ST AN2807 APPLICATION NOTE

AN2807

STMPE811 reference code for touchscreen controller operation

Introduction

The purpose of this application note is to provide the reference code for the STMPE811
touchscreen controller operation.

The first chapter describes how to initilialize the STMPE811 device while the second part
gives two practical methods to handle the data coming from the device.

For additional information, please refer to the STMPE811 datasheet.

September 2008 Rev 1 1/11

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Device initialization AN2807

1 Device initialization

In the startup of the system, the initialization of each hardware is performed.

For STMPE811 operating as touchscreen controller, the registers listed below need to be
initialized:

1. SYS_CTRL_2 register (0x04), to turn on the necessary blocks.
To turn on the touchscreen controller block, we need to turn on both the touchscreen
controller and ADC blocks by writing 0x0C to this register.

2. INT_EN register (0x0A), to enable the required interrupt source.
For the touchscreen function, only three interrupt sources need to be enabled:
TOUCH_DETECT and FIFO_TH.

- Write 0x03 to this register for group data reading.

- Write 0x02 to this register for single data reading.

3. ADC_CTRL1 (0x20), to select sample time, bit number and ADC reference.

a) Sample time: it is recommended to select 80 clock cycles for ADC conversion in

order to get reasonably fast data rate and accurate reading.

b) ADC bit number: 12-bit ADC is used in this document.

c) Reference selection: in this document, internal reference is used.

4. ADC_CTRL2 (0x21), to select the ADC clock speed.

3.25 MHz is the recommended value for touchscreen operation, although the device
can be pushed to operate at 6.5 MHz. Write 0x01 for this setting.

5. GPIO_AF (0x17), to program the I/O pins to the correct function.
By default, Power on reset (POR) sets the device in primary function
(ADC/ touchscreen controller application), but it is a good practice to make sure the
I/Os are in the correct mode. Write 0x00 to set all the I/O in ADC/touchscreen controller
mode.

6. TSC_CFG (0x41), to set Average, Touch Detect Delay and Settling Time setting.

a) Touch Detect dDelay and Settling Time: the setting for these two features depend

on the filter capacitor used in the touch screen pins (X+, Y+, X-, Y-). For small
screens, less than 6”, a filter capacitor of 1-2 nF is recommended to be placed in
each of the touchscreen pins, while for bigger screens, 5-10 nF capacitors can be
used.

– In case of 1-2 nF capacitors:

500 uS settling time and touch detect delay are recommended.

– In case of 5-10 nF capacitors:

1 mS settling time and touch detect delay are recommended.

In this document, 2 nF filter is assumed. Hence 500 uS settling time and 500uS
Touch Detect Delay are used.

b) Average control: average of 2 or 4 is recommended. In this document, 4 point

averaging is used. Write 0x9A to this register for the above mentioned setting.

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AN2807 Device initialization

7. FIFO_TH (0x4A), to set FIFO threshold level.
FIFO threshold defines the minimum number of samples in the FIFO memory before it
gives interrupt. The setting depends on the interrupt handling architecture to be used.
Refer to Section 1.1. In overall there are two options for data fetching from FIFO
memory:

● Single point reading.

Read a single point data in each I

2

C transaction to read data from FIFO memory. In this
case the FIFO threshold is set to 1. Hence, in every data acquired, the device will issue
FIFO threshold interrupt to inform host to read the data. Write 0x01 for this setting.

● Group points reading.

Read a group of data in each I

2

C transaction to read data from FIFO memory. In this
case, the FIFO threshold may be set to recommended value of 5. When the number of
data in FIFO memory is more than 5, it will issue FIFO threshold interrupt. Write 0x05
for this setting.

8. FIFO_CTRL_STA (0x4B), to reset FIFO.
Write 0x01 to clear the FIFO memory content.
Write 0x00 to put the FIFO back into operation mode.

9. TSC_FRACT_Z (0x56), to set the data format for Z value.
Write 0x07 to set to recommended value (7 fractional part and 1 whole part).

10. TSC_I_DRIVE (0x58), to set the driving capability of the device for touchscreen
controller pins.
Two setting may be chosen, 20 mA or 50 mA maximum current setting.
In this document, 50 mA setting is used. Write 0x01 for this setting

11. TSC_CTRL (0x40), to set tracking index, set touchscreen controller operation mode
and enable the TSC.

– No tracking Index will be used.

– The device operate at X, Y, Z mode

Write 0x01 to write the setting and enable the touch screen controller.

12. INT_STA (0x0B), to clear all the interrupt status.
Write 0xFF to clear all the interrupt status

13. INT_CTRL (0x09), to set interrupt mode and enable interrupt.
Write 0x01 to enable the interrupt.

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Device initialization AN2807

1.1 Initialization summary

1. Write: register Add = 0x04, data = 0x0C

2. Write: register Add = 0x0A, data = 0x07

3. Write: register Add = 0x20, data = 0x49
delay, 2mS

4. Write: register Add = 0x21, data = 0x01

5. Write: register Add = 0x17, data = 0x00

6. Write: register Add = 0x41, data = 0x9A (2 nF filter capacitor)

– a. Write: register Add = 0x4A, data = 0x01 (for single point reading)

– b. Write: register Add = 0x4A, data = 0x05 (for group points reading)

7. Write: register Add = 0x4B, data = 0x01

8. Write: register Add = 0x4B, data = 0x00

9. Write: register Add = 0x56, data = 0x07

10. Write: register Add = 0x58, data = 0x01

11. Write: register Add = 0x40, data = 0x01

12. Write: register Add = 0x0B, data = 0xFF

13. Write: register Add = 0x09, data = 0x01

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