ST AN2825 Application note

AN2825

Application Note

S-Touch

STMPE811 resistive touchscreen controller advanced features

Introduction

This application note provides details on the advanced features of the S-Touch® STMPE811
touchscreen controllers.

In particular, the STMPE811 device is a GPIO (general purpose input/output) port expander
equipped with an advanced touchscreen controller which helps to reduce the load of the
host and simplify data processing.

The document is divided into three sections which respectively describe: touchscreen
advanced features, available touchscreen controller settings and basic guidelines on the use
of interrupts.

®

September 2011 Doc ID 15023 Rev 1 1/11

www.st.com

Contents AN2825

Contents

1 STMPE811 advanced features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Autonomous operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 FIFO memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Movement tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Averaging data to optimize noise cancellation . . . . . . . . . . . . . . . . . . . . . . 5

1.5 Window tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Settings description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 ADC sample time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 ADC bit mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 ADC clock frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 TSC operating mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.5 Tracking index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.6 Averaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.7 Touch detect delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.8 Driver settling time delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Interrupt user guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 Touch detect interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 FIFO interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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AN2825 STMPE811 advanced features

1 STMPE811 advanced features

The touchscreen advanced features such as autonomous operation, movement tracking
algorithm to avoid excessive data, a 128-depth FIFO buffer and a programmable active
window feature that may be used in applications without increasing the host load.

1.1 Autonomous operation

The STMPE811 device is integrated with a hard-wired touchscreen controller, for a 4-wire
resistive type touchscreen, able to operate in an autonomous way, and interrupts the
connected CPU only when a pre-defined event occurs.

Figure 1. Touchscreen controller block diagram

FIFO

The STMPE811 touchscreen controller converts the ADC output into digital location data
(coordinates) and stores these in a FIFO for system access. This FIFO can be accessed
directly by the system as and when needed. This design reduces the system loading and

2

the I

C bus utilization.

1.2 FIFO memory

The FIFO memory has a depth of 128 sectors, enough for 128 sets of touch data at
maximum resolution (2 x 12 bits). The FIFO can be programmed to generate an interrupt
when it is filled to a pre-determined level.

Movement

and

window tracking

FIFO
and

interrupt control

10/12 bit

ADC

Driver

and

switch control

Switch

and

drivers

AM00745V1

The devices convert the data automatically when a touch is detected. The data is stored in
the FIFO memory for the host to read.

The FIFO memory is equipped with a control and status register where the host may refer to
it during its operation.

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STMPE811 advanced features AN2825

Below is a list of the various status of the FIFO memory reported in the control and status
registers (FIFO__CTRL_STA register, address 0x4B):

● FIFO overflow:

reports ‘1’ when FIFO memory is full and new data is coming.

● FIFO full:

reports ‘1’ when FIFO memory is full.

● FIFO empty:

reports ‘1’ when there is no data in FIFO memory.

● FIFO threshold trigger:

reports ‘1’ when the number of data in FIFO memory exceeds the number specified by
user in FIFO_SIZE register.

In the same register there is 1 bit for the user to put FIFO memory in reset mode by writing 1
to this bit. The data in the FIFO will be flushed when FIFO is in reset mode.

The host needs to read the data in the FIFO memory consistently. For example, in XYZ
mode, a single data consists of 3 bytes where X, Y and Z data is included. Hence, each data
read transaction must consist of multiples of 3 bytes in length to avoid confusion and data
loss.

Please refer to the STMPE811 datasheet for a detailed description of each register.

1.3 Movement tracking

The movement tracking feature reduces the number of samples generated by the
STMPE811 device. Using this feature, the device will receive only data that satisfies the
criteria set by the host.

The system host may select one of the 8 tracking index options: 0, 4, 8, 16, 32, 64, 92 and

127. The value of movement index refers to the native resolution of the ADC, which is 4096
points for full range.

In XY mode, for example, the “127 points” option is selected, valid data form a square that is
127 points in size where the valid data being the center of the square. New data which is
inside this box is discarded (not stored in the FIFO memory). If new data is located outside
the square, the data is taken and used as the center of new square. This is illustrated in

Figure 2 which shows how the tracking index eliminates the data inside the tracking area

which is formed in the surrounding of valid data.

Figure 2. Tracking index

In XYZ mode, there is another factor that affects the tracking index function; the Z value. The
tracking index algorithm still considers new data inside the tracking area as valid data if the
Z value is lower than the previous valid data (pressure of new data is higher than previous
valid data).

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