V
VDDA
VSSA
Touch
occurs
Checks for
Pen-up
t
Idle stepStep 1
Step N-1 Step N
Charge step Idle step
. . .
Usage Notes and Known Design Exceptions to Functional Specifications
Advisory 1.0.30
OSC0 and OSC1: Noise Immunity Improved When Crystal Circuit is Connected
Directly to PCB Digital Ground
Revisions Affected
2.1, 2.0, 1.0
Details
The
Sitara AM335x ARM Cortex-A8 Microprocessors (MPUs)
data manual (literature
number
), revision F and earlier, has specified OSC0 crystal circuit grounds
only be connected to VSS_OSC in both ZCE and ZCZ packages, and OSC1 crystal
circuit grounds only be connected to VSS_RTC in the ZCZ package. The VSS_OSC and
VSS_RTC terminals are connected to the VSS terminals inside the AM335x device,
which connects the crystal circuit to the PCB digital ground, but this ground connection is
a higher impedance connection than a direct connection to the PCB digital ground.
The higher impedance connection through the AM335x device to the PCB digital ground
makes it easier for electrical noise to couple into the crystal circuit. If the noise is large
enough, it is possible that the oscillator output may produce clock glitches to various
internal logic circuits. These clock glitches may cause unexpected behavior.
Workarounds
Connect the VSS_OSC and VSS_RTC terminals and respective crystal circuit
component grounds directly to the nearest PCB digital ground, making it more difficult for
noise to couple into the crystal circuit.
Advisory 1.0.31
TSC_ADC: False Pen-up Interrupts
Revisions Affected
2.1, 2.0, 1.0
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.
illustrates the effect
on the analog voltage when constant pen pressure is applied to the touchscreen and
held through the Charge step.
Figure 5. TSC Generates False Pen-up Interrupt
Workarounds
There are two possible workarounds for this problem:
32
SPRZ360F – October 2011 – Revised November 2013
Sitara™ AM335x ARM
®
Cortex™-A8 Microprocessors (MPUs) (Silicon
Revision 2.1, 2.0, 1.0)
Copyright © 2011–2013, Texas Instruments Incorporated
