SPRZ408D June   2014  – June 2021 AM4372 , AM4376 , AM4377 , AM4378 , AM4379

 

  1.   Trademarks
  2. 1Introduction
    1. 1.1 Device and Development Support Tool Nomenclature
    2. 1.2 Revision Identification
  3. 2All Errata Listed With Silicon Revision Number
  4. 3Usage Notes and Known Design Exceptions to Functional Specifications
    1. 3.1 Usage Notes
      1. 3.1.1 LPDDR2/DDR3: JEDEC Compliance for Minimum Self-Refresh Command Interval
      2. 3.1.2 DDR3/DDR3L: JEDEC Specification Violation for DDR3 RESET Signal When Implementing RTC+DDR Mode
    2. 3.2 Known Design Exceptions to Functional Specifications
      1. 3.2.1 Advisory List
      2.      Advisory 1
      3.      Advisory 2
      4.      Advisory 3
      5.      Advisory 4
      6.      Advisory 5
      7.      Advisory 6
      8.      Advisory 7
      9.      Advisory 8
      10.      Advisory 9
      11.      Advisory 10
      12.      Advisory 11
      13.      Advisory 12
      14.      Advisory 13
      15.      Advisory 14
      16.      Advisory 15
      17.      Advisory 16
      18.      Advisory 17
      19.      Advisory 19
      20.      Advisory 20
      21.      Advisory 21
      22.      Advisory 22
      23.      Advisory 24
      24.      Advisory 25
      25.      Advisory 26
      26.      Advisory 27
      27.      Advisory 28
      28.      i2223
      29.      i2224
      30.      i912
      31.      i2225
      32.      i2226
  5. 4Revision History

Advisory 19

TSC_ADC: False Pen-up Interrupts

Revisions Affected

1.1, 1.2

Details

The touchscreen controller (TSC) determines the pen state (up or down) by checking the respective analog input (AIN0 for 4-wire or AIN4 for 5-wire) voltage level immediately after the Charge step. This does not provide enough time for the analog input voltage to return to the normal Pen-down voltage before testing the pen state. This will cause the TSC to generate a false Pen-up interrupt if the Charge step is enabled with the strong pull-up turned on when using hardware synchronized steps. Figure 3-4 illustrates the effect on the analog voltage when constant pen pressure is applied to the touchscreen and held through the Charge step.

GUID-9F0D0FE3-A80A-4986-9AA1-1B988E7457AE-low.gifFigure 3-4 TSC Generates False Pen-up Interrupt

Workarounds

There are two possible workarounds for this problem:

  • The first workaround is implemented by configuring the Charge step exactly like the Idle step, where the internal strong pull-up is not turned on. This workaround will remove the strong pull-up on the analog input during the Charge step and leave only the internal weak pull-up. The weak pull-up is not strong enough to effect the normal Pen-down voltage. Therefore, the voltage applied to the analog input will not rise and cause the TSC to generate a false Pen-up interrupt. There is a drawback to this approach; this workaround will cause the Pen-down event to remain active until the weak pull-up turned on in the Idle step charges the touchscreen capacitance, as shown in Figure 3-5.
    GUID-1DA93781-8981-4599-8D4A-B1995B3619F7-low.gif Figure 3-5 Pen-down Remains Active in Idle Step
  • Another method is available for 4-wire touchscreen implementations. It this case, it is possible to enable and configure one of the 16 steps to emulate the Charge step with the strong pull-up turned on as the step before the actual Charge step that has been configured like the Idle step with the strong pull-up turned off. The advantage of this implementation is that you are able to charge the touchscreen quicker and avoid the possible false Pen-down interrupts. The disadvantage is that you lose one of the 16 programmable steps and during this step there will be a garbage result stored in the FIFO. You will need to set the Step_ID_tag bit in the CTRL register so that you can identify the garbage result in the FIFO and discard it.