MXT448UD-CCUBHA1 datasheet(21/76 Pages) MICROCHIP

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mXT448UD-CCUBHA1 1.0

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DS40002352A-page 21

6.6

Digital Filtering and Noise Suppression

The mXT448UD-CCUBHA1 supports on-chip filtering of the acquisition data received from the sensor. Specifically, the

Noise Suppression T72 object provides an algorithm to suppress the effects of noise (for example, from a noisy charger

plugged into the user’s product). This algorithm can automatically adjust some of the acquisition parameters on-the-fly

to filter the Analog-to-Digital Conversions (ADCs) received from the sensor.

Additional noise suppression is provided by the Self Capacitance Noise Suppression T108 object. Similar in both design

and configuration to the Noise Suppression T72 object, the Self Capacitance Noise Suppression T108 object is the

noise suppression interface for self capacitance touch measurements.

Noise suppression is triggered when a noise source is detected.

• The host driver code can indicate when a noise source is present.

• The noise suppression is also triggered based on the noise levels detected using internal line measurements.The

Noise Suppression T72 and Self Capacitance Noise Suppression T108 object selects the appropriate controls to

suppress the noise present in the system.

6.7

Functional Safety – UL/IEC 60730 Class B Compliance

• The device is designed with functional safety applications in mind (for example, for the home appliance market).

Specifically, the device complies with the UL/IEC 60730 Class B safety specification:

- User-configurable self diagnostics to detect internal system errors (for example, CPU, memory and system

clocks), power, pin fault errors and signal errors for immediate action by the host system (see Section 12.3

“Self Test”). These tests can be configured to run at power-on/reset and/or as periodic testing during

operation.

- Other built-in automatic system-level tests (for example interrupts, CTE and peripheral I/O)

- Heartbeat (alive) signal output to host

- Enhanced communications that includes sequence numbers (timing) and CRC error checking

For more details on the self tests available on the mXT448UD-CCUBHA1, see Section 12.3 “Self Test”.

6.8

EMC Reduction

The mXT448UD-CCUBHA1 has the following mechanisms to help reduce EMC emissions and ensure that the user’s

product operates within the desired EMC limits:

• Spread Spectrum – Varies the burst frequency on each measurement pulse to spread the EMC energy over the

frequency domain. This feature is configured by the CTE Configuration T46 object.

• Configurable Voltage Reference Mode – Allows for the selection of voltage swing of the self capacitance

measurements.This feature is configured by the Self Capacitance Global Configuration T109 object.

• Input Buffer Power Configuration – Controls the positive/negative drive strength of the Input Buffer for self

capacitance measurements. This feature is configured by the Self Capacitance Global Configuration T109 object.

• Configurable Input Amplifier Bias – Controls the Input Amplifier Bias. This feature is configured by the Self

Capacitance Global Configuration T109 object.

• Configurable Wave Shaping – Controls the voltage modulation on self capacitance scans allows wave shaping

of the edge for EMC harmonic control. This feature is configured by the Self Capacitance Voltage Modulation T133

object.

6.9

Shieldless Support and Display Noise Suppression

The mXT448UD-CCUBHA1 can support shieldless sensor design even with a noisy LCD.

The Optimal Integration feature is not filtering as such, but enables the user to use a shorter integration window. The

integration window optimizes the amount of charge collected against the amount of noise collected, to ensure an optimal

SNR. This feature also benefits the system in the presence of an external noise source. This feature is configured using

the Shieldless T56 object.

Display noise suppression allows the device to overcome display noise simultaneously with external noise. This feature

is based on filtering provided by the Lens Bending T65 object (see Section 6.12 “Lens Bending”).

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