Abstract: The QT1101 is a QTouch charge transfer (QT) device, a complete digital controller capable of detecting signals when approached or touched up to 10 independent buttons. It can be widely used in MP3 players, mobile phones, PC peripherals, television control, fixed-point devices, and remote control. This paper details the principle of the QT1101 and its application in touchscreens. Keywords: QT1101; touch sensor; charge transfer; touchscreen 1 Introduction Traditional human-computer interaction is achieved through keyboards or mice, which is relatively slow and requires operators to have certain professional knowledge, significantly diminishing the purpose of information exchange. Therefore, a non-keyboard, non-mouse communication method is needed, among which touchscreen technology has the most application value. Touchscreen technology is a new human-computer interaction technology that emerged in the early 1990s, mainly divided into resistive, capacitive, infrared, and surface acoustic wave types. The basic principle is that when a finger or other object touches the screen, the touchscreen controller detects the touch position and sends it to the CPU through an interface to determine the input information. Touchscreens have advantages such as durability, fast response, space saving, flexible operation, and ease of use, and their application range is very wide. They are mainly used for multimedia public information queries, such as business queries in telecommunications bureaus and banks, information queries in airports, train stations, hotels, tourist attractions, and shopping malls and supermarkets; secondly, they can also be applied to leadership offices, industrial control, military command, video games, multimedia teaching, etc. In addition, touchscreens are entering homes, such as touchscreen telephones and smart computer telephones. This paper proposes a touchscreen design method based on the QT1101 touch sensor and describes its basic principles in detail. [b]2 Touchscreen Design[/b] The design principle of the touchscreen is very simple. Its main components are the QT1101 and the microcontroller. The QT1101 completes the detection and confirmation of the buttons, and then sends the detected button information to the microcontroller. The microcontroller processes the received data accordingly. The basic block diagram is shown in Figure 1. The core part of this design is the touch sensor QT1101, so understanding its working principle is very helpful for the design. [b]3 QT1101 Basic Principles[/b] The QT1101 charge transfer device is a standalone, patented digital controller capable of detecting signals from the proximity or touch of up to 10 individual buttons. The QT1101's electrodes emit independent sensing fields that penetrate any dielectric material (such as glass or plastic) and feature continuous self-calibration, requiring no adjustments. It is designed for applications such as control panels, control devices, gaming devices, lighting controls, or any human-machine interface with mechanical switches or buttons. It can also be used in material sensing and control applications. Each channel can operate independently of the others, and by adjusting the corresponding external capacitor Cs, each channel can be tuned to a specific sensitivity level, achieving high flexibility. Quantum's patented Adjacent Key Suppression (AKS) technology suppresses touches from weak signals, allowing only one primary button touch to be detected, preventing interference from fingers covering other adjacent empty keys. This feature is especially important when using small control panels, as fingers may touch more than one button. Spread spectrum burst pulse technology provides excellent noise suppression. The device's SYNC/LP pin can be connected to similar devices or external information synchronization, or LP mode can be selected to save power. The QTll01 features the following characteristics: Requires 450 ms calibration initialization after reset or restart; Patented charge transfer (QT) design; 10 independent QT sensing channels (buttons); Single voltage supply from 2.8 V to 5.5 V; Typical current consumption of 40 μA at 3 V in 360 ms low-power mode (LP); 100% fully automatic, no adjustment required; Serial 1-wire or 2-wire interface with automatic baud rate, completely jitter-free; Spread spectrum burst pulse mode for noise suppression; Pins effectively suppress low-frequency noise; "Fast mode" for sliding applications; 32-QFN or 48-SSOP lead-free package. Figure 2 shows the pinout of the QTll01 (using the 32-QFN package as an example), and Table 1 lists the functions of each pin. 3.1 QTll01 Pin Description 3.1.1 DETECT Pin DETECT is the logical OR of the 10 buttons, which can be used to wake up a battery-powered product when touched. The output polarity and driving mode of this pin are shown in Table 2. 3.1.2 CHANGE Pin This pin informs the host that a change in touch state has been detected (e.g., a button is touched or deactivated), and the host then reads the new button state through the serial interface. When a button state changes, CHANGE goes low, preventing the transmission of duplicate data. If CHANGE is not used, the host will continue to query QT1l01 even without a touch change. When a button press is detected, CHANGE goes low and remains low until the host queries the button through the serial port. Then CHANGE is released and goes high, remaining high until the next change in button state (any button goes high or low). CHANGE is open-drain and requires a 100 kΩ pull-up resistor connected to VDD. 3.1.3 SYNC/LP Pin As shown in Table 3, the function of the SYNC/LP pin is determined by whether SL_0 and SL_1 are connected to VDD or VSS. (1) Sync Mode Sync mode allows the burst pulse to be synchronized with an external signal source (such as the main frequency 50/60 Hz), suppressing interference. It can also synchronize two QT devices working in close proximity, so that when two or more buttons of two devices are close to each other, they will not interfere with each other. The SYNC input is triggered by a positive pulse. If the SYNC input is not changed, the device will work at the self-excited frequency after 150 ms. When there is a trigger pulse for SYNC, the device will activate three burst pulses in the order of ABC: Burst pulse A: buttons O, 1, 4, 5 Burst pulse B: buttons 2, 3, 6, 7 Burst pulse C: buttons 8, 9 (2) Low Power (LP) Mode In this mode, the power consumption device enters a slow mode with lower power consumption. The three nominal response times are: 120ms, 200ms and 360ms. When a positive pulse is detected on the SYNC/LP pin, the device enters LP mode. Once an LP pulse is detected, the device enters and maintains this microamplitude mode until a touch is sensed and confirmed. At this point, it automatically switches to normal (full-speed) mode. The typical response time is less than 40 ms (related to the burst pulse duration). When SYNC/LP remains high or another LP pulse is received, the device re-enters LP mode. The response time is determined by optional resistors SL_1 and SL_2; a shorter response time results in lower power consumption. The SYNC/LP pulse duration should be greater than 150 μs. If the SYNC/LP pin is permanently set high, the device enters normal mode upon a button touch and returns to low-current mode after the button status is detected and read by the host. If SYNC/LP is permanently set low, the device will maintain normal full-speed mode operation. 