Texas Instruments FDC2214 Manuel utilisateur

User's Guide

SNOU139–October 2015

FDC2214 Proximity and Capacitive Touch EVM User’s

Guide

Contents

1 Overview...................................................................................................................... 2

2 Hardware...................................................................................................................... 3

2.1 Grounded versus Floating Systems.............................................................................. 3

2.2 Proximity Sensor.................................................................................................... 4

2.3 Capacitive Touch Buttons ......................................................................................... 4

2.4 Reset Button......................................................................................................... 5

2.5 Optional Features Descriptions ................................................................................... 5

2.6 Additional Debugging Features ................................................................................... 6

3 Firmware...................................................................................................................... 6

3.1 FDC2214 Register Configuration................................................................................. 6

3.2 Programming........................................................................................................ 7

4 Setup .......................................................................................................................... 7

5 Operation ..................................................................................................................... 8

6 Board Layout................................................................................................................ 10

7 Schematic ................................................................................................................... 13

List of Figures

1 FDC2214 Proximity and Capacitor Touch Sensing EVM............................................................... 2

2 System Level Block Diagram............................................................................................... 3

3 Component Layout Location of Board on Top Side and Bottom Side................................................ 3

4 Sensor Size Area Versus Sensing Distance............................................................................. 4

5 Comparison of Capacitance vs Target Position for Covered and Non-covered Sensors .......................... 5

6 : JTAG Pin Out Diagram.................................................................................................... 7

7 Proximity Sensor Waveform (Yellow), Cap Touch Button 1/2 Waveforms (Green and Magenta)................ 8

8 Proximity Startup Waveform (Yellow), Cap Touch Button 1/2 Startup Waveforms (Green, Magenta) .......... 9

9 Sensor Multiplexing – Proximity (Yellow), Cap Touch Button 1 and 2 (Green, Magenta) ......................... 9

10 Composite Top and Bottom Layer Plot (Viewed From the Top) ..................................................... 10

11 Top Layer.................................................................................................................... 10

12 Midlayer 1 (Power Plane) ................................................................................................. 11

13 Midlayer 2 (GND Plane)................................................................................................... 11

14 Bottom Layer................................................................................................................ 12

15 Schematic ................................................................................................................... 13

List of Tables

1 Optional Feature Use Cases............................................................................................... 5

2 FDC2214 Register Configuration.......................................................................................... 6

3 JTAG Connector Pin Assignment.......................................................................................... 7

4 Proximity and Capacitive Touch Sensor Properties..................................................................... 8

All trademarks are the property of their respective owners.

SNOU139–October 2015 FDC2214 Proximity and Capacitive Touch EVM User’s Guide

Overview

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1 Overview

The FDC2214 proximity and capacitive touch button board demonstrates the use of TI’s capacitive

sensing technology to sense and measure the presence of various objects. The board is a complete

hardware and firmware solution that integrates the FDC2214, MSP430FR5969, and TPS61029 into one

design. The firmware included processes the data from one proximity sensor and two capacitive touch

buttons via the FDC2214 to determine whether an object is in the intended sensing area. All of the

processing is done on the MSP430 allowing the board to be a standalone system. Dedicated colored

LEDs light up once the device detects a target in close proximity to the board or detects a press on the

buttons. The board is designed to operate with one AA-battery. The maximum sensing distance that can

be reliably achieved with the board is 10 cm (4 inches) as a floating system and 50 cm (19.6 inches) as a

grounded system. The cap touch buttons and firmware have been optimized to be very responsive to

quick user touch interactions

The hardware uses the 4-channel FDC2214 to multiplex through each of the three capacitive sensors: one

proximity sensor and two capacitive touch buttons. As the firmware in the MSP430 detects a user

approaching the board, a green LED lights up to indicate that an object has been detected. If the user

touches Button1, Button2, or both of them simultaneously, their corresponding red LED lights up. The

board integrates the FDC2214, MSP430FR5969 and the TPS61029 as a complete standalone system.

Figure 1 shows the top side of the reference design.

Figure 1. FDC2214 Proximity and Capacitor Touch Sensing EVM

2FDC2214 Proximity and Capacitive Touch EVM User’s Guide SNOU139–October 2015

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RESET

LED LED

Button 1 Button 2

FDC2214

LED

LED LED

Proximity Sensor

GND SHLD

JTAG Connector

AA Battery

Holder

TPS61029

MSP430FR5969

GND SHLD

Top Side of EVM Bottom Side of EVM

IN0A

IN0B

IN2A

IN2B

FDC2214

VDD

GND

SCL

SDA

Int. Osc.

