uses one
switched capacitor (SC) block and three basic digital blocks. Refer the ADCINCVR user module datasheet for details
on the ADC parameters and their impact. With the configuration used in the project, the ADC sample rate is
approximately 45.7 Samples per Second (SPS).
Figure 4. ADCINCVR UM Configuration
The project also uses a PWM8 UM clocked from the 32 KHz low frequency clock. This PWM is used to control the
intensity of an LED based on the ambient light intensity. 32 KHz clock is selected as the input to make sure the PWM
output is generated even during sleep. During device sleep, the high frequency clock is turned OFF to save power.
The output of the PWM is routed to P1[5].
Figure 5. PWM UM Configuration
In addition to the PWM, the ambient light output is displayed in a character LCD and sent out through I2C.
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
Figure 6. ALS_ADC PSoC Block Layout
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
Figure 7. PSoC Design Block Diagram
P1[2]
100 Ω
1
P2[0]-P2[6]
LX1972A
2
P0[5]
10µF
LCD
+
PGA
5.1 kΩ
M8C
ADC
PWM
-
P1[5]
1 kΩ
EzI2C
PSoC 1
P1[0]-P1[1]
USB-I2C
Bridge
PC
Table 3. Resource Consumption
PSoC Family
CY8C23x33
CY8C24x33
CY8C29x66
CY8C27x43
CY8C24x94
CY8C24x23A
CY8C21x34
CY8C21x23
Analog UM
PGA,
ADCINC
PGA,
ADCINC
PGA,
ADCINC
PGA,
ADCINC
PGA,
ADCINC
PGA,
ADCINC
ADC8
ADC8
Digital UM
PWM8
PWM8
PWM8
PWM8
PWM8
PWM8
PWM8
PWM8
DBB
4
4
4
4
4
4
4
4
DCB
-
-
-
-
-
-
-
-
ACB
1
1
1
1
1
1
1
1
ASC
1
1
1
1
1
1
1
1
ASD
-
-
-
-
-
-
-
-
I2C
I2C
I2C
I2C
I2C
I2C
I2C
I2C
SW UM
Char LCD
Char LCD
Char LCD
Char LCD
Char LCD
Char LCD
Char LCD
Char LCD
I/Os used
5+6 (LCD)
5+6 (LCD)
5+6 (LCD)
5+6 (LCD)
5+6 (LCD)
5+6 (LCD)
5+6 (LCD)
5+6 (LCD)
Fixed
resources
Note
Appendix A shows the power calculations.
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Document No. 001-52491 Rev. *B
7
Implementing Ambient Light Sensing Using PSoC® 1
6.2
6.3
Hardware Requisites
1.
CY3210 PSoC1Eval board with CY8C29466-24PXI device (comes with CY3210)
2.
CY3217 MiniProg1 (comes with CY3210) or CY8CKIT-002 MiniProg3 for programming
3.
LX1972A or equivalent Ambient light sensor
4.
CY8CKIT-002 MiniProg3 for testing over I2C bridge
5.
USB cables, connecting wires and 5.1 K resistor
Test Procedure
1.
Connect the ALS sensor and the 5.1 KΩ resistor as shown in Figure 7.
2.
Connect the ALS sensor output to the PSoC pin P0[5] on the CY3210 board
3.
Connect P1[5] to the LED. Optionally you can connect P1[2] to the ALS Vcc pin for turning ON/OFF the ALS
through firmware.
4.
Mount the CY8C29466-24PXI device onto CY3210
5.
Connect Miniprog1 or Miniprog3 to the 5-Pin programming header on board
6.
Open AN52491_ALS_ADC project and Program the device. Use Power Cycle programming option if Powering
using Miniprog
7.
After programming, power the board using Miniprog or power jack; the ADC count is displayed on the LCD and
LED varies brightness depending on the intensity of ambient light. You can bring a mobile phone LED flash on
closer to the ambient light sensor to observe the effect.
8.
The project also implements the I C slave interface into a PSoC project through EzI2Cs user module. Any I C
2
master can read this ADC count of ALS sensor from this I C slave device.
2
2
Table 4. Setup on the CY3210 Evaluation board
PSoC 1 Pins
CY3210 Connections
Description
P0[5]

