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I'm using an STM32L053 MCU to interface to a poorly-documented PCB module (DRA818V). There are a couple of GPIO driven signals as well as a UART involved, and I want to make sure I'm doing things safely. It seems that I can connect the UARTs directly (RXD to TXD) and nothing else is needed there. For the GPIOs, I'm not so sure. I believe, in the module, the GPIOs just go directly to the MCU inside it. I'm concerned that, when I pull the lines low in the STM32, the other module might source too much current (though, configured as input, I would hope not). Should I have a current-limiting resistor in line on the GPIOs? Would there be any harm in putting a 1k? (i.e., preventing the input from seeing "low")

Additionally, I'd like to drive an LED from one of these lines. I'd just planned to connect the base of a PNP in addition to the connection to the module (the other MCU) -- do I need to do anything special here, beyond the normal base resistor? Specifically, I would branch the GPIO to drive the transistor base and also signal the other MCU.

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You can and probably want to put these 1k resistors in series with your GPIO lines. Remember that if you need blazing speeds series resistors can be a problem because it creates a low pass filter together with the input capacitance of the pins.

About your second question, you can't put the base of a PNP in parallel with anything because you need two terminals to speak of parallel. This site and the whole internet is full of examples on how to turn on a led with a transistor so please try to search a bit and ask another question if you have problems. Nothing more than a base resistor should be needed, you will also need a resistor for the LED obviously.

You probably do not need a BJT at all though, and I don't see any additional safety that comes from it.

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  • \$\begingroup\$ The BJT is because the pins on the STM32L0 are specified to sink or source 8mA only. The LED alone requires 10mA. By parallel, I mean split the GPIO both to the other MCU and to turn on the LED, from a single GPIO signal. \$\endgroup\$
    – David
    Commented May 2, 2015 at 22:44
  • \$\begingroup\$ that's a huge led you've got. if you need only to signal something you can throttle down the current a lot and still see light. you can do so to save power (and components) \$\endgroup\$
    – Vladimir Cravero
    Commented May 3, 2015 at 10:50

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