Cypress CY8C20xx7/S CapSense Series, Design Manual

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CapSense Performance Tuning with User Modules
CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 43
10 to 23 1360 17 to 35 1360 10 to 23 680
23 to 41 2730 35 to 41 2730 23 to 41 1360
41 to 45 5460 41 to 45 5460 41 to 45 2730
Table 4-6. Sampling Time for a Sensor with IMO = 6 MHz
Sensitivity = 0.2 pF Sensitivity = 0.3 pF Sensitivity = 0.4 pF
C
P
(pF) ST (µs) C
P
(pF) ST (µs) C
P
(pF) ST (µs)
8 to 11 680 8 to 10 680 8 to 11 680
11 to 23 1360 10 to 17 1360 11 to 23 1360
23 to 42 2730 17 to 35 2730 23 to 41 2730
42 to 45 5460 35 to 41 5460 41 to 45 5460
41 to 45 10920
Table 4-7 shows the value for processing time with various CPU frequencies.
Table 4-7. Processing Time for a Sensor
CPU CLK Processing Time (PT) in µs
24 71
12 142
6 284
3 568
For example, if a CapSense system is designed with a 24-MHz IMO frequency, a 6-MHz CPU clock (IMO/4), and a
SmartSense_EMCPLUS sensitivity level of 0.3 pF, the scan time of the sensor that has parasitic capacitance around
15 pF can be calculated from the previous tables using Equation 4-1.
Sampling time for the previously mentioned configuration (24 MHz of IMO and 0.3 pF of sensitivity) is chosen from
Table 4-4 ; it is 680 µs. Processing time for the previously mentioned configuration (CPU clock of 6 MHz) Is chosen
from Table 4-7 ; it is 284 µs.
Thus, the total scan time in this configuration is 680 + 284 = 964 µs. Scan time for more than one sensor is the sum
of the scan time of each sensor.
4.3.7 SmartSense_EMCPLUS Response Time
Consider the following application with standard CSD along with typical CapSense scanning firmware.

Three CapSense sensors with parasitic capacitance of sensor between 5 pF and 10 pF

IMO of 12 MHz and CPU clock of 12 MHz

Sensor sensitivity level of 0.4 pF

Debounce = 3
According to the previous tables, scanning of each sensor requires 482 µs and three sensors have a scan time of
1.45 ms. The following firmware example requires 1 ms for additional firmware execution; thus, the loop execution
time will be 2.45 ms.
while (1)
{
SmartSense_EMC_PLUS_ScanAllSensors();
SmartSense_EMCPLUS _UpdateAllBaselines();
if(SmartSense_EMCPLUS _bIsAnySensorActive() )
{
//1ms firmware routines
}
}