AT42QT2160-MMU

Features • Number of keys: up to 16 keys, and one slider (constructed from 2 to 8 keys) • Number of I/O lines: 11 (3 dedicated - configurable for input or output, 8 shared output only), PWM control for LED driving • Technology: patented spread-spectrum charge-transfer (transverse mode) • Key outline sizes: 6 mm x 6 mm or larger (panel thickness dependent); widely different sizes and shapes possible • Key spacings: 8 mm or wider, center to center (panel thickness dependent) • Slider design: 2 to 8 keys placed in sequence, same design as keys • Electrode design: two-part electrode shapes (drive-receive); wide variety of possible • • • • • • • • • • • • • layouts PCB layers required: one layer (with jumpers), two layers (no jumpers) Electrode materials: PCB, FPCB, silver or carbon on film, ITO on film Panel materials: plastic, glass, composites, painted surfaces (low particle density metallic paints possible) Adjacent metal: compatible with grounded metal immediately next to keys Panel thickness: up to 3 mm glass, 2.5 mm plastic (key size dependent) Key sensitivity: individually settable via simple commands over I2C-compatible interface Interface: I2C-compatible slave mode (100kHz) Moisture tolerance: best in class Power: 1.8 V to 5.5 V Package: 28-pin 4 x 4 mm MLF RoHS compliant Signal processing: self-calibration, auto drift compensation, noise filtering, Adjacent Key SuppressionTM technology Applications: laptop, mobile, consumer appliances, PC peripheral etc. Patents: AKS™ (patented Adjacent Key Suppression™) technology QMatrix™ (patented charge-transfer method) QSlide™ (patented charge-transfer method) (patent-pending QSlide sensing configuration) This datasheet is applicable to revision 4R0 chips only QSlide™, 16-key QMatrix™ Sensor IC AT42QT2160 • 1. Overview The A T42QT2160-MMU ( QT2160 ) is designed for use with up to 16 keys and a slider (constructed from 2 keys up to 8 keys). There are three dedicated General Purpose Input/Outputs (GPIOs) which can be used as inputs for mechanical switches etc. or as driven outputs. There are eight shared General Purpose Outputs (GPOs) (X0...X7) which are driven outputs only. There is PWM control for all GPIO/GPOs. The QMatrix™ technology employs transverse charge-transfer sensing electrode designs which can be made very compact and are easily wired. Charge is forced from an emitting electrode into the overlying panel dielectric, and then collected on a receiver electrode. This directs the charge into a sampling capacitor which is then converted directly to digital form, without the use of amplifiers. The keys are configured in a matrix format that minimizes the number of required scan lines and device pins. The key electrodes can be designed into a conventional Printed Circuit Board (PCB) or Flexible Printed Circuit Board (FPCB) as a copper pattern, or as printed conductive ink on plastic film. 9502A–AT42–07/08 2. Pinout and Pin Listing Description 2.1 Pinout Description GPIO1 I2CA1 SDA RST SCL Y1A Y0A GPIO2 GPIO3 VDD VSS X6 X7 CHANGE 1 2 3 4 5 6 7 28 27 26 25 24 23 22 21 20 19 I2CA0 Y1B Y0B VSS VDD VDD X5 QT2160 18 17 16 89 SMP VRef 15 10 11 12 13 14 X1 X3 X4 X0 X2 2.2 Pin Listing Description Table 2-1.Pin Listing Pin 1 2 3 4 5 6 7 8 9 10 11 12 13 Function GPIO2 GPIO3 Vdd Vss X6 X7 CHANGE Vref SMP X0 X1 X2 X3 I/O I/O I/O P P O O OD P O O O O O Comments General purpose input/output 2 General purpose input/output 3 Power Supply ground X matrix drive line / shared GPO X6 X matrix drive line / shared GPO X7 State change notification Supply ground Sample output. X matrix drive line / shared GPO X0 X matrix drive line / shared GPO X1 X matrix drive line / shared GPO X2 X matrix drive line / shared GPO X3 If Unused, Connect To... Leave open Leave open Leave open Leave open Leave open Leave open Leave open 2 AT42QT2160 9502A–AT42–07/08 AT42QT2160 Table 2-1.Pin Listing (continued) Pin 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Function X4 X5 Vdd Vdd Vss Y0B Y1B I2CA0 I2CA1 SDA SCL RST Y0A Y1A GPIO1 I/O O O P P P I/O I/O I I OD OD I I/O I/O I/O Comments X matrix drive line / shared GPO X4 X matrix drive line / shared GPO X5 Power Power Supply ground Y line connection Y line connection I2C-compatible address select I2C-compatible address select Serial Interface Data Serial Interface Clock Reset low; has internal 30k - 60k pullup Y line connection Y line connection General purpose input/output 1 If Unused, Connect To... Leave open Leave open Leave open Leave open Leave open or Vdd Leave open Leave open 3. Introduction The QT2160 device is a digital burst mode charge-transfer (QT) sensor designed specifically for matrix layout touch controls; it includes all signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only a few external parts are required for operation. The entire circuit can be built within a few square centimeters of single-sided PCB area. CEM-1 and FR1 punched, single-sided materials can be used for the lowest possible cost. The PCB’s rear can be mounted flush on the back of a glass or plastic panel using a conventional adhesive, such as 3M VHB two-sided adhesive acrylic film. The QT2160 employs transverse charge-transfer (QT) sensing, a technology that senses changes in electrical charge forced across two electrode elements by a pulse edge (see Figure 3-1). The QT2160 allows a wide range of key sizes and shapes to be mixed together in a single touch panel. The device uses an I2C-compatible interface to allow key data to be extracted and to permit individual key parameter setup. The command structure is designed to minimize the amount of data traffic while maximizing the amount of information conveyed. In addition to normal operating and setup functions the device can also report back actual signal strengths. 3 9502A–AT42–07/08 Figure 3-1. Field Flow Between X and Y Elements overlying panel X element Y elem ent 3.1 Keys and Slider The QT2160 is capable of a maximum of 16 keys. These can be located anywhere within an electrical grid of 8X and 2Y scan lines. A lesser number of enabled keys will cause any unused acquisition burst timeslots to be pared from the sampling sequence, to optimize acquire speed and lessen power consumption. Thus, if only 8 keys are actually enabled, only 8 timeslots are used for scanning. Additional processing can be done on the keys to form a slider. The slider will have to start at X0 and use only Y0. The slider can consist of a minimum of 2 keys and a maximum of 8 keys. 3.2 Enabling/Disabling Keys Keys can be enabled by setting a nonzero burst length. A zero burst length disables the key. 4. Hardware and Functional 4.1 Matrix Scan Sequence The circuit operates by scanning each key sequentially, key by key. Key scanning begins with location X = 0, Y = 0 (key 0). X axis keys are known as rows while Y axis keys are referred to as columns although this has no reflection on actual wiring. Keys are scanned sequentially by row, for example the sequence X0Y0 X1Y0...X7Y0, X0Y1, X1Y1... etc. Keys are also numbered from 0...15. Key 0 is located at X0Y0. Table 4-1 shows the key numbering. Table 4-1. Y0 Y1 Key Numbers X7 7 15 X6 6 14 X5 5 13 X4 4 12 X3 3 11 X2 2 10 X1 1 9 X0 0 8 Key numbers Each key is sampled in a burst of acquisition pulses whose length is determined by the Setups parameter BL (Section 4.2 on page 5); this can be set on a per-key basis. A burst is completed entirely before the next key is sampled; at the end of each burst the resulting signal is converted to digital form and processed. The burst length directly impacts key gain; each key can have a unique burst length in order to allow tailoring of key sensitivity on a key-by-key basis. 4 AT42QT2160 9502A–AT42–07/08 AT42QT2160 4.2 Burst Paring Keys that are disabled by setting their burst length to zero have their bursts removed from the scan sequence to save scan time and thus power. The QT2160 operates on a fixed 16 ms cycle and will go to sleep after all acquisitions and processing is done till the next 16ms cycle starts. As a consequence, the fewer keys, the less power is consumed. 4.3 Cs Sample Capacitor Operation Cs capacitors (Cs0...Cs1) absorb charge from the key electrodes on the rising edge of each X pulse. On each falling edge of X, the Y matrix line is clamped to ground to allow the electrode and wiring charges to neutralize in preparation for the next pulse. With each X pulse charge accumulates on Cs causing a staircase increase in its differential voltage. After the burst completes, the device clamps the Y line to ground causing the opposite terminal to go negative. The charge on Cs is then measured using an external resistor to ramp the negative terminal upwards until a zero crossing is achieved. The time required to zero cross becomes the measurement result. The Cs capacitors should be connected as shown in Figure 4-8 on page 15. The value of these capacitors is not critical but 4.7 nF is recommended for most cases. They should be 10 percent X7R ceramic. If the transverse capacitive coupling from X to Y is large enough the voltage on a Cs capacitor can saturate, destroying gain. In such cases the burst length should be reduced and/or the Cs value increased. See Section 4.4. If a Y line is not used its corresponding Cs capacitor may be omitted and the pins left floating. 