Very new to EDA and PCB work so please bear with me. Implementing the circuit in the image and clueless as to the value of the three resistors connected to VDD and IRQ / SCL / SDA. The datasheet provided by NXP for the MPR121 doesn't explain why you would attach them.

 

hi cyth,
Welcome to AAC.
This PDF should help you determine the Pull-Up resistor values for your circuit.
Ask if you have a problem.
E

 

Those resistors are pullups for I2C SCL and SDA, and /IRQ.

The data sheet does mention 4.7k resistors for the I2C pins, and the same value or 10k should work for /IRQ.

 

Ah, I see. Thank you!

 

The I2C pullups are somewhat critical.

The /IRQ pullup up to 10K will be fine.

The is a 3.3 volt part, so you'll need level shifters if using a 5v micro.

 

Calculating the value of pull-up and pull-down resistors in a circuit is a standard exercise students are given in an introductory digital electronics course.

There is no single solution. Instead of arriving at a single value, students are asked the determine the minimum and maximum values that still present valid logic low and high voltages given what is known of the source sink and source capability and the load requirements.

For high frequency applications, signal frequency, rise and fall times, and circuit capacitances need to be taken into consideration.

Finally, with bussed signals, determining fan-in and fan-out is also important.