A Fully Integrated Touch Screen Controller Based on 12Bit 825kSps SAR ADC

A Fully Integrated Touch Screen Controller Based on 12b 825kS/s SAR ADC

Stefano Pietri, Alfredo Olmos, Michael BerensFreescale Semiconductor 6501 William Cannon Dr. Austin, Texas 78735, USA Andre Vilas Boas, Marcos GoesFreescale SemiconductorRod. Anhanguera, km 104 Campinas - SP - Brazil

Abstract - This paper describes a low-power 12 bits 825kS/s Suc-cessive Approximation Register Analog to Digital Converter (SARADC) for personal digital assistants, portable instruments, andtouch-screen monitor applications. The proposed SAR ADC isbased on 3-stage 4-4-4 C-DAC architecture and a high-speed 3-stages comparator. It features 8 general-purpose input channelsand 8 dedicated channels supporting 4, 5, 7, and 8-wire resistivetouch-screen panels. It has a full-scale input range, power-downmode for low current consumption, and a novel pen-touch detec-tion control with fast response able to support any touch-screenpanel. The ADC performs ±1.3 LSB DNL and ±5 LSB INL whentested up to 125C at supply voltages ranging from 2.7V to 3.6V.Fabricated in a 0.13um CMOS with fringe capacitors, it occupiesan area of 0.17 mm2 and has a total dissipation of 200uA at full

speed. The circuit has been integrated on Freescale MCF5227xMicro-Controller Unit (MCU) family of devices with a 166 MHzColdFire(R) core processor and an LCD controller.

Keywords - SAR ADC, DAC, Successive Approximation, touch screen, pen-touch detection.

I. INTRODUCTIONLonger battery life and new features are expected at everynew generation of portable electronic appliances. The means toincrease active device time are many, from advanced powermanagement, to system on chip integration to limit powerhungry off chip communications. Analog IC designers arechallenged to reduce cost and power budget, still meeting orimproving performance. Considering that analog componentdo not scale with the new CMOS technology, the challengebecomes even more demanding. The SuccessiveApproximation Register Analog to Digital Converter (SARADC) is a family of data converter, known to be second best inclass for each single specification [1-10], therefore widelyreusable in most applications and preferred for the ones thatrequire reasonable speed/performance trade off with low powerconsumption and small silicon area.A resistive touch screen is a sensor that translates thephysical location of a touch at a point in a panel area intovoltages that represents the X and Y coordinates of the touch.Many LCD modules come equipped with resistive touchscreens, which may use 4 wires, 5 wires, 7 wires, or 8 wires for

generating screen bias voltages and reading the touch pointvoltage [11]. In the past, reading touch coordinates from aresistive touch screen required MCU bandwidth or an externaldedicated touch screen controller device, an ADC and severalswitches. Cost saving requirements led to dedicated MCU’sthat integrate on-chip ADC, touch screen biasing circuitry andtouch screen controller that allow to perform position readingson a panel without loading the CPU and without requiringboard components. In this paper the design of a low power, area effective 12bits SAR ADC for a touch-screen application is presented. Thetouch-screen interface to ADC is described in Section 2.Results of high-level model simulation of the CapacitiveDigital to Analog Converter (C-DAC) array are reported inSection 3. Section 4 depicts the high-speed comparator, andthen silicon results are described in Section 5, followed byconclusions in Section 6.

II. TOUCH-SCREEN INTERFACE TO ADCThe analog section of the touch screen interface isillustrated in Fig. 1 and includes all circuitry required to controland read touch position on almost all kinds of touch paneldevices: bias network, input multiplexer, reference multiplexer,12 bit DAC, and comparator. The SAR control block notshown in Fig. 1 implements the function of successiveapproximation data conversion and provides the data to theCPU by means of a dedicated asynchronous communicationchannel.The ADC block can be used as a touch screen or genericpurpose ADC. It can also perform single-ended or ratiometricconversions. Single-ended conversions consume less powerfrom battery because the screen excitation voltage can beswitched-off once the signal is acquired. Ratiometricconversions use the screen excitation voltage as reference forthe ADC. They provide greater accuracy since voltage dropacross switches has no effect. No matter if the block is configured as touch screencontroller or generic purpose ADC, its behavior can bedescribed by a sequence of operations such as: (a) Pen detection phase: the period to detect the pen down state;(b) X measurement phase: the period to take the X coordinatemeasurement; (c) Y measurement phase: the period to take the