4.4 Sample Capacitor Saturation Cs voltage saturation at a pin YnB is shown in Figure 4-1. Saturation begins to occur when the voltage at a YnB pin becomes more negative than -0.25V at the end of the burst. This nonlinearity is caused by excessive voltage accumulation on Cs inducing conduction in the pin protection diodes. This badly saturated signal destroys key gain and introduces a strong thermal coefficient which can cause phantom detection. The cause of this is either from the burst length being too long, the Cs value being too small, or the X-Y transfer coupling being too large. Solutions include loosening up the key structure interleaving, more separation of the X and Y lines on the PCB, increasing Cs, and decreasing the burst length. Increasing Cs will make the part slower; decreasing burst length will make it less sensitive. A better PCB layout and a looser key structure (up to a point) have no negative effects. Cs voltages should be observed on an oscilloscope with the matrix layer bonded to the panel material; if the Rs side of any Cs ramps more negative than -0.25 volts during any burst (not counting overshoot spikes which are probe artifacts), there is a potential saturation problem. Figure 4-2 shows a defective waveform similar to that of Figure 4-1, but in this case the distortion is caused by excessive stray capacitance coupling from the Y line to AC ground; for example, from running too near and too far alongside a ground trace, ground plane, or other traces. The excess coupling causes the charge-transfer effect to dissipate a significant portion of the received charge from a key into the stray capacitance. This phenomenon is more subtle; it can be best detected by increasing BL to a high count and watching what the waveform does as it descends towards and below -0.25V. The waveform will appear deceptively straight, but it will slowly start to flatten even before the -0.25V level is reached. A correct waveform is shown in Figure 4-3. Note that the bottom edge of the bottom trace is substantially straight (ignoring the downward spikes). 5 9502A–AT42–07/08 Unlike other QT circuits, the Cs capacitor values on QT2160 devices have no effect on conversion gain. However, they do affect conversion time. Unused Y lines should be left open. Figure 4-1. VCs – Nonlinear During Burst (Burst too long, or Cs too small, or X-Y transcapacitance too large) X Driv e YnB Figure 4-2. VCs – Poor Gain, Nonlinear During Burst (Excess capacitance from Y line to Gnd) X Driv e YnB Figure 4-3. VCs – Correct X Driv e YnB 6 AT42QT2160 9502A–AT42–07/08 AT42QT2160 Figure 4-4. Drive Pulse Roll-off and Dwell Time X drive Lost charge due to inadequate settling before end of dwell time Dwell time Y gate Note: The Dwell time is a minimum of ~250ns - see Section 4.7 4.5 Sample Resistors The sample resistors (Rs0...Rs1) are used to perform single-slope ADC conversion of the acquired charge on each Cs capacitor. These resistors directly control acquisition gain; larger values of Rs will proportionately increase signal gain. For most applications Rs should be 1MΩ. Unused Y lines do not require an Rs resistor. 4.6 Signal Levels The signal values should normally be in the range of 200 to 750 counts with properly designed key shapes and values of Rs. However, long adjacent runs of X and Y lines can also artificially boost the signal values, and induce signal saturation; this is to be avoided. The X-to-Y coupling should come mostly from intra-key electrode coupling, not from stray X-to-Y trace coupling. The signal swing from the smallest finger touch should preferably exceed 8 counts, with 12 being a reasonable target. The signal threshold setting (NTHR) should be set to a value guaranteed to be less than the signal swing caused by the smallest touch. Increasing the burst length (BL) parameter will increase the signal strengths, as will increasing the sampling resistor (Rs) values. 4.7 Matrix Series Resistors The X and Y matrix scan lines can use series resistors (Rx0...Rx7 and Ry0...Ry1 respectively) for improved EMC performance (Figure 4-8 on page 15). X drive lines require Rx in most cases to reduce edge rates and thus reduce RF emissions. Values range from 1 kΩ to 20 kΩ, typically 1 kΩ. Y lines need Ry to reduce EMC susceptibility problems and in some extreme cases, ESD. Typical Y values are about 1 kΩ. Y resistors act to reduce noise susceptibility problems by forming a natural low-pass filter with the Cs capacitors. It is essential that the Rx and Ry resistors and Cs capacitors be placed very close to the chip. Placing these parts more than a few millimeters away opens the circuit up to high frequency interference problems (above 20 MHz) as the trace lengths between the components and the chip start to act as RF antennae. The upper limits of Rx and Ry are reached when the signal