
1. Re: Regarding how to check CSX mutual capacitance.
AH_96 Oct 30, 2019 8:14 AM (in response to YuMa_1534086)Hello YuMa_1534086
We don't have an API that directly gives the mutual capacitance value. Please refer this KBA Mutual Capacitance Calculation of a CapSense Sensor Through Firmware  KBA222425 for details on how to calculate Cm for your design.
Thanks and regards
Hari

2. Re: Regarding how to check CSX mutual capacitance.
YuMa_1534086 Oct 30, 2019 6:42 PM (in response to AH_96)Harisan.
Q1)
In KBA222425, Cm is calculated based on RawCount is outputted from PSoC4.
But I think that PSoC4 should have calculated Cm before calculating RawCount.
Because Cm value is included in CSX RawCount formula.
How does PSoC4 calculate Cm internally before calculating RawCount?
Q2)
Does Cypress have any plans to add API to calculate Cm directly in the future?
Unlike CSD, CSX cannot calculate Cm directly, so it is difficult to judge whether PSoC can drive sensors.
Best Regards.
Yutaka Matsubara

3. Re: Regarding how to check CSX mutual capacitance.
AH_96 Nov 3, 2019 9:59 PM (in response to YuMa_1534086)Hello Yutakasan
1. Yes. Cm is a physical parameter which dictates the value of rawcounts. PSoC does not calculate raw counts with value of Cm, rather, the value of Cm determines the raw counts. When Cm increases, raw counts also increase and when Cm decreases, raw counts decrease.
2. We have forwarded this to our internal team who will look into this request. Thank you for the suggestion and your interest in Cypress products.
Please note that driving the sensors still depend on parasitic capacitance (Cp) of the Tx electrode and not on the mutual capacitance formed (Cm).
Thanks and regards
Hari

4. Re: Regarding how to check CSX mutual capacitance.
YuMa_1534086 Nov 5, 2019 6:54 PM (in response to AH_96)Harisan.
Does PSoC not calculate Cm internally?
If PSoC does not calculate Cm, I think that CSX RawCount formula in design guide is wrong.
Because RawCountCounter formula contains Cm parameter.
And CSX RawCount formula is “MaxCount – RawCountCounter".
So if design guide formula is correct, PSoC should calculate Cm value at first.
Because PSoC can not calculate CSX RawCount if PSoC can not calculate Cm.
Is CSX RawCount formula in design guide wrong?
Or if formula is correct, how does PSoC4 calculate Cm internally before calculating RawCount?
Best Regards.
Yutaka Matsubara

5. Re: Regarding how to check CSX mutual capacitance.
AH_96 Nov 5, 2019 10:05 PM (in response to YuMa_1534086)Hello Yutakasan,
The formula given in design guide is a mathematical representation of the physical parameter's calculation. The CapSense component in the controller does not calculate raw counts using this formula. Rather, there is an electrical circuit that converts the physical parameter (Cm) into digital quantity (raw counts).
In CSX design, there is a Tx electrode and an Rx electrode and a mutual capacitance is formed between them (Cm). The Tx electrode is switched with a square wave that switches between VDDIO and ground. There are two switches, SW1 and SW2 which connect to integrating capacitors. During one half cycle, the voltage on CintA increases above Vref. IDAC is configured in sinking mode which brings the voltage back to Vref and during this time, the counter is ON. During the next cycle, CintB is connected and voltage drops below Vref (since Tx is LOW). IDAC is in sourcing mode and brings the voltage back to Vref. Again, the counter is ON during the time in which the CintB is not Vref.
This happens for a number of cycles (called number of subconversions and is set in the component) and the sum of all subconversions is the raw counts that we observe.
This raw counts is indeed based on the mutual capacitance formed by the equation given, but the equation is not the way we obtain the raw counts in the controller. The equation provides the mathematical approximation for the raw counts that is expected and is provided in the design guide to help in tuning the sensors.
You can refer CapSense Design guide for more details on CSX operation.
Thanks and regards
Hari

6. Re: Regarding how to check CSX mutual capacitance.
YuMa_1534086 Nov 10, 2019 10:47 PM (in response to AH_96)Harisan.
Thank you for response.
>>> The CapSense component in the controller does not calculate raw counts using this formula.
Same as CSX, does not CSD use below formula when CSD calculates RawCount?
CSD has API for calculate Cp value.
Does this API just calculate Cp by use above formula?
Best Regards.
Yutaka Matsubara

7. Re: Regarding how to check CSX mutual capacitance.
AH_96 Nov 11, 2019 3:43 AM (in response to YuMa_1534086)Hi Yutakasan
Yes. That is correct. CSD also has hardware component that measures the raw counts.
The API just back calculates Cp from the equation since raw counts is already present. The method of calculation is like the KBA that was shared (since all other parameters are known to the controller, Cp can be calculated).
Best regards
Hari

8. Re: Regarding how to check CSX mutual capacitance.
YuMa_1534086 Nov 13, 2019 4:59 PM (in response to AH_96)Harisan.
Thank you for your response.
I understand.
Best Regards.
Yutaka Matsubara