ADDR

INTB

GND

MCU

VDD

3.3V

GPIO

Core

Resonant

circuit

driver

Resonant

circuit

driver

I2C

peripheral

3.3V

CLKIN

40MHz

Button1

3.3V

GPIO

IN1A

IN1B

Resonant

circuit

driver

Proximity

Button2

GND PGND

LBO

LBI

VBAT

SW VOUT

VBAT

3.3V

TPS61029

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Hardware

2 Hardware

Figure 2. System Level Block Diagram

Figure 3 illustrates the features of the top side and bottom side of the board. The proximity sensor

consists of a copper bezel arranged around the perimeter of the board. The board is 10.16 cm by 12.7 cm

(4 x 5 inches). The proximity sensor is 19.05 mm (0.75 inches) wide and conforms to the edges of the

board. The buttons are 20.32 mm (0.8 inches) diameter sensors arranged in the center of the board. A cut

out on the proximity sensor where the battery is located is necessary since the battery acts as a ground.

This limits the amount of direct capacitive coupling seen by the sensor.

Figure 3. Component Layout Location of Board on Top Side and Bottom Side

2.1 Grounded versus Floating Systems

There are two typical scenarios in how a capacitive sensing system is designed relative to its ground

reference: a grounded system and a floating system. A grounded system means the system is referenced

to or very close to earth ground. A floating system means the system is referenced to the “ground”

reference of the source that is generating the power for the system, for example a battery. As an example,

a laptop that is operating with its charger connected to a wall outlet is grounded whereas a laptop

powered solely on its battery is a floating system.

SNOU139–October 2015 FDC2214 Proximity and Capacitive Touch EVM User’s Guide

Sensor Area - square sensor (cm2)

Sensing Distance (cm)

0 5 10 15 20 25 30 35 40 45 50

D002

Hardware

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The sensitivity of a grounded system is more significant compared to a floating system, especially if the

intended target is the human body. The human body will have a voltage potential close to earth ground

(same or close to system ground) while the voltage potential of the floating system can be significantly

different. This difference in potential directly corresponds to a difference in sensitivity or sensing distance.

This EVM uses a AA-battery as its power source so

2.2 Proximity Sensor

The proximity sensor on the board was designed to detect a human hand at 50 cm using a grounded

system and 10 cm with a floating system. Sensitivity is defined as maximum sensing distance, at which

point the shift in the measured capacitance exceeds the detection threshold, which is determined by the

system noise level. Figure 4 shows a plot of the sensing distance the FDC2214 can achieve with a given

sensor size area in a grounded system. A solid square sensor was used to collect the data in Figure 4.

Figure 4. Sensor Size Area Versus Sensing Distance

The proximity sensor on the EVM is designed around the edges of the board in a bezel configuration. The

actual sensor size area is 59 cm2with the spatial sensor size area spanning 121 cm2. All of the electronics

are located in the middle of the proximity sensor.

One of the major factors that affect sensitivity of a proximity sensor is the size and location of the nearest

ground potential. The layout of the board minimizes the ground plane as much as possible. A ground

plane underneath the sensor significantly reduces sensitivity. For example, if a ground shield is directly

below and the same size as the sensor, sensitivity is reduced by 50%. Noise in the system and in the

environment also affects sensing performance.

The system was optimized and tested in a low noise environment. Variations and false triggers may occur

depending on the surrounding environment. The algorithm thresholds in the firmware may need to be

adjusted accordingly to fit a specific environment condition

2.3 Capacitive Touch Buttons

The circular buttons measure 20.32 mm (0.8 inches) in diameter. Capacitive touch buttons require less

attention to sensitivity in terms detecting small changes in capacitance simply because the intent is to

detect a touch event. Typically, changes in capacitance on the order of picofarads are required in

detecting button presses. The same factors that affect sensitivity for proximity still apply for capacitive

touch buttons. A solid or hatched ground pad/shield underneath the button sensor can be used to

moderate the sensitivity of the button and mitigate unintentional touch events.

In most applications, the user interacts with the button sensor directly. In some applications, there may be

a protective cover or a piece of isolation material to prevent direct contact with the sensor (for quality and

reliability purposes). Sensitivity performance is dependent on the material and thickness of the material.

Figure 5 shows a simulated sensitivity analysis for a proximity sensor. Capacitive touch buttons rely on a

large response to determine when a press is detected directly on the sensor. The simulation shows that at

close range, including the touch condition, a protective cover (in this case PVC) can actually enhance the

response. However, this effect is observed over a limited range and at longer ranges shows little impact.

