ESP8266 Community Forum

Hi folks, fist post.

I'm following the schematics that are ESP8266.ru for using the UART pins to flash firmware.

The link is this: https://translate.google.com/translate? ... &sandbox=1

My Question has to do with the following schematics:



As we can see the RTS pin connects to the Reset pin but also it has a Reset button that pulls that pin to ground.
My question is, normally RTS pin is HIGH and pressing RESET won't short circuit the UART?

Hi, RST can be connected directly to Vcc or GND without the risk of dammage. It is one of the few pins this is true for though.

You're correct there is potential for a short here, depending on the type of serial adapter.

I've seen RTS on many adapters is a push/pull output, meaning it can drive high to 3.3V or low to 0V. On other adapters its an open drain output, which means it only drives low and the "off" state is floating (no connection) and relies on a pull up resistor.

With a push/pull output, you're correct that when the RTS output is driving high the reset button will short RTS to ground and may damage the RTS pin. You can prevent this by inserting a 470 ohm (or similar lowish value) series resistor to limit the current from the RTS pin.

(Note that if RTS is push/pull and always connected then you also don't need the 10K pullup on the reset line either, because RTS will drive the line high by itself. Keeping this pullup resistor and adding the series resistor means the circuit will work well with both kinds of RTS output, though.)

Thanks for the info.

I'm using the CP2102/9 USB adapter, and in fact I've added a resistor in series with the Reset pin to limit the current. I completely forgot of the open drain output type, but as far as I can see on the CP2102/9 only the Reset pin is of such type.

So to be safe it is better to have a current limiting resistor.