BU21024FV-M Datasheet, PDF (6/18 Pages) Rohm – Resistive Type Touch Screen Controller ICs

BU21023GUL,BU21023MUV,BU21024FV-M
Technical Note
●Background Information
A resistive touch panel is made up of a multilayer sandwich of resistive films and protective coatings all sitting on top of an
LCD display. Resistive touch panels work by direct contact of a stylus or a finger flexing a pair of resistive films, hence any
blunt pointing instrument or a gloved finger may be used.
Touch panel resolution
The resolution of a touch panel is typically measured in dots per inch (dpi) and is a function of the physical size of the touch
panel and the ADC used in the conversion circuitry. For example, a 3”x5” panel used with the BU21023/BU21024 devices
can provide a theoretical resolution of 1024 /5 = 204 dpi. In the case of resistive touch panels, the direct contact nature of its
operation and finger thickness often impose an upper limit on the effective system resolution that may be achieved,
regardless of the resolution of the ADC itself.
●Functional Description
The BU21023/BU21024 devices connect to a standard 4-wire resistive touch screen on one side and to a host processor on
the other side. The BU21024 includes four additional sense terminals allowing it to be used with either 4-wire or 8-wire
resistive touch screens. The BU21023/BU21024 include the analog and digital circuitry to process and provide dual touch
coordinate data and pinch, spread, rotate-right and rotate-left gesture information to the host CPU.
The BU21023/BU21024 devices include an internal CPU and provide a high degree of programmability by exposing a set of
registers that can be accessed by a host CPU through SPI or I²C like serial interfaces. The IFSEL pin determines whether
SPI or I²C like interface is selected, The BU21023/BU21024 devices include an INT pin whose polarity can be programmed
via registers. When an interrupt occurs, for example, due to touch detection, the host processor is required to read an
interrupt status register to determine the cause of the interrupt and take appropriate action.
The program memory of the internal CPU may be initialized via the host interface or via an external EEPROM. This is
selected by a register setting.
The BU21023/BU21024 devices support two sensing modes; continuous and interval sensing modes, that are register
selectable. In the continuous sensing mode, the embedded CPU reads Z, X & Y coordinates continuously when a touch is
detected. The interval sensing mode allows a delay to be inserted between each cycle. The continuous sensing mode is
used more often though the interval sensing mode minimizes power consumption. The continuous sensing mode typically
completes a sampling cycle consisting of Z, X & Y measurements in approximately 2.3 mS.
The BU21023/BU21024 devices enable optimization of touch detection threshold for a given panel. They also include
several sophisticated calibration algorithms.
This document includes a description of the registers followed by flow charts that describe specific steps that a host
processor must follow. Often, a flow chart requires other flow charts to explain the steps in finer detail.
The document also includes information on touch screen parameters that one should to look for while selecting resistive
touch panels for multi-touch.
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2011.08 - Rev.A