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Sensing Technologies

jcsb1994
Contributor

Hi,

I am using CSX touchpads with the PSoC 4100S series, and I always have been using Rx lines for the longest side, ie in a 6 x 9 matrix, I use 9 RX lines and 6 TX lines.

This has the disadvantage of using more resistors on the input lines

jcsb1994_0-1623246462597.png

1. But are there any advantages or other considerations when making this choice? Should I just swap my Rx and Tx lines so that I can reduce the footprint?

2. My application requires the best of the best signal integrity for capsense, so I am scared having more Tx lines will increase the overall noise in the circuit, does that make sense?

*You can also tell me if Cypress did not acquire enough data on this and the answer is unknown

Thanks!!

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3 Replies
Hari
Moderator
Moderator

Hello @jcsb1994 

 

It is recommended to use the series resistor on the Tx and the Rx lines. The purpose of the series resistor is to reduce the RF emissions by CapSense and to protect the GPIOs from any ESD events. Since the Tx lines are also prone to be in human contact, we recommend placing the series resistor on the Tx lines as well. 

Since signal integrity is a primary concern, it is recommended to go with a 560ohm series resistor initially and if noise is observed, then you can increase the value of the series resistor as well. 

 

To answer the question regarding Tx and Rx lines, we recommend having more Rx lines than Tx. The reason is to keep the Cp of the Rx sensors lower than Tx sensors that have the following advantages - 

1. Lower Cp of the Rx sensors - This will help in tuning since Tx has higher Cp, we only need to worry if Tx is charging and discharging completely. 

2. Lower Cp also reduces the finger injected noise as the coupling to finger would be lower per Rx line. 

 

You can take a look at CapSense design guide, section 7.5.1 for details on finger injected noise. 

 

Best regards, 
Hari

jcsb1994
Contributor

Thanks, this means my previous versions were flawed in that regard.

1. However, the design guideline clearly states the CSX resistor values should be 2k ohm here

jcsb1994_0-1623424221389.png

but then goes on explaining it is a matter of balancing RF filtering to SNR optimization. I guess this table said 2k because CSX is more prone to parasitic capacitance?

2. I also wanted to know if there is a difference in the trackpad orientation concerning linking the RX lines to the tracks without vias vs linking them to the tracks with vias.

jcsb1994_1-1623424659323.png

Here we see the RX are linked to the tracks that are connected on the top layer, because I thought the vias from the vertical connections would add up to the noise, and I thought it was best practice to keep the cleanest signal in favor of RX vs TX. Does that really change anything or it could have been the other way around (rx as columns and tx as rows in this scenario)

3. I was told in another post that the distance between the pad electrodes can be as low as 0.15 to 0.22mm, this is what I have done here. I used to make it much higher like 0.5 mm for 2.5mm electrodes and the signal was clean. Why is 0.15mm considered safe when we advice 10mil (0.25mm) distance between the tracks linking the capsense pins to the touchpads? I am worried it induces noise when adjacent inactive pins are grounded, could it steal charges from the active lines? I wanted to ask you because you always explain very well when you answer my questions. 

jcsb1994_3-1623425065617.png

 

jcsb1994_2-1623425036865.png

 

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Hari
Moderator
Moderator

Hi @jcsb1994 

 

1. However, the design guideline clearly states the CSX resistor values should be 2k ohm here

A 2kohm is mentioned there as the Tx lines in CSX is digitally driven. So the internal 500ohm series resistance is not present for the CSX electrodes. However, you can use the value of 560ohm as well if you don't expect a lot of noise in the system. 

 

2. That does not impact the system. You can have it either way. In general, it is recommended to route Rx traces such that Cp of Rx is smaller than that of Tx electrodes. But connecting using vias vs the direct connection should not cause a large difference. 

 

3. Could you please clarify this? Is this the distance between the two diamond patterns? Or the distance between the traces and the sensor (diamond pattern)?

The distance between the sensors should be a function of the Cm and the dcm that you obtain. Note that we need to place it 'just right' so that you get linearity in the touch positions without increasing the Cm a lot. This is dependent on the overlay thickness and material as well. 

 

Best regards, 
Hari

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