ESP8266 Community Forum

- Tue Mar 22, 2016 5:38 am #43773 I wrote (greek-mu-symbol)F. Plain UTF-8, very standard Exactly like you did Code point 0x3bc, 0xce+0xbc in UTF-8.

Anyway, I wrote it wrong, I meant nF, not μF. 1 μF is way too large.

- Tue Mar 22, 2016 6:33 am #43777 Fine. I was certain you didn't mean what you'd typed but thought it best to correct it in case anyone else took it literally.

While I'm here in this topic I'd like to point out that I keep seeing references to on-chip "weak pull-ups" being inadequate so people hang lower-value external resistors in attempt to "better define" a high-impedance input. While it may sound plausible, it has little real meaning. Radio Frequency Interference isn't going to be effectively attenuated, only Electric field - i.e. capacitively coupled charge. This is much less of a problem unless large effective electrical areas are involved.

RFI problems are resolved by presenting low-impedance (@RF frequencies) parallel paths to the supply rails close to the pins, fed by high-impedance (@RF frequencies) series paths i.e. inductance of external wiring etc.

This means for example, that a (allegedly "weak") 27K internal pull-up with external 1nF to GND is going to get you out of trouble where a 10K pull-up alone would not. I think the community may be less familiar with RF issues in general yet with the proliferation of unshielded ESP8266's and bread-boarded prototypes many people are indeed getting into trouble.

- Tue Mar 22, 2016 6:38 am #43778 Thanks for your explanation.

The internal "weak" pull is too weak for I2C usage though, anyway

- Tue Mar 22, 2016 7:15 am #43780 That's often the case but I have to say not always. The I2C protocol relies on open-drain active pull-down and passive resistor pull-up. The latter must charge the capacitance of the bus line in a fraction of the shortest bit-period. This amount of Capacitance depends on the sum of all input/output capacitances and PCB/wiring traces in between.

Sure, simply lob in a 1K pull-up and it's pretty much guaranteed to work for most applications but if what you want is the lowest possible power-consumption for a battery powered design it can be well worth spending time on the analysis.