I have some beginner-level questions about this ESP-WROOM-32 schematic.

[seanmchugh513]

I found the shematic here https://github.com/StudioSophisti/ESP-WROOM-32-Eagle-Breakout. Here is the esp-wroom-32 data sheet https://www.espressif.com/sites/default/files/documentation/esp32-wroom-32_datasheet_en.pdf.

Questions:

1. In the data sheet they say the operating voltage is 3-3.6 volts. But on the github page they say the operating voltage is 2.2-3.6 volts. How is that possible?
2. What is the point of the resistors I highlighted?
3. How did they know to put a capacitor on the VCC? I happened to know that from playing around with ESP32's but if you were just looking at the data sheet how would you know? It doesn't say anywhere to use a capacitor.
4. Super dumb. In this schematic current is flowing from VCC into ground, right? Does it matter if the capacitor is before or after the ESP-WROOM-32's VCCs pin?

 

[wwfeldman]

1) i expect there is a typo on github
manufacturer's data sheet is boss.

2) the two highlighted resistors are pull ups.
EN and IO0 (?) connect somewhere on the ESP-WROOM-32 (U3)
when the switch is open, the resistors connect those pins to VCC
and so are pulled high, logic 1 (hence pull up)
when the switch is pressed, those pins are connected to ground (logic 0)

3) it is standard practice to put a small bypass capacitor on every power pin of every
digital IC.
sometimes it is 0.1 uF, sometimes 1.0 uF, sometimes they alternate.

the purpose is to help hold up the VCC when the chip draws current
and to bypass high freqency signals to ground
sometimes, small series resistor is included, with the capacitor
between the resistor and the power pin

look up bypass capacitor

4) I don't know what "... the capacitor is before or after the ESP-WROOM-32's VCCs pin?" means.
Bypass capacitors should be placed as close to the VCC pin as possible.
what do you mean by before or after?

 

[JuhaH]

2. to prevent VCC being shorted to ground when button pressed.
3. common practise, won't hurt to add a cap.
4. close to pin on pcb is what matters

 

[KlausST]

Hi,

How did they know to put a capacitor on the VCC? I happened to know that from playing around with ESP32's but if you were just looking at the data sheet how would you know? It doesn't say anywhere to use a capacitor.

The informtion in the datasheet is the power supply voltage specification. The power supply needs to be within it's limits all the time.
A DVM is not useful to verify this.
Because when the circuit (controller) is working it will draw short current spikes...maybe down to the 10s of picoseconds. These current spikes .. without capacitors ... will cause the voltage to drop below the specified range. ...because of the power supply impedance. A DVM can't recognize this high speed voltage drops.
It's even hard to measure the voltage spikes with a scope. And every mm of trace will modify the signal waveform on the VCC pin ... and on the scope.

Thus it's a general rule to use
* a solid, low impedance GND plane (as clean and reliable voltage reference for supply as well as logic signals)
* one fast ceramics capacitor as close as possible at each supply pin of every IC
This does not need to be mentioned in every IC's datasheet.

Klaus