AN957 Datasheet, PDF (16/25 Pages) List of Unclassifed Manufacturers – Measuring HEXFETCharacteristics

AN957: TouchXpress™ Configuration and Profiling Guide
Configuration Tips
3. Configuration Tips
While TouchXpress devices are designed to provide responsive, reliable performance at optimally low current draw for all valid configu-
rations, most TouchXpress-configurable parameters do require some degree of trade-off between robustness and current draw. In gen-
eral, more scanning and sensing time means more time in Active or Optimized Active states of operation, which results in a higher
average current draw than a system that can remain in Sleep mode for a higher percentage of operating time.
The table below provides recommendations on configurable parameters that present a current draw vs. robustness trade-off. Note that
the term robustness in this context refers both to responsiveness and to robustness against coupled interference events.
Table 3.1. Recommendations for Optimal Current vs. Optimal Robustness
Parameter
[Active mode scan type]
[Active mode scan period]
[Counts before sleep]
[Accumulation]
Optimal Current Notes
Optimal Robustness Notes
Choose [one scan per period].
Choose [free run mode].
This setting will allow the device to drop in-
to a Sleep state after being in Active/Opti-
mized Active during a scan of enabled sen-
sors.
This setting prevents the device from ever
going into Sleep mode, instead executing
successive enabled input scans as quickly
as possible. Higher scan frequency means
faster touch qualification and a more re-
sponsive system.
When coupled with Active mode scan type
set to [one scan per period], a higher ac-
tive mode scan period means that the sys-
tem will stay in a low power Sleep state for
a higher percentage of operating time, low-
ering average current draw.
Lower Active mode scan periods result in
faster touch qualification.
Lower [counts before sleep] results in a A higher [counts before sleep] value re-
system that enters Sleep mode more quick- sults in a system that stays in Active mode
ly to reduce current consumption.
scanning for longer, ensuring optimal re-
sponsiveness during user interaction.
Lower accumulation means less time spent Higher accumulation increased digital filter-
in Active and Optimized Active power
ing, which increases signal-to-noise ratio
states.
(SNR).
If Active mode scan type is set to [one
scan per period], the device can spend a
higher percentage of time in a Sleep state
between scans.
3.1 Debounce vs. Active Mode Scan Period Considerations
The debounce parameter in [Xpress Configurator] is the setting that controls the number of contiguous sensor scans that must fall
above or below thresholds in order for the sensor to be qualified as active or inactive, respectively. The higher the debounce number,
the more scans must result in threshold crossings contiguously before an event is qualified. While this parameter improves robustness
against spurious interference events, increasing this value also has an effect on responsiveness.
A system optimized for responsiveness and robustness can have a combination of high debounce value and either a fast active mode
scan period or a device’s active mode scan type set to [free run mode]. In a system where scans occur more frequently, the terminal
debounce value can be reached faster, preserving response time.
A device’s responsiveness is primarily a function of the debounce setting and the active scan mode period. A good rule of thumb is to
make sure that the debounce setting times the active mode scan period is less than 100 ms, if active mode scan type is set to [one
scan per period].
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