3.1.4 The circuit connection diagram of QT1101 in full-select mode on OSC pin is shown in Figure 3. The internal oscillator of QT1101 is implemented through an external network connected to the OSC and SS pins. The recommended values ​​are determined by the nominal operating voltage and spread spectrum mode, as shown in Tables 4 and 5. If spread spectrum is not used, only resistor Rb1 is used, and capacitor CSS is not required. The SS pin is connected to VSS through a 100 kΩ resistor. 3.2 Introduction to special functions of QT1101 3.2.1 AKSTM function The adjacent key suppression (AKSTM) function of QT1101 has two modes, which can be controlled by an optional resistor. AKS can be disabled, allowing any combination of keys to be active at the same time. When AKS is working, the two modes are shown in Tables 6 and 7. Global: The AKS function works for all 10 keys at the same time, that is, only one key is active at any given time. Grouping: The AKS function operates on three groups of buttons (0-1-4-5, 2-3-6-7, and 8-9), meaning that a maximum of three buttons can be active at any given time. In grouping mode, buttons in one group do not interact with buttons in other groups via AKS. Note: AKS can only be disabled in fast detection mode. 3.2.2 MOD_0 and MOD_1 Inputs In full selection mode, MOD_0 and MOD_1 are used to set the recalibration timeout for the maximum duration. If a button is pressed for too long, exceeding the set time, that specific button will be automatically recalibrated. The settings are 10 s, 60 s, and infinity. The maximum duration feature operates on a single button basis; when one button is pressed continuously, its recalibration has no effect on other buttons. The logic combination of the MOD optional pins sets the timeout delay, as listed in Table 8. In simplified mode, the maximum duration is fixed at 60 s. 3.2.3 Fast Detection Function In many devices, rapid touch sensing is required; examples include scrolling sliders or disconnect buttons. This allows the device to operate in fast detection mode, which typically requires a response time of less than 15 ms. In LP mode, fast detection does not accelerate the initial delay (typically up to 360 ms depending on optional settings), but once a button is detected, the device is forced back to normal speed mode. The device remains in fast mode until another LP pulse arrives. In slider applications, it is best to avoid using AKS when the button is active. The processing time for a button release is the same in both normal and fast modes, requiring six timing confirmations to turn off a button. Fast detection mode can be activated using the combinations shown in Tables 6 and 7. 3.2.4 Simplified Mode Simplified mode does not require a large number of optional resistors. This mode is set by connecting a resistor between pins SNS6K and SNS7. In this mode, only one option is available—AKS enabled or disabled. When AKS is disabled, fast detection mode is available; when AKS is enabled, fast detection mode is not available. Furthermore, AKS in this mode operates globally, meaning it applies to all function buttons. Other optional features are as follows: DETECT pin: push-pull, active high; SYNC/LP: LP mode, 200 ms response time; Maximum duration: 60 s. 3.3 Serial Communication of QTll101 The QTll101 has two serial communication modes: 1-wire mode (1W) and 2-wire mode (2W). The basic principles of each part are introduced below. 3.3.1 Serial 1-Wire (1W) Interface The serial 1W interface is an RS-232-based automatic baud rate serial asynchronous interface. It only requires one connection between the host MCU and QTll101. The serial data is short and easy to understand. The 1W bus is bidirectional, and its basic timing for serial operation is shown in Figure 4(a). During transmission, the host can drive the 1W bus through an open-drain or push-pull driver. However, if the host uses a push-pull driver, the 1W bus must be released after processing the stop bit to avoid drive conflicts when QTll101 sends an acknowledgment. If the host uses open-drain transmission, the pull-up resistor value should be optimized for the desired baud rate. High rates require small resistors to prevent excessively fast rise times; a typical value is 100 kΩ for a baud rate of 19,200 kb/s. Figure 4(b) shows the format of the host request bit (“Pt”) (RS-232 format), 8 data bits, no parity bit, and a baud rate of 8,000 b/s to 38,400 b/s. The first “S” is the start bit, and the second “S” is the stop bit. This bit format cannot be changed. The baud rate of the QT1101 response is the same as the received “P” character. Figure 4(c) shows the acknowledge byte format on the QT1101. After sending the “P” character, the host should immediately leave the 1W signal floating to prevent drive conflicts between the host and the QT1101. The delay time from receiving the stop bit to the OT1101 driving the 1W pin is 1 to 3 bit cycles; therefore, the host should leave the pin floating for one bit cycle to prevent drive conflicts. 3.3.2 2W Operation As described above, in 1W bus operation, when waiting for a response from QT1101, the host should leave the 1W bus floating. However, this is generally not feasible. To solve this problem, QT1101 can also receive the character "P" from the host via the RX pin, thus separating it from the 1W pin, as shown in Figure 5. Since QT1101 never drives RX, the host does not need to leave RX floating. After receiving "P" on RX, QT1101 will send the same response format via the 1W bus as in pure 1W mode. [b]4 Conclusion[/b] This paper presents a design method for a touchscreen and introduces the basic principle of the touch sensor QT1101. This device has the characteristics of high performance and low cost, and can be widely used in MP3 players, mobile phones, PC peripherals, TV control, fixed-point devices, remote control, and other fields. The touchscreen designed based on QT1101 can be used in multimedia public information query and other occasions that require fast human-computer interaction.