level and hence key sensitivity are clearly reduced. The limits of Rx and Ry will depend on key geometry and stray capacitance, and thus an oscilloscope is required to determine optimum values of both. 7 9502A–AT42–07/08 Dwell time is the duration in which charge coupled from X to Y is captured (Figure 4-4 on page 7). Increasing Rx values will cause the leading edge of the X pulses to increasingly roll off, causing the loss of captured charge (and hence loss of signal strength) from the keys. The dwell time is a minimum of 250 ns. If the X pulses have not settled within 250 ns, key gain will be reduced; if this happens, either the stray capacitance on the X line(s) should be reduced (by a layout change, for example by reducing X line exposure to nearby ground planes or traces), or, the Rx resistor needs to be reduced in value (or a combination of both approaches). One way to determine X line settling time is to monitor the fields using a patch of metal foil or a small coin over the key (Figure 4-5). Only one key along a particular X line needs to be observed, 250 ns dwell time should exceed the observed 95 percent settling of the X-pulse by 25 percent or more. In almost all cases, Ry should be set equal to Rx, which will ensure that the charge on the Y line is fully captured into the Cs capacitor. Figure 4-5. Probing X-Drive Waveforms With a Coin 4.8 Key Design Circuits can be constructed out of a variety of materials including conventional FR-4, Flexible Printed Circuit Boards (FPCB), silver silk-screened on PET plastic film, and even inexpensive punched single-sided CEM-1 and FR-2. The actual internal pattern style is not as important as the need to achieve regular X and Y widths and spacings of sufficient size to cover the desired graphical key area or a little bit more; ~3mm oversize is acceptable in most cases, since the key’s electric fields drop off near the edges anyway. The overall key size can range from 6mm x 6mm up to 100mm x 100mm but these are not hard limits. The keys can be any shape including round, rectangular, square, etc. The internal pattern can be interdigitated as shown in Figure 4-6. For small, dense keypads, electrodes such as shown in the lower half of Figure 4-6 can be used. Where the panels are thin (under 2 mm thick) the electrode density can be quite high. 8 AT42QT2160 9502A–AT42–07/08 AT42QT2160 For better surface moisture suppression, the outer perimeter of X should be as wide as possible, and there should be no ground planes near the keys. The variable “T" in this drawing represents the total thickness of all materials that the keys must penetrate. Figure 4-6. Recommended Key Structure Y0 X0 Y1 Note: “T" should ideally be similar to the complete thickness the fields need to penetrate to the touch surface. Smaller dimensions will also work but will give less signal strength. If in doubt, make the pattern coarser. The lower figure shows a simpler structure used for compact key layouts, for example for mobile phones. A layout with a common X drive and two receive electrodes is depicted 4.9 4.9.1 Setting the Slider Introduction Groups of keys can be configured as a slider, in addition to their use as keys. The slider uses the Y0 line of the matrix and must start at X0, with the keys placed in consecutive numerical order. The slider can take up a programmable number of keys on the Y0 line. The remaining keys on that Y line behave as normal. Positional data is calculated in a customizable range of 2 bits (0-3) to 8 bits (0-255). Geometric constraints may mean that the data will not reach the full range. Thinner dielectric or the use of more keys in a slider will increase the data range towards the ends. Stability of the reported position will be dependent on the amount of signal on the slider keys. Running at higher resolutions, with a thick panel might produce a fluctuating reported position. 9 9502A–AT42–07/08 Key sizes should be in the 5-7mm range when used in the slider to get the best linearity. The slider should be made up of however many of these elements are required to fit their dimensions. The slider will be treated as an object in the Adjacent Key Suppression (AKS) groupings. The keys in the slider would normally be set to the same burst length and threshold, although adjustments can be made in these at the expense of linearity. 