Datasheet

CY8C20xx6A/H CapSense Design Guide Doc. No. 001-65973 Rev. *A 38

Table 4-5. Sampling Time for a Sensor with IMO = 12 MHz

Sensitivity = 0.2 pF Sensitivity = 0.3 pF Sensitivity = 0.4 pF

ST (µs) (pF) C

ST(µs) (pF) C

ST (µs) (pF)

8 to 10 680 8 to 17 680 8 to 10 340

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

ST (µs) (pF) C

ST(µs) (pF) C

ST (µs) (pF)

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 24-MHz IMO frequency, 6-MHz CPU clock (IMO/4), and

SmartSense sensitivity level of 0.3 pF, the scan time of the sensor that has parasitic capacitance ~15 pF can be

determined from the previous tables using Equation 12.

Sampling time for the above-mentioned configuration (24 MHz of IMO, 0.3 pF of sensitivity) is chosen from Table 4-4;

it is 680 µs. Processing time for the above-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

4.4.5 SmartSense 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 above 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 would be 2.45 ms.

while (1)

{