Connect to ALS / 5.1 KΩ Resistor junction
P1[5]
LED1
PWM LED output
P1[2]

Connect to ALS VDD pin
-
Char LCD header
Character LCD output (P2[0] to P2[6]) are routed in CY3210

ISSP header (J11)
Connect MiniProg1 or MiniProg3 for programming

ISSP header (J11)
Connect MiniProg3 for I2C communication to PC; P1[0] and P1[1] are
routed in CY3210
2
Note To see results using an I C to USB Bridge, comment the code that is used to put PSoC into sleep mode
(“M8C_Sleep”) Use the read command “r 01 @1ADCCount @0ADCCount” from the Bridge Control Panel software
to read the ADC output from the ALS sensor (see Figure 8). Refer to AN2397 – CapSense Data viewing tools for
details on sending and receiving commands through BCP. Make sure the I2C data rate selected (Tools > Protocol
Configuration > I2C) does not exceed 100 KHz.
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
Figure 8. Bridge Control Panel configuration
Command to be sent
from BCP
Variable settings (Chart
> Variable Settings)
6.4
Expected Results
The LED brightness controlled by P1[5] is at its minimum when the ALS is in dark (covered with hand). The LED
brightness will gradually increase when a mobile phone LED flash light is brought near it. The brightness will be at its
maximum when the flash light is around 1 cm above the ALS. See Figure 10 for CY3210 board connections and
output. You can refer Table 4 and Figure 7 for detailed pin connections on the CY3210.
Figure 9 shows the expected ADC output seen in Bridge control panel.
Figure 9. ADC Output in Bridge Control Panel
Dark
Condition
– Sensor
covered
with hand
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Output
saturation
when LED
flash light is
at 1 cm
from
Sensor
sensor
response for
approaching
LED flash light
Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
Figure 10. CY3210 Connections and Output
LX1972A
>Sensor Anode on the
left (5.1 KΩ + Capacitor +
P0[5] junction)
>Sensor Cathode on
right (100 Ω to P1[2])
LED Output
ADC output
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
7
Example Project: ALS_Comparator Example
This example project demonstrates a simple application of turning on and off the lights depending on the ambient
light intensity measured (such as headlamp control in cars by automatically turning on the lights in twilight or when
entering a tunnel). This is done by using just a comparator. The comparator output drives a digital buffer, which in
turn controls the LED ON/OFF. This example also uses the PSoC CY8C29x66 family to demonstrate the
implementation. The example project performs the following tasks:
Input

Converts output current from the ALS to voltage signal using a resistor (R = 5.1 K, as shown in Figure 7.
Note Capacitor (C = 10 µf) is placed across the current sample resistor to reduce voltage spikes (Figure 7).
Signal Processing Stage

Compares the voltage output to a known threshold using a hardware comparator
Output

