I have completed my customized Arduino schematics using KiCad. I have been using MCP2200 for usb-to-uart conversion. After completing it, i was having a look at Arduino Uno Schematics available at Arduino website. I have following questions:

  1. Arduino uno has used resistor of 1K in between TX/RX pins of ATMEGA8U2-MU and Atmega328. I am not quite sure why they have been used? To limit the current when Arduino TX/RX pins are used as GPIO?

  2. I want to power my board from USB power. I am using capacitor of 10uF to remove noise. Is it a right way to do? Although, i am having a regulator LM1117-3.3 and it's output is being connected to a pin. It is there to power other off-board modules and hence, i am not using it for on-board components.

If you find any other problem with my schematics or layout, please mention them in your comments/answers.

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1 Answer 1

  1. Correct. In that case, you want the signal from the socket header to driver stronger than the signal from the USB controller, which will always drive its Tx line. If you don't connect this signal to the outside, the resistor is unnecessary. There is no such resistor in the other direction, because the Arduino port is fully controllable.

  2. One large cap (10u or more), and one small per-chip cap (100n) directly next to it is what I would do.

You also need something on the USB side (most likely, DTR connected to RESET through a 100n cap) so you can trigger a reset for programming.

  • \$\begingroup\$ I have a confusion for your first point. What if i changed mode of TX pin to GPIO and make it LOW? Then, there will be a short circuit between Vdd of MCP2200 and GND of arduino? \$\endgroup\$
    – abhiarora
    Commented Apr 15, 2017 at 4:57
  • 1
    \$\begingroup\$ The Arduino's Tx pin is already an output, connected to an input port on the USB controller, so this is safe. The resistor is to avoid output vs output if you change the Rx pin from input to output. \$\endgroup\$ Commented Apr 15, 2017 at 23:47

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