QTLIB_BURST_AGAIN (burst_flag) question

[arendc]

Hi guys,

After pestering the Atmel support people I have finally got my board up and running but have a few more questions.

My board is currently running the code that the Atmel support people tested on their own hardware according to my board's layout. They said it worked fine with them but when I run their code on my hardware my touch sensing is quite sporadic. I debugged the code and found that the burst flag was being set quite frequently, meaning that the code was getting stuck in the do/while loop quite a bit.

I couldn't find much information about the burst flag, only that if it is set then the qt_measure_sensors() function did not work properly and needs to be called again. My question is, what causes the burst flag to be set and what can I do to make the function work more consistently?

Not sure if I should post code and/or schematics as this is a rather general question but if you want me to just let me know. Thanks for your help,

Arend

[Sai]

Hi,

Status flag stores the qt_measure_sensors() return status. Burst flag stores the result of (status flag & QTLIB_BURST_AGAIN). If one more burst operation on the sensors is required, then qt_measure_sensors() set the flag to 1. Burst sequence/operation does happens on the sensors as long as the burst flag is set to 1. Refer to page32 of the Library user guide for more details on qt_measure_sensors() return flags.

What is the sensors behavior that you have been observed on the hardware. Share your observations with us.
It seems the issue is with the hardware design.I am referring to the SE_APO2_FRONT_001.PDF file that you have shared with our support team.

Following are my comments as far as your design is concerned.

1. Series resistor(Rs) on each sensor electrode is placed away from the IC. This creates lot of noise issues on the sensor electrode. Since the sensor track between IC pin and the Rs is susceptible to high frequency noise. It is recommended to reduce the gap between IC pin and the Rs. So place Rs and Cs as close as possible. This acts as a RC low pass filter and removes high frequency signals on the sensor tracks.

2. Never place the Vdd or Gnd lines near the sensor electrodes/tracks. It reduces the SNR of the sensor. In your design, Vdd line is running very close to the sense lines. Always route the sense lines on another PCB planes which is away from the touch.

3. Ensure the power supply output is stable. Any transient spikes on the voltage source or if the same supply shares with other switching components( like LCD,LEDs etc) on the PCB, then sensor behavior might be erratic.

For proper operation of sensor electrodes, optimized design is required. This can be achieved by following the guide lines provided in the BSW sensor design guide.

Feel free to send the Layout design files to touch@atmel.com. So we can provide more suggestions on your design.