7.1
Sends the comparator output to an LED through a digital buffer
Device Configuration
The block layout of the PSoC project is shown in Figure 12. The project is tested using CY3210 PSoC1Eval board
with CY8C29466-24PXI device. It can also be implemented in any PSoC with continuous time block by configuring
them to implement comparator.
The project uses comparator UM to implement the voltage comparison. The ambient light signal, P0[5], is routed to
comparator non-inverting input through AnalogColumn_InputMUX_0. The threshold is kept at 210 mV (0.042 * VDD)
and can be varied by changing the RefValue parameter of the UM. This output controls an LED connected to P1[5]
through a Digital buffer UM.
Figure 11. Comparator UM configuration
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
Figure 12. ALS Comparator PSoC Block Layout
7.2
7.3
Hardware Requisites
1.
CY3210 PSoC1Eval board with CY8C29466-24PXI device (Comes with CY3210)
2.
CY3217 MiniProg1 (comes with CY3210) or CY8CKIT-002 MiniProg3 for programming
3.
LX1972A or equivalent Ambient light sensor
4.
USB cables, connecting wires and 5.1 K resistor
Test Procedure
1.
Connect the ALS and the 5.1 KΩ resistor as shown in Figure 7.
2.
Connect the ALS output to the PSoC pin P0[5] on the CY3210 board
3.
Connect P1[5] to the LED. Optionally you can connect P1[2] to the ALS Vcc pin for turning ON/OFF the ALS
through firmware.
4.
Mount the CY8C29466-24PXI device onto CY3210
5.
Connect Miniprog1 or Miniprog3 to the 5-Pin programming header on board
6.
Open AN52491_ALS_Comparator project and Program the device. Use Power Cycle programming option
7.
After programming, power the board using Miniprog or power jack; The LED connected to P1[5] should turn ON
when the ALS is in dark conditions (covered with hand) and OFF when in bright conditions (mobile phone LED
flash).
Table 5. Setup on the CY3210 Evaluation Board
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PSoC 1 Pins
CY3210 Connections
Description
P0[5]
-
Connect to ALS / 5.1 KΩ Resistor junction
P1[5]
LED1
Comparator LED output
P1[2]
-
Connect to ALS VDD pin
-
ISSP header (J11)
Connect MiniProg1 or MiniProg3 for programming
Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
7.4
Expected Results
LED connected to P1[5] should turn ON when the ALS is in dark conditions (covered with hand) and OFF when in
bright conditions (mobile phone LED flash). The threshold at which the LED turns ON can be controlled by changing
the RefValue parameter of the Comparator UM.
8
Summary
This application note discusses the analog signal conditioning of ambient light sensor using PSoC. The
implementation proves how easy and effective it is to interface and process the sensor signals with PSoC.
About the Author
Name:
Jaya Kathuria
Title:
Applications Engineer Sr.
Background:
Jaya Kathuria is a Senior
Applications
Engineer
in
Cypress
Semiconductor’s
Consumer and Computation
Division, focused on PSoC
solutions.
Contact:
[email protected]
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
9
Appendix A
9.1
Power Calculations
Figure 13. Screen Shot of Output Voltage in Dark Conditions
  =






  ∗  +   ∗ 

PSoC Active Current = 1.62 mA at 6 MHz
PSoC Sleep Current = 3 µA.
ALS Sensor Active Current = 410 µA at 100 Lx
ALS Sensor Sleep Current = 0 A.
See Figure 13 for time information.
Based on the previous values and equation:

PSoC I average = 39.2 µA

Sensor I average = 205 µA.

I average total = 244.2
Note The sensor on time is high as its settling time is high. Settling time also varies with light intensity.
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Document No. 001-52491 Rev. *B
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Implementing Ambient Light Sensing Using PSoC® 1
10
Document History
®
Document Title: AN52491 – Implementing Ambient Light Sensing Using PSoC 1
Document Number: 001-52491
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
2678058
XKJ
03/24/2009
New application note
*A
3567112
MSUR
03/30/2012
Updated template. Completing sunset review.
Updated the title to: Implementing Ambient Light Sensing Using PSoC® 1.
Added a Note in Example Project: ALS_ADC Example.
Replaced APDS-9002 ALS with LX1972A as APDS-9002 is obsolete and not
available for purchase
*B
4732178
ASRI
05/07/2015
Updated projects to CY8C29x66 as CY8C23x33 CY3210 POD is not available for
customer to test the project.
Replaced SAR ADC project with incremental ADC as CY8C29x66 does not
support SAR ADC and the application does not require SAR ADC
Updated the example projects section to be consistent – Updated Project
description, Device Configuration, Hardware Requisites, Test procedure and
expected results
Moved examples explanation from Appendix to main body.
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Document No. 001-52491 Rev. *B
15
Implementing Ambient Light Sensing Using PSoC® 1
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