Analog Devices AN-738 Application Notes

AN-738

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • Fax: [phone] • www.analog.com

Using the AD7877 Touch Screen Controller and the Intel PXA250 Processor Under

Windows CE.NET

by Paul Perrault and Susan Pratt

INTRODUCTION

The AN-738 Application Note details one way to connect
an Analog Devices AD7877 touch screen controller to
an Intel
CE operating system. The user will be able to quickly
develop a touch screen driver based on the example
software accompanying this note. The exact code base
will vary depending on the processor. Table 1 lists the
software with a brief description.

DRIVER OVERVIEW

The driver supports the basic touch controller function­ality of the AD7877. The driver makes use of several
advanced features, including programmable acquisition
delays, averaging, conversion sequencing, and contact
resistance calculation. The driver requires an SPI inter­face on the host processor and a single GPIO pin.

The sample AD7877 touch driver is written for the Intel
PXA25x CPU family (XScale
the CPU via the hardware SPI controller on the PXA25x.

File Name Description

Tchpdd.cpp The PDD (platform dependent driver) layer of the touch screen driver. The MDD (model device driver) layer of
the driver is written by Microsoft. Together, these two fi les form a driver for Windows CE.

Pxa255.h This is a header fi le to describe the register defi nitions of the PXA255 processor. This also defi nes the bit defi nitions
for the synchronous serial port (SSP) on this processor so that the SPI interface works with the AD7877.

ad7877.h This is a header fi le to deal with the registers inside the AD7877.

sources A sources fi le is a text fi le that sets the macro defi nitions for the source code in a subdirectory. Build.exe uses
these macro defi nitions to determine how to compile and link the source code.

makefi le In an application development environment, such as Microsoft
commands, macro defi nitions, and options needed. In this application, the makefi le simply references the
standard Windows CE makefi le.

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PX A25x processor for use under a Windows®

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), and communicates with

Table 1. Files Included and Referenced in the AN-738 Application Note

The Intel PXA25x processor family (XScale) has been
very popular with Windows CE developers in recent
years, and this driver is written specifi cally for that pro ­cessor family.

However, any processor with a hardware SPI controller
and an available GPIO could use this driver for touch
screen interactions with minimal changes. The methods
described within this application note have been tested
to pass the Windows CE Test Kit (CETK) using a slightly
modified PXA25x integrated development platform
(IDP) and an AD7877 evaluation board. The software is
intended to be used as a sample driver, or as a basis to
build up a production - grade driver for your system, but
is not intended for use “as is” in production systems.

The touch driver is interrupt driven, using the PENIRQ
output to signal the processor when the screen is touched.
Subsequent samples are scheduled using the OS timer.
The driver queries the PENIRQ output at each sample
point to determine if the screen remains touched.

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Platform Builder, a makefi le contains all

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AN-738

Data conversions utilize the built- in channel sequencing
support in slave mode. A conversion is initiated at each
sample point so that it w ill b e com plete by th e nex t sam ­ple point. Thus, at each sample point, the driver reads
the results of the conversion initiated at the prior sam­ple point. This mechanism minimizes time spent in the
driver, eliminates the need to use GPIO resources for the
DAV status signal, and requires only that the sample rate
guarantees enough time for whatever acquisition delays,
averaging, and channel sequencing are in ef fect.

DRIVER CONFIGURATIONS

The touch driver requires interrupt support from the
OAL (OEM adaptation layer). The OAL must return a
valid sysintr number when PENIRQ asserts. This sysintr

Table 2. Sample GPIO Register Settings of PXA255 for Use with the AD7877

Intel
PXA255
Register Bits Setting Description

SSCR0 SSE [7] 0 Disable SSP so that
SSP changes can
happen

SSCR0 DSS [3:0] 0b1111 S e t the data size to
16 bits

SSCR0 FRF [5:4] 0b00 Set the frame
format to
Motorola’s SPI

SSCR0 SCR [15:8] 0b0000 Set the SSP clock
rate to 1.8 MHz

SSCR0 ECS [6] 0 Use the on-chip
clock for SSPCLK

SSCR1 RIE [0] 0 Disable receive
FIFO interrupt

SSCR1 TIE [1] 0 Disable transmit
FIFO interrupt

SSCR1 LBM [2] 0 Disable loopback
mode

SSCR1 SPO [3] 0 The SPI clock
idles low

SSCR1 SPH [4] 0 SPI clock polarity

GPSR0 PD24 1 Set GPIO24
(SSPSFRM)
output high

number must be known to the touch driver at driver
initialization. The sysintr value is currently specifi ed
via a #define configuration option in the driver and
defaults to 0x00000019. This value must match the
value returned by the OAL. Implementation of the OAL
requirements is beyond the scope of this document and
will vary from code base to code base. It is suffi cient
to note that the OAL impacts will be limited to the pin
operation of the GPIOs on the PXA25x and the interrupt
support. Table 2 shows sample GPIO register settings
to enable the AD7877 to be used with the PXA25x.

Intel
PXA255
Register Bits Setting Description

GPDR0 PD26 0 Set GPIO26
(SSPRXD) pin
direction (input)

GPDR0 PD25 1 Set GPIO25
(SSPTXD) pin
direction (output)

GPDR0 PD24 1 Set GPIO24
(SSPSFRM) pin
direction (output)

GPDR0 PD23 1 Set GPIO23
(SSPSCLK) pin
direction (output)

GAFR0_U AF23 [15:14] 0b10 Set GPIO23 to be
SSP CLK

GAFR0_U AF24 [17:16] 0b10 Set GPIO24 to be
SSP SFRM

GAFR0_U AF25 [19:18] 0b10 Set GPIO25 to be
SSP TXD

GAFR0_U AF26 [21:20] 0b01 Set GPIO26 to be
SSP RXD

SSCR0 SSE [7] 1 Enable SSP

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