Rainbow Electronics AT42QT1040 User Manual

Features

• Number of Keys:

–Up to 4

• Discrete Outputs:

– 4 discrete outputs indicating individual key touch

• Techn ology:

– Patented spread-spectrum charge-transfer (direct mode)

• Electrode Design:

– Simple self-capacitance style (refer to the Touch Sensors Design Guide)

• Electrode Materials:

– Etched copper, silver, carbon, Indium Tin Oxide (ITO)

• Electrode Substrates:

– PCB, FPCB, plastic films, glass

• Panel Materials:

– Plastic, glass, composites, painted surfaces (low particle density metallic paints

possible)

• Panel Thickness:

– Up to 10 mm glass, 5 mm plastic (electrode size dependent)

• Key Sensitivity:

– Fixed key threshold, sensitivity adjusted via sample capacitor value

• Adjacent Key Suppression

– Patented Adjacent Key Suppression™ (AKS™) technology to enable accurate key

detection

• Interface:

– Pin-per-key outputs, plus debug mode to observe sensor signals

• Moisture Tolerance:

–Good

• Power:

– 1.8V ~ 5.5V

• Package:

– 20-pin 3 x 3 mm VQFN RoHS compliant

• Signal Processing:

– Self-calibration, auto drift compensation, noise filtering, Adjacent Key

Suppression technology

• Applications:

– Mobile, consumer, white goods, toys, kiosks, POS, and so on

™

QTouch™ 4-key Sensor IC

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1. Pinout and Schematic

N/C

VSS

VDD

N/C

SNS2

SNSK1

SNS1

SNSK0

SNS0

OUT0

OUT1

QT1040

OUT3

OUT2

SNSK3

SNS3

N/C

SNSK2

N/C

1.1 Pinout Configuration

Table 1-1. Pin Listing

Pin Name Type Function Notes If Unused...

1 SNS2 I/O Sense pin To Cs2 Leave open

2 SNSK1 I/O Sense pin To Cs1 + key Leave open

3SNS1I/O

Sense pin and

option detect

To Cs1 and/or option resistor Connect to option resistor*

4 SNSK0 I/O Sense pin To Cs0 + key Leave open

5SNS0I/O

Sense pin and

option detect

To Cs0 and/or option resistor Connect to option resistor*

6N/C– – –

7N/C– – –

8 Vss P Supply ground –

9 Vdd P Power –

10 N/C – – –

11 OUT0

12 OUT1

13 OUT3

14 OUT2

OD Out 0 Alternative function: Debug CLK Leave open

OD Out 1 Alternative function: Debug DATA Leave open

OD Out 3 Leave open

OD Out 2 Leave open

15 SNSK3 I/O Sense pin To Cs3 + key Leave open

16 SNS3 I/O Sense pin To Cs3 Leave open

17 N/C – – –

18 N/C – – –

19 N/C – – –

20 SNSK2 I/O Sense pin To Cs2 + key Leave open

* Option resistor should always be fitted even if channel is unused and Cs capacitor is not fixed.

I/O CMOS input and output OD CMOS open drain output P Ground or power

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1.2 Schematic

SLOW

FAST

OFF

LED3

LED2

LED0

LED1

VDD

VSS

N/C

OUT2

SNSK3

SNSK2

SNSK1

SNSK0

N/C

OUT1

OUT0

SNS3

SNS1

N/C

OUT3

SNS0

SNS2

SPEED SELECT

AKS SELECT

NOTES:

1) The central pad on the underside of the VQFN chip is a Vss pin and should be connected to ground. Do not put any other tracks underneath the body of the chip.

2) It is important to place all Cs and Rs components physically near to the chip.

Add a 100 nF capacitor close to pin 9.

QT1040

Creg Creg

VREG

Follow regulator manufacturer's recommended values for input and output bypass capacitors (Creg).

