Datasheet

575
32133D–11/2011
UC3D
Figure 27-3. CAT Acquisition and Processing Sequence
27.6.5 Spread Spectrum Sensor Drive
To reduce electromagnetic compatibility issues, the capacitive sensors can be driven with a
spread spectrum signal. To enable spread spectrum drive for a specific acquisition type, the
user must write a one to the SPREAD bit in the appropriate Configuration Register 1 ( ATCFG1,
TGACFG1, or TGBCFG1).
During spread spectrum operation, the length of each pulse within a burst is varied in a deter-
ministic pattern, so that the exact same burst pattern is used for a specific burst length. The
maximum spread is determined by the MAXDEV field in the Spread Spectrum Configuration
Register (SSCFG) register. The prescaler divisor is varied in a sawtooth pattern from
(2(DIV+1))-MAXDEV to (2(DIV+1))+MAXDEV and then back to (2(DIV+1))-MAXDEV. For exam-
ple, if DIV is 2 and MAXDEV is 3, the prescaler divisor will have the following sequence: 6, 7, 8,
Idle
Acquire
autonomous
touch count
Acquire counts
Update
autonomous
touch detection
algorithm
Wait for all
acquired counts
to be transferred
AISR written flag set?
YesNo
Clear AISR
written flag
No Yes
Autonomous touch
enabled (ATEN)?