Interlink Electronics FSR 400 Series - Manual (Page 24)

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FSR
®
Integration Guide
Multi-Channel FSR-to-Digital Interface
Figure 11: Multi-Channel FSR-to-Digital Interface
Sampling Cycle (any FSR channel):
The microcontroller switches to a specific FSR channel, toggling it high, while all other
FSR channels are toggled low. The RESET channel is toggled high, a counter starts and
the capacitor C1 charges, with its charging rate controlled by the resistance of the FSR (t
~ RC). When the capacitor reaches the high digital threshold of the INPUT channel, the
counter shuts off, the RESET is toggled low, and the capacitor discharges.
The number of “counts” it takes from the toggling of the RESET high to the toggling of the
INPUT high is proportional to the resistance of the FSR. The resistors RMIN and RMAX
are used to set a minimum and maximum “counts” and therefore the range of the
“counts.” They are also used periodically to re-calibrate the reference. A sampling cycle
for RMIN is run; the number of “counts” is stored and used as a new zero. Similarly, a
sampling cycle for RMAX is run and the value is stored as the maximum range (after
subtracting the RMIN value). Successive FSR samplings are normalized to the new zero.
The full range is “zoned” by dividing the normalized maximum “counts” by the number of
desired zones. This will delineate the window size or width of each zone.
Continual sampling is done to record changes in FSR resistance due to changes in force.
Each FSR is selected sequentially.