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QTouch
Any value from 1- 255 for 8bit AVR
3,4,5,10,25,50 for UC3
libraries
and ATSAM
QMatrix
1,2,3,4,5,10,25,50
Example:
When operating at 4 MHz, 1~10 cycle charge times are recommended (0.125us to 1.25us).
Table 2 : Frequency of operation
Note :
Frequency of
Microcontroller
(MHz))
1
2
4
8
10
16
20
48
>48
microcontroller Cycle time
(us)
1
0.5
0.25
0.125
0.1
0.0625
0.05
0.02083
<0.02083
Suitable Charge Cycle times (or)
Suitable Dwell Cycle times
(us)
1 to 2 cycles (1us to 2us)
1 to 5 cycles (0.5us to 2.5us)
1 to 10 cycles (0.25us to 2.5us)
1 to 10 cycles (0.125us to 1.25us)
2 to 25 cycles (0.2us to 2.5us)
2 to 25 cycles (0.125us to 1.5625us)
3 to 50 cycles (0.15us to 2.5us)
5~50 cycles (0.104us to 1.04us)
5 to < 50 (up to 255 cycles for 8bit AVR)
?
For UC3 and ATSAM devices, 1 & 2 charge cycle delay times are not supported.
If the microcontroller is only used for Touch detection then running at the lowest frequency
possible for the desired touch response may provide the best power and EMC performance. If it
is also used for other functions then running at a higher frequency may be necessary. In some
power critical applications it may be worth switching the frequency on the fly, such as lowering the
frequency during touch detect API instead of using long cycle times, and then switching to a
higher frequency for non-touch code. It is necessary to carefully design timer operation when
change frequencies.
5.6.9 Interrupts
This section illustrates the usage of interrupts during qt_measure_sensors call.
The library disables interrupts for time-critical periods during touch sensing. These periods are
generally only a few cycles long, and so host application interrupts should remain responsive
during touch sensing. However, any interrupt service routines (ISRs) during touch sensing should
be as short as possible to avoid affecting the touch measurements or the application
responsiveness.
Interrupts are disabled once for each signal count/burst pulse and this is typically 65 instruction
cycles when Delay cycles (QT_DELAY_CYCLE=1).
The number of times interrupts are disabled during one measurement will depend on signal count
of a channel as well as the number of channels and port configuration like interport (SNS and
SNSK on different port)/intraport (SNS and SNSK on same port).
Example:
4 channel intraport case:
Channel0 is formed by PA0 and PA1 pin with signal count 300.
Channel1 is formed by PA2 and PA3 pin with signal count 200.
Channel2 is formed by PA4 and PA5 pin with signal count 250
47
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