ESP8266 Community Forum

- Wed Jan 03, 2018 3:51 pm #72864 Hi guys,

So, Im better with SW than with HW, so here's a real newbue question;

I would like my ESP8266 device to detect whether a 3v3 power-supply (not the one powering my device) is on or off.

I only have TX and RX free on my device. Would I be able to just connect the 3v3 power supply to the RX pin, and if so, how should I wire this? (+ to RX?)
Thanks for your patience

- Mon Jan 08, 2018 11:44 am #72969 Sorry for bumping my own post, but it took a while for it to appear so it turned up far down the list and hardly got any views. Would be grateful for any tips.

- Mon Jan 08, 2018 1:12 pm #72971 My experience with RX is that if it is not high at startup/reset the ESP module doesn't work correctly. Now I always have a pull up resistor on RX. I know the RX pin can be used as an I/O pin but the program must be running before that can happen.

I think you would be better off trying to use TX as the input pin. Because TX is an output I would put a resistor in series with it. I think that 470 Ohms to 2k2 would be good values. One side to the TX pin and the other to the 3.3v you want to monitor. In addition to that I would add a resistor from that 3.3v to ground to make sure that it goes low when the power is not present. Without some resistance the voltage may go low but not to zero and the ESP8266 may not see it as a low.

- Mon Jan 08, 2018 5:19 pm #72980 I am intrigued by this RX dependency as I have seen it reported before but haven't experienced it myself at all. There have also been long threads discussing this but without seeming to come to a definitive conclusion. I wonder whether it is dependent on the overall module design rather than the esp8266 chip itself.

On the ESP12-F modules I normally use I don't see it. I also just tried loading the RX with various external pull down resistors to 0V and got down to a 1K pull down without any reset /start up issues. The RXD pin was at a value close to 0V with this.

As far as I know looking at ESP-12F schematics there are no external pullups inside the module itself and when I measure the characteristic of the RX pin it seems to have a 37K pull up during reset and also when running normally just like the other pins which default to the standard input pull up weak value.