Resistor pull-ups for ESP8266

Question

I'm using 4.7kOhm resistor as pullups for GPIOs 2,0,2,16, CH_PD and RESET, and also 4.7kOhm pulldown for GPIO 15. The ESP8266 boots and works fine. Would it be okay for a long shot? Should I use 10kOhm instead of 4.7kOhm?

Answer 1

In general it makes little or no difference what the value of resistor you use is.

The only things it can affect are:

• The current consumption while a button is pressed - a lower value resistor will cause more current to flow while for the time the button is pressed (it makes no difference at other times).
• The LOW voltage (V_IL) read at the IO pin if the button has a high resistance (for example if it's on the end of a 500m length of cable).
• Noise immunity - a lower value resistor will pull the pin up "harder" making it more difficult for noise induced in a long wire (see the previous point, too) from causing a false button press (although this is less of a concern with modern digital inputs that tend to all have Schmitt triggers).

10k is often chosen not because it's "better" than any other value, but purely because it's more common that many other values, and it's in the right kind of ball-park for most setups. It's low enough to give good noise immunity while being high enough to not cause undue current flow while the button is pressed. But more importantly, it's a resistor that everyone has in their kit, and is most likely the most common resistor used in a number of other places in the circuit as well. It's a good all-rounder as far as resistors go.

To take a closer look at the current flow aspect - if you are running (as with an ESP8266) at 3.3V and you have a 10k resistor the current flow, when the button is pressed and the resistor connected directly to ground, though the resistor would be (I=V/R) 3.3/10,000 = 330uA. If you used a 4.7k resistor (as you are doing at the moment) the current rises to 3.3/4,700 = 702uA. Yes, that's just over double the current, but twice peantus is still peanuts. So 4.7k, under most general circumstances, is perfectly fine.

However if power consumption is a real concern and you want to keep all consumption to an absolute minimum switching to 100k (as you often see in low-power circuits) would be beneficial, since 3.3/100,000 is 33uA.

If your input is connected to a momentary button which will be pressed for fractions of a second at a time it's not that much of an issue. But if the input is connected to a switch that could be activated for days or weeks at a time it obviously becomes of greater interest.

So in short: when running on batteries higher value pullup resistors are better, but when running off a power supply it really makes no difference.