Suppose a device with MSP430 or AVR microcontroller and floating UART lines (e.g. to external connector). Need I add an external pull-up to the RXD line?

  • \$\begingroup\$ Isn't this information in the respective data sheets? Have you looked? \$\endgroup\$
    – Joe Hass
    Commented Aug 23, 2013 at 15:34
  • \$\begingroup\$ @JoeHass: Yes, I've looked it. I didn't find any information for my question, but there are no pull-up resistors showed in block diagrams. \$\endgroup\$
    – user54579
    Commented Aug 23, 2013 at 15:45
  • \$\begingroup\$ Please add a link to the datasheets of both chips. \$\endgroup\$
    – jippie
    Commented Aug 23, 2013 at 15:52
  • \$\begingroup\$ Perhaps you need to clarify your question. Are you asking whether specific devices have internal pullups or are you asking in general whether a UART RX input needs a pullup? If it's the former, you need to dig deeper in the literature. \$\endgroup\$
    – Joe Hass
    Commented Aug 23, 2013 at 15:59
  • \$\begingroup\$ @JoeHass: Almost sure it don't differ inside a family. But maybe it differ between AVR and MSP430 families. \$\endgroup\$
    – user54579
    Commented Aug 23, 2013 at 16:02

1 Answer 1

  1. Find the datasheet of the device
  2. Figure out which pins have the UART functionality
  3. Figure out if the output drivers for these pins support pull up / down resistors
  4. Check the text of the I/O pin drivers in the datasheet if any exceptions are made for UART mode

For AVR all pins support pull up (except Vcc, GND and except RESET to some extent) you can turn on pull up resistors by setting Data Direction Register for a pin as input and enable the pull up by setting the corresponding pin in the PORT register.

  • \$\begingroup\$ In MSP430 user can switch pin between perephiral modules. If I set up internal pull-up resistor in GPIO, does it stay when I switch pin to UART? \$\endgroup\$
    – user54579
    Commented Aug 23, 2013 at 16:05
  • \$\begingroup\$ Not really familiar with MSP430 series. \$\endgroup\$
    – jippie
    Commented Aug 23, 2013 at 16:09
  • \$\begingroup\$ You should be looking at the INPUT docs, not the output driver ones. \$\endgroup\$ Commented Aug 23, 2013 at 16:45
  • \$\begingroup\$ Updated the text. \$\endgroup\$
    – jippie
    Commented Aug 24, 2013 at 20:43

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