Regarding MBR signal level and series resistance.

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YuMa_1534086
Level 7
Level 7
Distributor - Macnica (Japan)
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Hello.

Q1)

Below is description of “Getting Started with CapSense”(AN64846).

Do the following contents also apply to MBR3?

In other words, does MBR3 signal level become lower if series resistance increased from 560ohm?

But MBR3 tuning mode is smartsense.

Smartsense automatically tunes parameters such as Fsw etc by MBR3 itself.

Since parameters are auto-tuned, will not user feel too much sensitivity reduction even if series resistance increased?

I think that MBR3 can drive enough sensor-pad if MBR3 uses slower Fsw by auto-tuning even if series resistance becomes bigger.

In this time, I think that signal level does not become lower.

Document description

==========================================

If the series resistance value is set high, the slower time constant of the switching circuit suppresses the emission but limits the amount of charge that it can transfer.

Thus the sensors may not get charged and discharged completely.

This lowers the signal level, which in turn lowers the SNR.

Smaller values are better, but are less effective at blocking RF emissions and interference.

==========================================

Q2)

Below is description of “Getting Started with CapSense”(AN64846).

Do the following contents also apply to MBR3?

Could you please let us know recommended series resistance(min/max) of MBR3?

560ohm to 4.7kohm?

Document description

==========================================

The recommended series resistance for CapSense input lines on general copper PCBs is 560 Ω.

An ITO panel already provides a high resistance; one may not have too much flexibility in the value selection (range 100 Ω–1 kΩ).

Series resistors are generally used in the range of 560 Ω–4.7 kΩ for EMC purpose.

The actual maximum value of the series resistor that can be used varies from device to device.

This depends on multiple factors such as the resistance of the GPIO used as sensor, the switching frequency used to scan sensors, and the SNR required.

==========================================

Q3)

When does MBR3 decide Fsw?

Does MBR3 decide Fsw every scan timing?

(So MBR3 Fsw changes from scan to scan. MBR3 Fsw is not constant value.)

Or does MBR3 decide Fsw when power-up or reset?

(So MBR3 Fsw is constant value until power-up or reset)

Best Regards.

  Yutaka Matsubara

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1 Solution

Hi,

Sorry about the calculation. There is a mistake in the formula.

Fsw = 1 / (10 *R *C)

Also the resistor in the formula is not only the external resistor but it is also the internal resistance that gets added up.

Yes, you are right. There are only 2 methods to change sensitivity in software (Ez- click):

1. Increase finger threshold

2. Varying the sensitivity parameter (fF)

Other than these two you can change the sensitivity only in hardware.

Regards,

Bragadeesh

Regards,
Bragadeesh

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11 Replies
BragadeeshV
Moderator
Moderator
Moderator
First question asked 1000 replies posted 750 replies posted

Hi,

The "Getting Started with CapSense (AN64846)" applies to the MBR3 devices also.

1. The SmartSense adjusts the Sense frequency (Fsw) in accordance to the time constant (RC). So, the signal (raw counts) would get reduced.

If your application requires a higher resistance to be used (> 560 ohm), I would recommend you to check if SNR > 5:1 and ensure that the sensors are completely charged and discharged.

2. The recommended series resistance is 560 Ω. However, the resistance range could be from 560Ω to 4.7 k .

3. Fsw is a constant value that is set during the power up or reset.

Hope this helps.

Thanks,

Bragadeesh

Regards,
Bragadeesh
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YuMa_1534086
Level 7
Level 7
Distributor - Macnica (Japan)
500 replies posted 250 sign-ins 10 likes received

Dear Bragadeesh-san.

Thank you for your answer.

Could you please let us know all Fsw setting value that MBR3 can select?

Best Regards.

Yutaka Matsubara

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Hi,

The Sense frequency can be from 45 kHz to 12000 kHz.

Regards,

Bragadeesh

Regards,
Bragadeesh
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YuMa_1534086
Level 7
Level 7
Distributor - Macnica (Japan)
500 replies posted 250 sign-ins 10 likes received

Dear Bragadeesh-san.

Q1)

Max series resistance value is 4.7kohm.

Could you please let us know why cypress specified 4.7kohm as max value?

Could you please let us know a criteria for determining max value?

Q2)

If series resistance becomes more than 4.7kohm, what will become of IC behavior?

What kind of problems occurs?

Best Regards.

Yutaka Matsubara

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Hi,

1. Let us assume the maximum possible Cp value is 45pF and the minimum sense frequency is 45kHz. You can calculate the Rmax from the formula:

Fsw = 1/(R*C)

R = 1/ (Fsw * C)

From the above calculations we obtain the Rmax value to be 4.7kohms.

The maximum resistor value is given for the worst case scenario.

But in case your Cp value is less that 45pF you can adjust it with a greater series resistance.

Make sure your SNR is greater than 5:1 for a satisfactory performance.

May I know why do you need a higher series resistance value? What is your application?

2) There would not be any problem with the IC behaviour. But if  sense frequency is lesser than 1 / (time constant), then the capacitor would not charge and discharge completely.

This will affect your raw count and thereby the sensor level decreases.

Regards,

Bragadeesh.

Regards,
Bragadeesh
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YuMa_1534086
Level 7
Level 7
Distributor - Macnica (Japan)
500 replies posted 250 sign-ins 10 likes received

Hi

If used 45pF and 45kHz, I think that Rmax becomes 493.8kohm(= 1 / (45kHz*45pF)).

Please confirm it.

Now, application has too sensitivity.

So we want to decrease MBR3 sensitibility to prevent false touch.

But there are few ways to lower MBR sensitibility due to autotuning.

I think that the way to lower MBR sensitibility is as follows.

Is my understanding correct?

Is there any other method other than following?

=====================================================

HW

=> Changed overlay(thickness, material)

=> Increased series resister

SW

=> Changed finger threshold

=> Changed sensitibity config parameter

=====================================================

Best Regards.

Yutaka Matsubara

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Hi,

Can you please share the following information:

1. Your configurations in Ez- Click.

2. Cp value

3. Expected Cf value.

Regards,

Bragadeesh

Regards,
Bragadeesh
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YuMa_1534086
Level 7
Level 7
Distributor - Macnica (Japan)
500 replies posted 250 sign-ins 10 likes received

Hi

We do not have customer's config file.

My previous questions is general question.

So you can answer without customer's information.

Please give me the answer on the theoretical operation of MBR3.

Best Regards.

Yutaka Matsubara

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Hi,

You can reduce sensitivity by increasing the Cp value (Not more than 45pF).

Refer to this document for more information:

http://www.cypress.com/file/46366/download

Regards

Bragadeesh

Regards,
Bragadeesh
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YuMa_1534086
Level 7
Level 7
Distributor - Macnica (Japan)
500 replies posted 250 sign-ins 10 likes received

Hi

If used 45pF and 45kHz, I think that Rmax becomes 493.8kohm(= 1 / (45kHz*45pF)).

Please confirm it.

Is there any way other than the following when lowering sensitivity by config setting?

=================================

=> Changed finger threshold

=> Changed sensitibity config parameter

=================================

Best Regards.

Yutaka Matsubara

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Hi,

Sorry about the calculation. There is a mistake in the formula.

Fsw = 1 / (10 *R *C)

Also the resistor in the formula is not only the external resistor but it is also the internal resistance that gets added up.

Yes, you are right. There are only 2 methods to change sensitivity in software (Ez- click):

1. Increase finger threshold

2. Varying the sensitivity parameter (fF)

Other than these two you can change the sensitivity only in hardware.

Regards,

Bragadeesh

Regards,
Bragadeesh
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