FUJITSU SK-TSC-1125S - Manual (Page 14)

SK-TSC-1125S User Guide
- 14 - © Fujitsu Microelectronics Europe GmbH
Typically, to start tuning a new application, first of all the Reference Delay and Alpha settings
need to be set. For this, the ‘mon2’ monitor mode can help to directly show the effect of
changed settings.
The Reference Delay is set by the ‘srd nn’ command, where ‘nn’ is the desired value from 0-
127. A warm reset command is issued automatically to activate the new setting. Typical
starting value will be ~50-80, depending on the touch electrodes connected.
The Reference Delay value moves all of the bars (mon2 mode) to the left (higher RD value)
or to the right (lower RD value). Using the Reference Delay, the average (idle) impedance of
all touch sensors can be placed somewhere in the middle of the FMA1125 dynamic range,
indicated by the bars. Incrementing the Reference Delay by one step has a similar effect as
attaching an external capacitor of 0.078pF to the PAREF pin of the TSC (depending on the
R_SEL settings of the TSC, please refer to the FMA1125 register description for details). So,
in case the reference delay maximum value of 127 is not sufficient, an additional capacitor of
max. ~10 pF can be connected to the PAREF pin to increase the reference impedance
value.
If the difference between the sensor idle impedances of the channel is too high (e.g. some
channels smaller ~30, others bigger ~100), small tuning capacitors (some few pf; 1 count
step = 0.078pF depending on R_SEL settings) should be added (from the touch pads to
ground), so that the idle capacitances of the different channels are in the same range.
However, a difference of 40-50 counts still is not critical as long as there is enough room left
to 0 and 127. Therefore, most typical applications will not require tuning capacitors.
After setting the Reference Delay, the Alpha of the channels can be set. Alpha is the
difference between measured impedance and the reference in idle (not touched) state. A
smaller value of Alpha makes the pad more sensitive, while increasing Alpha decreases
sensitivity. Typical values for a directly touched (only solder stop or thin foil between
electrode and finger) pad will be ~20 or higher. If some mm of plastic is used as front cover,
typical Alpha values will be in the range of 5-15.
NOTE:
When a front panel with high dielectric constant, such as glass, is pressed against the touch
electrodes during operation (no front panel present at power-up or reset), the sudden
increase in capacitance might be detected as touch. As the Automatic Impedance
Calibration is suspended as long as a touch is detected, a calibration should be forced (e.g.
by using the ‘wr’ command to issue a warm reset) after the front panel was added during
operation and when this was detected as touch. If the front panel is already present during
power-up or reset, the initial calibration will automatically adjust sensitivity.
Based on these Settings, the Alpha values can be set for all input channels (‘sax’), for one
channel only (‘sa’), or it can be incremented / decremented for all channels (a+nn / a-nn).
The current values of Alpha and Reference Delay can be checked with the ‘sv’ command.
All remaining registers can be accessed by the ‘set’ and ‘setx’ commands using their
addresses. Please note that after writing to certain registers, a ‘warm reset’ has to be
performed e.g. using the ‘wr’ command. This also will trigger a calibration.
Up to 10 register settings (all read/writable registers) can be stored to and loaded from the
internal Flash memory of the Microcontroller using the ‘stp’ and ‘ldp’ commands.
Please refer to the application note ‘FMA1125_Init_Reg_Description’ for further register
description.