Choosing which lines are Rx and Tx with CSX Capsense

jcsb1994 · ‎Jun 09, 2021

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

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!!

Hari · ‎Jun 10, 2021

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

View solution in original post

Choosing which lines are Rx and Tx with CSX Capsense

Choosing which lines are Rx and Tx with CSX Capsense

Re: Choosing which lines are Rx and Tx with CSX Capsense