Key0

Key1

Key2

Key3

VUNREG

GND

Cs0

Cs1

Cs2

Cs3

RL0

RL1

RL2

RL3

RAKS

RFS

Rs0

Rs1

Rs2

Rs3

Example use of output pins

Figure 1-1. Typical Circuit

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Suggested regulator manufacturers:

• Torex (XC6215 series)

• Seiko (S817 series)

• BCDSemi (AP2121 series)

Re Figure 1-1 check the following sections for component values:

• Section3.1 on page6: Cs capacitors (Cs0 – Cs3)

• Section3.5 on page7: Voltage levels

• Section3.3 on page6: LED traces

2. Overview of the AT42QT1040

2.1 Introduction

The AT42QT1040 (QT1040) is a digital burst mode charge-transfer (QT™) capacitive sensor driver designed for touch-key applications. The device can sense from one to four keys; one to three keys can be disabled by not installing their respective sense capacitors. Any of the four channels can be disabled in this way.

The device includes all signal processing functions necessary to provide stable sensing under a wide variety of changing conditions, and the outputs are fully debounced. Only a few external parts are required for operation.

The QT1040 modulates its bursts in a spread-spectrum fashion in order to heavily suppress the effects of external noise, and to suppress RF emissions.

2.2 Signal Processing

2.2.1 Detect Threshold

The internal signal threshold level is fixed at 10 counts of change with respect to the internal reference level. This in turn adjusts itself slowly in accordance with the drift compensation mechanism. See Section 3.1 on page 6 for details on how to adjust each key’s sensitivity.

When going out of detect there is a hysteresis element to the detection. The signal threshold must drop below 8 counts of change with respect to the internal reference level to register as untouched.

2.2.2 Detection Integrator

The device features a detection integration mechanism, which acts to confirm a detection in a robust fashion. A per-key counter is incremented each time the key has exceeded its threshold, and a key is only finally declared to be touched when this counter reaches a fixed limit of 5. In other words, the device has to exceed its threshold, and stay there for 5 acquisitions in succession without going below the threshold level, before the key is declared to be touched.

2.2.3 Burst Length Limitations

Burst length is the number of times the charge transfer process is performed on a given channel; that is, the number of pulses it takes to measure the key’s capacitance.

The maximum burst length is 2048 pulses. The recommended design is to use a capacitor that gives a signal of <1000 pulses. Longer bursts take more time and use more power.

Note that the keys are independent of each other. It is therefore possible, for example, to have a signal of 100 on one key and a signal of 1000 on another.

Refer to Application Note QTAN0002, Secrets of a Successful QTouch™ Design (downloadable from the Atmel hence determine the burst length. Refer also to the Touch Sensors Design Guide.

2.2.4 Adjacent Key Suppression Technology

The device includes Atmel’s patented Adjacent Key Suppression (AKS) technology, to allow the use of tightly spaced keys on a keypad with no loss of selectability by the user.

website), for more information on using a scope to measure the pulses and

There is one global AKS group, implemented so that only one key in the group may be reported as being touched at any one time.

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The use of AKS is selected by connecting a 1 M resisitor between Vdd and the SNSK0 pin (see Section 4.1 on page 8 for more information). When AKS is disabled, any combinations of keys can enter detect.

2.2.5 Auto Drift Compensation

Signal drift can occur because of changes in Cx and Cs over time. It is crucial that drift be compensated for, otherwise false detections, non-detections, and sensitivity shifts will follow.

Drift compensation is performed by making the reference level track the raw signal at a slow rate, but only while there is no detection in effect. The rate of adjustment must be performed slowly otherwise legitimate detections could be ignored.

Once an object is sensed and a key is in detect, the drift compensation mechanism ceases, since the signal is legitimately high and should not therefore cause the reference level to change.