4FDC2214 Proximity and Capacitive Touch EVM User’s Guide SNOU139–October 2015

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Target Distance From Sensor (cm)

Capacitance Change From Baseline (fF)

0 2 4 6 8 10 12

100

150

200

250

300

350

400

450

D001

Air Only

Air and PVC Cover

Difference

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Hardware

Figure 5. Comparison of Capacitance vs Target Position for Covered and Non-covered Sensors

2.4 Reset Button

A reset button (Figure 3) is included in the design to perform a soft reset to the system if there are any

disruptions to the board relative to its environment. For example, if the board is picked up and moved to a

different area, its baseline capacitance can dramatically change. The derivative-integration algorithm

implemented in this system may take some time to recover from this change in the baseline capacitance,

so the reset button can be used to eliminate this recovery time. Once the reset button is pressed, LED D2

and D3 near the button temporarily light up indicating that an initial calibration is being performed.

NOTE: A two second delay is implemented prior to the calibration process. During calibration, no

target or object should be in the intended sensing area. The LEDs turn off after calibration

and the system is fully operational.

2.5 Optional Features Descriptions

There are several optional features (Table 1) implemented in the design that can be suitable for future

system investigations:

Table 1. Optional Feature Use Cases

Part Designator Description Use Case

J3 3 GPIO pinouts on a 3 pin 100 mil header Debug and prototype

Allows for current measurements for low power

R24 Pull-up resistor on the supply pin of the FDC2214 (LP) applications

Pull-up resistor on the supply pin of the external

R25 Power gating to reduce current for LP applications

oscillator

Pull-down resistor on the enable pin of the external Enable gating to reduce current consumption for

R26 oscillator LP application

Series resistors on channel inputs of FDC2214 to Connecting external sensors for evaluation and

R28, R29, R31 sensors prototyping purposes

Understanding how ground shields affect sensitivity

R32, R34 Resistors to connect GND SHLD capabilities and interference

SNOU139–October 2015 FDC2214 Proximity and Capacitive Touch EVM User’s Guide

Hardware

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2.6 Additional Debugging Features

There are several hooks integrated in the design to debug the power supply rails and the I2C bus lines. To

debug the system in real time, the backchannel UART on the MSP-FET tool is connected to the UART_A1

module on the MSP430FR5969. The JTAG connector pins 12 and 14 (blue colored pins in Figure 6)

correspond to the transmit and receive signal lines respectively. The firmware implements functions to

transmit ASCII characters to a PC serial COM port host. The backchannel UART is disabled (relevant

code is commented out) in the firmware by default. To enable the UART functions, uncomment the UART

code, recompile the firmware, and load it to the MSP430 to use this functionality.

3 Firmware

3.1 FDC2214 Register Configuration

The full set of register contents used to configure the FDC2214 for this application can be found in

Table 2.

Table 2. FDC2214 Register Configuration

Address

0x08 RCOUNT_CH0 0x9C40 RCOUNT of 40000 (~16 ms conversion time)

0x09 RCOUNT_CH1 0x03E8 RCOUNT of 1000 (~400 µs conversion time)

0x0A RCOUNT_CH2 0x03E8 RCOUNT of 1000 (~400 µs conversion time)

0x10 SETTLECOUNT_CH0 0x0064 100 cycles before measurement taken

0x11 SETTLECOUNT_CH1 0x0064 100 cycles before measurement taken

0x12 SETTLECOUNT_CH2 0x0064 100 cycles before measurement taken

0x19 ERROR_CONFIG 0x0001 Report data ready flag by asserting INTB pin

Active mode enabled

Low power activation mode enabled

0x1A CONFIG 0x1E01 Reference frequency provided from CLKIN pin

INTB pin enabled

Normal sensor drive current enabled

Auto-scan conversions enabled

0x1B MUX_CONFIG 0xA20D Auto-scan sequence: Ch0, Ch1, CH2

Deglitch filter: 10 MHz

0x1E DRIVE_CURRENT_CH0 0x5000 Current drive set to 0.069 mA

0x1F DRIVE_CURRENT_CH1 0x5800 Current drive set to 0.081 mA

0x20 DRIVE_CURRENT_CH2 0x5800 Current drive set to 0.081 mA

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JTAG

13 5 7 9 11 13

4 6 8 10 12 142

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Firmware

3.2 Programming

Programming new firmware on to the MSP430 through Code Composer Studio (CCS) can be performed

in two ways:

• MSP-FET emulation tool via JTAG connector

• Spy-by-wire from an MSP430 LaunchPad

The JTAG connector (as shown in Figure 6) can be used with the MSP-FET tool for programming. Table 3

shows the signal names assigned to the pins of the JTAG connector. The JTAG is not populated by

default so the connector or 100 mil pitched headers need to be soldered on to the board to use for

programming and debugging

Figure 6. : JTAG Pin Out Diagram

Table 3. JTAG Connector Pin Assignment

Pin Signal Description

1 TDO JTAG data output

2 VCC_TOOL JTAG voltage selection from tool

3 TDI JTAG data input and TCLK input

4 VCC_TARGET Not Connected

5 TMS Signal to control the JTAG state machine

6 NC Not connected

7 TCK JTAG clock input

8 SBWTCK Spy-by-wire clock

9 GND Ground

10 NC Not connected

11 SBWTDIO Spy-by-wire bidirectional signal IO

12 A1_TX UART_A1 transmit signal (for debugging)

13 NC Not Connected

14 A1_RX UART_A1 receive signal (for debugging)

For spy-by-wire interface programming, Figure 6 indicates the yellow colored pins 8, 9, and 11 (SBWTCK,