4.9.2 AKS Technology and the Slider There can be up to three AKS groups, implemented so that only one key in each group may be reported as being touched at any one time. The AKS technique will lock onto the dominant key, and until this key is released, other keys in the group will not be reported as in detection. This allows a user to slide a finger across multiple keys with only the dominant key reporting touch. Each key may be in one of the groups 1...3, or in group 0 meaning that it is not AKS enabled. Keys in the slider are not able to use AKS technique against each other. This is necessary to enable smooth scrolling. Multiple keys within the slider can be in detect at the same time, regardless of the AKS settings. The AKS technique will, however, work against keys outside the object or within another object. For example, if a slider is in the same AKS group as keys, then touching anywhere on the slider will cause the AKS technique to suppress the keys. Similarly touching the keys first will suppress the slider. Note: For normal operation all keys in the slider should be placed in the same AKS group. 4.10 4.10.1 PCB Layout, Construction Overview It is best to place the chip near the touch keys on the same PCB so as to reduce X and Y trace lengths, thereby reducing the chances for EMC problems. Long connection traces act as RF antennae. The Y (receive) lines are much more susceptible to noise pickup than the X (drive) lines. Even more importantly, all signal related discrete parts (resistors and capacitors) should be very close to the body of the chip. Wiring between the chip and the various resistors and capacitors should be as short and direct as possible to suppress noise pickup. Ground planes, if used, should be placed under or around the QT chip itself and the associated resistors and capacitors in the circuit, under or around the power supply, and back to a connector. Ground planes can be used to shield against radiated noise, but at the expense of a reduction in sensitivity as described previously. Note: When using ground planes/floods, parasitic capacitance on Y lines can lead to reduced chargetransfer efficiency. For noise suppression, ground planes/floods can be beneficial around and between keys on the touch side of the PCB. However, it is advisable to route Y lines on the PCB layer furthest away from the plane/flood, to reduce parasitic capacitance. Cross-hatched ground patterns can act as effective shields, while helping to reduce parasitic capacitance. Ground planes/floods around the chip are generally acceptable, taking into account the same considerations as for the Y line parasitics. 4.10.2 LED Traces and Other Switching Signals Digital switching signals near the Y lines will induce transients into the acquired signals, deteriorating the SNR performance of the device. Such signals should be routed away from the Y lines, or the design should be such that these lines are not switched during the course of signal acquisition (bursts). 10 AT42QT2160 9502A–AT42–07/08 AT42QT2160 LED terminals which are multiplexed or switched into a floating state and which are within or physically very near a key structure (even if on another nearby PCB) should be bypassed to either Vss or Vdd with at least a 10nF capacitor to suppress capacitive coupling effects which can induce false signal shifts. The bypass capacitor does not need to be next to the LED, in fact it can be quite distant. The bypass capacitor is noncritical and can be of any type. LED terminals which are constantly connected to Vss or Vdd do not need further bypassing. 4.10.3 Tracks The central pad on the underside of the chip should be connected to ground. Do not run any tracks underneath the body of the chip, only ground. Figure 4-7. Position of Tracks Example of good tracking Example of bad tracking 4.10.4 PCB Cleanliness All capacitive sensors should be treated as highly sensitive circuits which can be influenced by stray conductive leakage paths. QT devices have a basic resolution in the femtofarad range; in this region, there is no such thing as “clean flux". Flux absorbs moisture and becomes conductive between solder joints, causing signal drift and resultant false detections or transient losses of sensitivity or instability. Conformal coatings will trap in existing amounts of moisture which will then become highly temperature sensitive. The designer should specify ultrasonic cleaning as part of the manufacturing process, and in cases where a high level of humidity is anticipated, the use of conformal coatings after cleaning to keep out moisture. 4.11 Power Supply Considerations See Section 10.2 on page 43 for the Vdd range and short-term power supply fluctuations. If the power supply fluctuates slowly with temperature, the device will track and compensate for these changes automatically with only minor changes in sensitivity. If the supply voltage drifts or shifts quickly, the drift compensation mechanism will not be able to keep up, causing sensitivity anomalies or false detections. As the device uses the power supply itself as an analog reference, the power should be very clean and come from a separate regulator. A standard inexpensive Low Dropout (LDO) type regulator should be used that is not also used to power other loads such as LEDs, relays, or other high current devices. Load shifts on the output of the LDO can cause Vdd to fluctuate enough to cause false detection or sensitivity shifts. 