CapSense Performance Tuning with User Modules
CY8C20xx7/S CapSense
®
Design Guide
Doc. No. 001-78329 Rev. *E
42
To implement a robust electromagnetic compliant algorithm, the SmartSense_EMCPLUS UM uses RAM
memory. As a result, the highest immunity mode (High) needs approximately three times the RAM memory used
in immunity mode Low. Immunity mode Medium uses only about two times more RAM memory than that of
immunity mode Low.
4.3.5.2 IMO Tolerance and Time Critical Task
IMO tolerance for SmartSense_EMC-enabled parts is +5 percent and
–20 percent.
When implementing time-critical algorithms and logic, you must consider IMO tolerance to make sure that the
firmware logic or algorithm does not break.
If a project uses interrupts, you should consider IMO tolerance while analyzing interrupt latency, ISR execution
time, and so on.
Every timing analysis that depends on the IMO (for example, a timer clocked by IMO, delay created using loop in
firmware, and API execution time) must consider the IMO tolerance to ensure robust application firmware.
4.3.5.3 I
2
C Operating Speed
I
2
C interface operation frequency is limited to a maximum of 80 percent of the actual operating frequency of the user
module in the SmartSense_EMC-enabled parts. This limitation is caused by the 20-percent IMO tolerance.
This means, when clock speed of 400 kHz is selected in the I
2
C user module, the I
2
C interface can be operated
to maximum of 320 kHz. Similarly, operating frequency is limited to a maximum of 80 kHz and 40 kHz when
100-kHz and 50-kHz clock modes, respectively, are selected in the I
2
C user module.
While using the I
2
C slave interface, the master clock should operate within the reduced specification mentioned
earlier. Not doing this will lead to data corruption, I
2
C bus conjunction, or inconsistent behavior from the I
2
C user
module.
Using the I
2
C master module impacts only the throughput of the interface.
When using I2CSBUF user module, it is strongly recommended use auto-nack mode while CPU accessing the 32-
byte dedicated buffer and when device put into sleep or deep sleep modes. See the I2CSBUF user module datasheet
details information.
4.3.6 Scan Time of a CapSense Sensor
To maintain the consistent finger response sensitivity over a wide range of parasitic capacitance, the
SmartSense_EMCPLUS User Module automatically determines the hardware parameters of the user module. As a
result, sensor scan time does not remain constant. For a design in mass production, it could vary based on the
parasitic capacitance variation of the PCB.
The total scan time of a sensor is decided by four factors. They are parasitic capacitance of sensor, IMO frequency,
CPU operating frequency, and sensitivity level of the SmartSense_EMCPLUS User Module.
Scan time of a sensor can be found using Equation 4-3 and the following tables.
𝑆𝑐𝑎𝑛 𝑡𝑖𝑚𝑒 = 𝑆𝑎𝑚𝑝𝑙𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 𝑆𝑇 + 𝑃𝑟𝑜𝑐𝑒𝑠𝑠𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 (𝑃𝑇)
Equation 4-3
The following tables show the sampling time value with various IMO and sensitivity levels.
Table 4-4. Sampling Time for a Sensor with IMO = 24 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 10
340
8 to 17
340
8 to 10
170
10 to 23
680
17 to 35
680
10 to 23
340
23 to 41
1360
35 to 41
1360
23 to 41
680
41 to 45
2730
41 to 45
2730
41 to 45
1360
Table 4-5. Sampling Time for a Sensor with IMO = 12 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 10
680
8 to 17
680
8 to 10
340