The QT1040's drift compensation is “asymmetric”: the reference level drift-compensates in one direction faster than it does in the other. Specifically, it compensates faster for decreasing (towards touch) signals than for increasing (away from touch) signals. The reason for this difference in compensation rates is that increasing signals should not be compensated for quickly, since a nearby finger could be compensated for partially or entirely before even approaching the sense electrode. However, decreasing signals need to be compensated for more quickly. For example, an obstruction over the sense pad (for which the sensor has already made full allowance) could suddenly be removed, leaving the sensor with an artificially elevated reference level and thus become insensitive to touch. In this latter case, the sensor will compensate for the object's removal very quickly, usually in only a few seconds.

AT42QT1040

Negative drift (that is, towards touch) occurs at a rate of ~3 seconds, while positive drift occurs at a rate of ~1 second.

Drifting only occurs when no keys are in detect state.

2.2.6 Response Time

The QT1040's response time is highly dependent on run mode and burst length, which in turn is dependent on Cs and Cx. With increasing Cs, response time slows, while increasing levels of Cx reduce response time. The response time will also be slower in slow mode due to a longer time between burst measurements. This mode offers an increased detection latency in favor of reduced average current consumption.

2.2.7 Spread Spectrum

The QT1040 modulates its internal oscillator by ±7.5 percent during the measurement burst. This spreads the generated noise over a wider band reducing emission levels. This also reduces susceptibility since there is no longer a single fundamental burst frequency.

2.2.8 Max On-duration

If an object or material obstructs the sense pad, the signal may rise enough to create a detection, preventing further operation. To prevent this, the sensor includes a timer known as the Max On-duration feature which monitors detections. If a detection exceeds the timer setting, the sensor performs an automatic recalibration. Max On-duration is set to ~30s.

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3. Wiring and Parts

3.1 Cs Sample Capacitors

Cs0 – Cs3 are the charge sensing sample capacitors; normally they are identical in nominal value. The optimal Cs values depend on the corresponding keys electrode design, the thickness of the panel and its dielectric constant. Thicker panels require larger values of Cs. Values can be in the range 2.2 nF (for faster operation) to 22 nF (for best sensitivity); typical values are 4.7 nF to 10 nF.

The value of Cs should be chosen such that a light touch on a key mounted in a production unit or a prototype panel causes a reliable detection. The chosen Cs value should never be so large that the key signals exceed ~1000, as reported by the chip in the debug data.

The Cs capacitors must be X7R or PPS film type, for stability. For consistent sensitivity, they should have a 10 percent tolerance. Twenty percent tolerance may cause small differences in sensitivity from key to key and unit to unit. If a key is not used, the Cs capacitor may be omitted.

3.2 Rs Resistors

The series resistors Rs0 – Rs3 are inline with the electrode connections (close to the QT1040 chip) and are used to limit electrostatic discharge (ESD) currents and to suppress radio frequency (RF) interference. A typical value is 4.7 k, but up to 20 k can be used if it is found to be of benefit.

Although these resistors may be omitted, the device may become susceptible to external noise or radio frequency interference (RFI). For details on how to select these resistors refer to Application Note QTAN0002, Secrets of a Successful QTouch Design Guide, both downloadable from the Touch Technology area of Atmel’s website, www.atmel.com.

™

Design, and the Touch Sensors

3.3 LED Traces and Other Switching Signals

For advice on LEDs and nearby traces, refer to Application Note QTAN0002, Secrets of a Successful QTouch

the Touch Technology area of Atmel’s website, www.atmel.com.

™

Design, and the Touch Sensors Design Guide, both downloadable from

3.4 PCB Cleanliness

Modern no-clean flux is generally compatible with capacitive sensing circuits.

CAUTION: If a PCB is reworked in any way, it is almost guaranteed that the behavior

of the no-clean flux will change. This can mean that the flux changes from an inert material to one that can absorb moisture and dramatically affect capacitive measurements due to additional leakage currents. If so, the circuit can become erratic and exhibit poor environmental stability.

If a PCB is reworked in any way, clean it thoroughly to remove all traces of the flux residue around the capacitive sensor components. Dry it thoroughly before any further testing is conducted.

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+ 12 hidden pages