Fujitsu SK-8FX-TSC-32PMC Manuale d'uso - Pagina 22

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SK-8FX-TSC-32PMC User Guide

Chapter 5 Basic TSC Tuning Process

5 Basic TSC Tuning Process

Following some basic rules during the tuning process of an FMA1125 touch application can

help to get the application running quickly. An example program is pre-programmed on the

SK-8FX-TSC-32PMC, which enables easy parameter tuning using a sort of command-line

interface to a host PC running a standard terminal program (8N1, 115200 baud).

Type 'help' followed by <ENTER> for a list of available commands.

Note:

Please keep in mind that the tuning firmware shows the Impedance and Reference

Impedance values as '127-Register Value' in order to have a more intuitive behaviour

(a touched key then gives a higher impedance value than an untouched one).

The first Parameter to adjust is the Reference Delay. This parameter is used to globally

compensate parasitic capacitance present on all tracks. It can be set in the example

Firmware using the 'srd' command, followed by a blank the desired value. The influence on

the Impedance and Reference Impedance values can be seen below:

RefDelay = 50

RefDelay = 0

RefDelay = max.

(Too low for this layout,

(Too high for this layout,

(ideal for this layout)

inputs are saturated)

inputs are too small)

The average value of the Impedance and Reference registers should be around the middle

of the scale, and a reserve of about 20-30 counts should be kept to zero and full-scale in

order to enable correct AIC (Automatic Impedance Calibration) operation.

In case the difference between biggest and smallest values is too big (some values close to

zero, others close to max.), tuning capacitors can be used to roughly match the

capacitances. Every count of the Impedance Register corresponds to a value of 0.078pF

(default settings of FMA1125) on the input pin, so that e.g. attaching a 4.7pF capacitor

between GND and the input pin with the smallest value will increase the bar by about 60

counts.

In case some values are close to the maximum even though Reference Delay is already set

to the maximum value (127), an additional capacitor can be connected to the PAREF pin.

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