11 9502A–AT42–07/08 Caution: A regulator IC shared with other logic devices can result in erratic operation and is not advised. A regulator can be shared among two or more QT devices on one board. Refer to page 15 for suggested regulator manufacturers. A single ceramic 0.1uF bypass capacitor, with short traces, should be placed very close to supply pins 3 and 4 of the IC. Failure to do so can result in device oscillation, high current consumption, erratic operation etc. Pins 16 and 17 do not require bypassing if the traces between these pins and power traces are short. 4.12 Startup/Calibration Times The device requires initialization times of approximately 70ms. The CHANGE line will go low and calibration will start (takes 15 matrix scans), after this start up period is over. 4.13 Calibration Calibration does not occur periodically. Keys are only calibrated on power-up and when: • Enabled AND – held in detect for too long. The negative recalibration delay (NRD) period is specified by the user OR – the signal delta value is greater than the positive threshold value, defined as reference value plus three-quarters of the negative threshold OR – the user issues a recalibrate command An interrupt on the CHANGE pin occurs when there is a change in the key status bytes. An interrupt will occur on calibration only if at least one of the keys or objects was in detect as recalibration will then cause a status change. 4.14 Reset Input The RST pin can be used to reset the device to simulate a power-down cycle, in order to bring the device up into a known state should communications with the device be lost. The pin is active low, and a low pulse lasting at least 10µs must be applied to this pin to cause a reset. If an external hardware reset is not used, the reset pin may be connected to Vdd. 12 AT42QT2160 9502A–AT42–07/08 AT42QT2160 4.15 Spread Spectrum Acquisitions QT2160 uses spread-spectrum burst modulation. This has the effect of drastically reducing the possibility of EMI effects on the sensor keys, while simultaneously spreading RF emissions. This feature is hard-wired into the device and cannot be disabled or modified. Spread spectrum is configured as a frequency chirp over a wide range of frequencies for robust operation. 4.16 Detection Integrator See also Section 4.2 on page 5. 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 stayed there for a number of acquisitions. When this counter reaches a preset limit the key is finally declared to be touched. For example, if the limit value is 10, then the device has to exceed its threshold and stay there for 10 acquisitions in succession without going below the threshold level, before the key is declared to be touched. If on any acquisition the signal is not seen to exceed the threshold level, the counter is cleared and the process has to start from the beginning. 4.17 Sleep The device operates on a fixed 16ms cycle time basis. The device will perform a set of measurements and then sleep for the rest of the cycle to conserve power. There are two user-configurable sleep modes; Low Power (LP) mode and SLEEP mode. The LP setting (see Section 4.2 on page 5) is used for conserving power when there are no touches and is set to be a long time period. This will determine how often the device wakes up to do drift compensation. It also determines the maximum response time to the first touch after inactivity. When a valid touch is registered, the device enters minimum cycle time (16ms) for a faster response to key touch and object operation. The device will stay in this mode if it continues to see keys being touched and released. There is a user-selectable inactivity timeout i.e. the awake timeout. The measurement period needs to be shorter than the 16ms fixed cycle time for optimum operation. If the measurement time exceeds the 16ms fixed cycle time, a CYCLE OVERRUN bit is set in the general status register. The QT2160 will still operate if the 16ms fixed cycle time is exceeded, but the timing for the timed parameters, e.g. drift compensation negative recalibration time out etc. will slightly change. A low power setting of zero causes the device to enter an ultra-low power mode (SLEEP), where no measurements are carried out. SLEEP mode also stops the internal watchdog timer, so that the part is totally dormant, and current drain is