Data Sheet

Copyright © Bridgetek Pte Ltd 35
BT81X (815/6) Advanced Embedded Video Engine Datasheet
Version 1.0
Document No.: BRT_000220 Clearance No.: BRT#126
These touch values are transformed into screen coordinates using the matrix in registers
REG_CTOUCH_TRANSFORM_A-F. The post-transform coordinates are available in register
REG_CTOUCH_SCREEN_XY. If the touch screen is not being pressed, both registers read -32768 (8000h).
The values for REG_CTOUCH_TRANSFORM_A-F may be computed using an on-screen calibration process.
If the screen is being touched, the screen coordinates are looked up in the screen's tag buffer, delivering
a final 8-bit tag value, in REG_TOUCH_TAG. Because the tag lookup takes a full frame, and touch
coordinates change continuously, the original (x; y) used for the tag lookup is also available in
REG_TOUCH_TAG_XY.
4.8.4 Extended Mode
Setting REG_CTOUCH_EXTENDED to 1b’0 enables extended mode. In extended mode a new set of
readout registers are available, allowing gestures and up to five touches to be read. There are two
classes of registers: control registers and status registers. Control registers are written by the MCU.
Status registers can be read out by the MCU and the BT815/6’s hardware tag system.
The five touch coordinates are packed in REG_CTOUCH_TOUCH0_XY, REG_CTOUCH_TOUCH1_XY,
REG_CTOUCH_TOUCH2_XY, REG_CTOUCH_TOUCH3_XY, REG_CTOUCH4_X and REG_CTOUCH4_Y.
Coordinates stored in these registers are signed 16-bit values, so have a range of -32768 to 32767. The
no-touch condition is indicated by x=y= -32768. These coordinates are already transformed into screen
coordinates based on the raw data read from the CTPM, using the matrix in registers
REG_CTOUCH_TRANSFORM_A-F. To obtain raw (x,y) coordinates read from CTPM, the user sets the
REG_CTOUCH_TRANSFORM_A-F registers to the identity matrix.
The BT815/6 tag mechanism is implemented by hardware, where up to 5 tags can be looked up.
4.8.5 Short-Circuit Protection
For resistive touch it is useful to protect the chip from permanent damage due to potential short-circuits
on the 4 XY lines. When a short circuit on the touch screen happens, the BT816 can detect it and stop the
touch detection operation, leaving the 4 XY pins in the high impedance state.
The short-circuit protection can be enabled/disabled by the REG_TOUCH_CONFIG.
4.8.6 Capacitive Touch Configuration
On a capacitive touch system some users may need to adjust the CTPM default values, such as the
registers affecting touch sensitivity. To do this the following sequence shall be executed once after chip
reset:
- Hold the touch engine in reset (set REG_CPURESET = 2)
- Write the CTPM configure register address and value to the BT815 designated memory location
- Up to 10 register address/value can be added
- Release the touch engine reset (set REG_CPURESET = 0)
The CTPM can be enabled in low power state when the touch function is not required by the application.
Setting the low-power bit in REG_TOUCH_CONFIG will enable the low power mode of the CTPM. When
the low-power bit is cleared, the BT815 touch engine will send a reset to the CTPM, thus re-enabling the
touch detection function.
4.8.7 Host Driven Multi-Touch
If the host MCU can provide touch inputs, it can supply them directly to the BT815 using touch host
mode. By using touch host mode, an application can choose to select a touch controller that is not in the
BT815 direct support list. For example, Touch Host mode would allow controllers to be used from other
manufacturers beyond Focaltech and Goodix.
To use the touch host mode, the host MCU shall be connected to the touch panel directly. The four touch
related pins of the BT815 can be left unconnected on the PCB. The host MCU is responsible for