Display Technology
Figure 5: Shown is a typical 4-wire USB controller board and host PC configuration. (Image source: NKK Switches)
continued from page 47
Both versions feature a flexible circuit tail that connects to a controller board.
The FTAS00-5.7AS-4A is a 4-wire, 5.7 in. model that draws 1 milliampere (mA) at 5 volts DC (VDC), has an XY resistive value of 250 to 850 ohms (Ω), a linearity of 1.5%, and an insulation impedance of 10 megaohms (MΩ). The touchscreen’s expected operational lifetime is 50,000 writing or one million tapping operations.
The FTAS00-10.4A-5 is a 5-wire, 10.4 in. model that draws 1 mA at 5.5 VDC, has an XY resistive value of 20 to 80 Ω, a linearity of 2%, and a minimum insulation impedance of 10 MΩ. The operational lifetime is 50,000 writing or 10 million tapping operations. For both the 4 and 5-wire touchscreen products, NKK offers a controller with either an RS232C or a USB interface. The controller boards are supplied with device driver software compatible with Windows 7, 8, and 10. The FTCS04C and the FTCU04B are the RS232C and USB interface controller
boards for the NKK 4-wire touchscreens, respectively, while the FTCS05B and the FTCU05B are the equivalents for the 5-wire touchscreens.
Getting started with a resistive touchscreen
The design process is similar for the 4 and 5-wire touchscreens. At the heart of the RS232C and the USB 4-wire controller board is the FTCSU548 controller chip. This 48-pin LFQFP IC has an asynchronous serial interface and a full-speed USB 2.0 interface. It is powered by a 3.3-to-5-volt supply for RS232C operation or a 5-volt supply for USB, with a rated output current of 170 mA, an operating frequency of 16-megahertz (MHz), and an analogue-to-digital converter (ADC) resolution of 10 bits. The chip features a built-in calibration function. When the touchscreen is pressed, the controller IC determines the coordinates using the value of the analogue voltage detected by the ADC and forwards them to
the host computer via the RS232C or USB interface (Figure 4).
The touchscreen’s 4-wire flexible circuit tail is connected to the controller board via CN1. The controller board connects to the host PC via CN4. The CN4 USB interface also supplies power to the board. The host runs the device driver and touchscreen application software (Figure 5).
Design tips
The resistive touchscreen requires calibration upon installation. The FTCSU548 controller IC includes a built-in calibration function. The controller IC must first be set to “source data mode” to calibrate. The PC then indicates a reference point (P1) on the touchscreen that the operator presses with a stylus, and the ADC voltage information is sent to the PC via the controller board. The process is repeated with a second point (P2) in a distant touchscreen area. The physical coordinates of P1 and P2 are sent to the PC as an eight-byte number. The touchscreen is then set into
“calibration data mode,” and the application software uses the voltage and coordinates reading for the two known points, plus a built-in “0,0” reference, to interpolate all other coordinates in the calibration data mode area (Figure 6).
The screen’s resistance changes as it ages, so recalibration is required throughout its operational life. It is essential to include grounding for the display device frame to prevent electromagnetic interference (EMI). It is also possible that the initial contact resistance from a finger might cause ‘chatter.’ To prevent chatter, a built-in delay can be used to allow the voltage to settle before the system calculates the coordinates.
Designers must also be careful not to include software instructing users to touch two touchscreen areas simultaneously. The technology cannot resolve two separate touches and defaults to a centre point between them. Finally, breaks will appear above the screen spacer when a line is drawn on a screen using a stylus, separating the two layers. Designers should ensure the application software fills in those gaps.
Conclusion
Figure 6: Calibration is required during initial configuration and periodically afterward because the resistance changes as the touchscreen ages. (Image source: NKK Switches)
48 September 2024 Components in Electronics
Resistive touchscreens are suitable HMIs in applications where cost, ruggedness, and operation with a bare or gloved hand or non-conductive stylus are essential. To simplify implementation, commercial solutions from NKK include the touchscreen overlay, controller boards with a dedicated controller IC, and device driver software.
https://www.digikey.co.uk www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
