For example, I'm using an Exar Exar SP3220EBCY-L RS-232 transceiver and the typical application loop back test shows that the EN pin is connected to GND and the SHDN pin is connected to VCC. Should I take this literally or am I suppose to assume that I'll need a pull-up/pull-down resistor? enter image description here


1 Answer 1


It is usually fine to directly connect inputs to either GND or VCC. The datasheet of the device should be clear on that otherwise.


  1. when using devices like microcontrollers that have tri-state pins that can be configured both as input or output, then it is good practice to use a resistor (eg. 1k) to prevent the GPIO pin from being overloaded when accidentally being mis-configured.
  2. when pulling for example a RESET pin inactive without a resistor, you can never activate it without changing the circuit. If you had a pull up/down resistor in place, you could manually override it.
  • \$\begingroup\$ Thanks jippie! 1. I've seen 1k resistors being used between GPIO pins, but how do they calculate the 1k? Is it the default value for a uC with 3.3V VCC? By misconfigured, do you mean that the GPIO pin is configured for higher than need current output? \$\endgroup\$ Commented Dec 6, 2013 at 21:50
  • \$\begingroup\$ Misconfigured as an output instead of an input. If configured as an output and driving the opposite direction as the short to Vcc/Ground, it will pass excessive current and may be damaged. \$\endgroup\$
    – DoxyLover
    Commented Dec 6, 2013 at 22:38
  • \$\begingroup\$ @JustinManuel 1k, 10k, 100k, all probably work fine. You have to check the datasheet for maximum pin output drive current and maximum package current. The higher the resistance, the more sensitive it gets. \$\endgroup\$
    – jippie
    Commented Dec 6, 2013 at 23:38

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