GND, and SBWTDIO) that need to be connected to the isolation block connections on the MSP430

LaunchPad. SBWRST and SBWTST pins on the LaunchPad need to be connected to SBWTDIO and

SBWTCK test points, respectively. A common ground is required between the board and LaunchPad for

proper operation.

4 Setup

The EVM is designed as a standalone system to run off of one AA-battery. All of the processing and

object detection indications occur on the EVM. To maximize sensing distance, it is recommended to

ensure that the surface is non-conductive and not grounded. The proximity sensing algorithm detects

changes in capacitances on the order of 1fF so it is recommended to fix the position of the board or not

move the board during evaluation to limit any false detection.

SNOU139–October 2015 FDC2214 Proximity and Capacitive Touch EVM User’s Guide

Operation

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5 Operation

Once the EVM is powered ON, the entire system starts running. LED D2 and D3 near the reset button

temporarily light up indicating that an initial calibration is being performed. A two second delay is

implemented prior to the calibration process. During calibration, no target or object should be in the

intended sensing area. The LEDs turns off after calibration and the system is fully operational.

Table 4 shows the signal properties of the proximity sensor and one of the capacitive touch buttons. They

were obtained by probing either INA or INB of each channel with an oscilloscope. Figure 7 through

Figure 9 illustrate the waveforms for the proximity sensor and the capacitive touch buttons. The

recommended voltage amplitude for the oscillation waveform is between 1.2 and 1.8 VPP. Anything over

1.8 VPP is not recommended because the ESD clamps turns on and become part of the L-C tank, causing

a frequency shift. The voltage amplitude is controlled by setting the current drive value for the channel.

The sensor still functions for voltage amplitudes to a minimum of 0.4 VPP, but the SNR will degrade.

The oscilloscope probe has a capacitive loading effect when probing the sensor signal lines. A FET probe

with low capacitance should be used to minimize the effects.

Table 4. Proximity and Capacitive Touch Sensor Properties

Proximity Sensor Capacitive Touch Button 1 Capacitive Touch Button 2

Sensor Frequency 5.30 5.41 5.60

Amplitude (mV) 1.22 1.44 1.42

Settling Time (us) 3.89 3.33 3.15

Current Drive Setting (mA) 0.069 0.081 0.081

Figure 7 and Figure 8 show the oscillation and startup waveforms for the proximity sensor and capacitive

touch buttons. The yellow waveform corresponds to the proximity sensor while the green and magenta

waveform corresponds to the capacitive touch button 1 and 2. The FDC2214 may be configured for

current overdrive at the beginning of a conversion cycle, as illustrated in Figure 8. This feature can be

used to reduce the startup up time of the oscillation and hence shorten the overall conversion time.

Figure 9 shows the multiplexing operation of the FDC2214 to collect the measurements from the various

sensors. The proximity sensor’s measurements (yellow) are significantly longer than the capacitive touch

buttons measurements (green and magenta) to obtain a better signal-to-noise ratio. There is a tradeoff

between sampling rate and resolution. For example, the higher the sampling rate, the lower the resolution.

Proximity sensing applications require a high resolution to detect small changes in capacitance (on the

order of a few femtofarads) from the baseline value (no target present). Capacitive touch buttons

requirements are the opposite, in which high resolution is not necessary for a change in capacitance on

the order of picofarads. The proximity sensor has a conversion time of approximately 16 ms while the

capacitive touch buttons have a conversion time of approximately 400 µs.

Figure 7. Proximity Sensor Waveform (Yellow), Cap Touch Button 1/2 Waveforms (Green and Magenta)

8FDC2214 Proximity and Capacitive Touch EVM User’s Guide SNOU139–October 2015

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Operation

Figure 8. Proximity Startup Waveform (Yellow), Cap Touch Button 1/2 Startup Waveforms (Green,

Magenta)

Figure 9. Sensor Multiplexing – Proximity (Yellow), Cap Touch Button 1 and 2 (Green, Magenta)

SNOU139–October 2015 FDC2214 Proximity and Capacitive Touch EVM User’s Guide

Board Layout

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6 Board Layout

Figure 10. Composite Top and Bottom Layer Plot (Viewed From the Top)

Figure 11. Top Layer

10 FDC2214 Proximity and Capacitive Touch EVM User’s Guide SNOU139–October 2015

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