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esp32-dev/pcb/electrical-constraints.md
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suyushi 5700eb4115 Initial: knowledge base
2026-08-05 17:25:09 +08:00

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Electrical Constraints Reference

类别：reference | 主题：电气约束通用规范 | 信源： 多平台汇总

Table of Contents

Platform Comparison Summary

  • Logic voltage：Raspberry Pi=3.3V, ESP32=3.3V
  • Max per-pin current：Raspberry Pi=16mA source/sink, ESP32=40mA max (20mA recommended)
  • Aggregate GPIO current：Raspberry Pi=50mA total, ESP32=~1200mA total (chip limit)
  • 5V tolerant：Raspberry Pi=NO, ESP32=NO
  • Internal pull-up：Raspberry Pi=~50kΩ, ESP32=~45kΩ typical
  • Internal pull-down：Raspberry Pi=~50kΩ, ESP32=~45kΩ typical
  • Drive strength：Raspberry Pi=Fixed, ESP32=Configurable (5-40mA)
  • Input threshold (VIH)：Raspberry Pi=~1.8V, ESP32=~2.0V
  • Input threshold (VIL)：Raspberry Pi=~0.8V, ESP32=~0.8V

Raspberry Pi Electrical Details

Voltage Levels

  • All GPIO pins operate at 3.3V ONLY
  • 5V on any GPIO pin WILL PERMANENTLY DAMAGE the SoC
  • No built-in overvoltage protection
  • No built-in ESD protection (handle with care)

Current Limits

  • Per-pin source：16mA, Consequence of Exceeding=Voltage droop, pin damage
  • Per-pin sink：16mA, Consequence of Exceeding=Voltage rise, pin damage
  • Total GPIO：50mA, Consequence of Exceeding=Instability, crashes, permanent damage

Critical: The 50mA limit is across ALL GPIO pins combined, not per-bank.

Power Rails

  • 3.3V：Source=Onboard regulator, Available Current=~50mA for peripherals, Notes=Shared with Pi's 3.3V needs
  • 5V：Source=USB/PSU direct, Available Current=1-2A minus Pi consumption, Notes=No regulation, direct pass-through
  • GND：Source=Common ground, Available Current=N/A, Notes=8 ground pins on header

Internal Pull Resistors

  • GPIO0-8：Default State=Pull-UP, Resistance=~50kΩ
  • GPIO9-27：Default State=Pull-DOWN, Resistance=~50kΩ

Note: Internal pulls are too weak for I2C (need 4.7kΩ external).

Safe Driving Patterns

  • Single LED：Method=330Ω-1kΩ series resistor, Notes=3-10mA safe
  • Multiple LEDs：Method=Transistor driver, Notes=If total >50mA
  • Relay：Method=Transistor/MOSFET + flyback diode, Notes=Never direct from GPIO
  • Motor：Method=Motor driver IC (L298N, DRV8833), Notes=Never direct from GPIO
  • Buzzer (passive)：Method=Transistor driver, Notes=Inductive load
  • Buzzer (active)：Method=Direct if <16mA, Notes=Check current draw

ESP32 Electrical Details

Voltage Levels

  • All GPIO pins operate at 3.3V ONLY
  • 5V on any GPIO pin WILL DAMAGE the chip
  • No built-in overvoltage protection
  • Some ESD protection but don't rely on it

Current Limits

  • Per-pin max：40mA, Notes=Absolute maximum
  • Per-pin recommended：20mA, Notes=For reliability/longevity
  • Total chip：~1200mA, Notes=Includes WiFi, BT, CPU

Power Consumption

  • Active + WiFi TX：Current Draw=80-240mA, Notes=Peaks during transmission
  • Active + WiFi idle：Current Draw=20-68mA, Notes=Connected but not transmitting
  • Active, no radio：Current Draw=20-68mA, Notes=CPU running
  • Modem sleep：Current Draw=3-20mA, Notes=WiFi paused, CPU active
  • Light sleep：Current Draw=0.8mA, Notes=CPU paused, RTC running
  • Deep sleep：Current Draw=10-150µA, Notes=Only RTC + ULP available

Internal Pull Resistors

  • Standard GPIO：Pull-up=Yes, Pull-down=Yes, Resistance=~45kΩ
  • GPIO34-39：Pull-up=NO, Pull-down=NO, Resistance=N/A (input-only)

Note: Internal pulls are too weak for I2C (need 4.7kΩ external).

Drive Strength Configuration

  • 5mA：Current=Weakest, Use Case=Low power, slow signals
  • 10mA：Current=Low, Use Case=General purpose
  • 20mA：Current=Default, Use Case=Most applications
  • 40mA：Current=Maximum, Use Case=Fast edges, heavy loads

Higher drive strength = faster edges but more EMI/noise.

Pull-up and Pull-down Resistors

When Required

  • I2C bus (SDA)：Pull Type=Pull-UP, Typical Value=4.7kΩ
  • I2C bus (SCL)：Pull Type=Pull-UP, Typical Value=4.7kΩ
  • 1-Wire bus (DQ)：Pull Type=Pull-UP, Typical Value=4.7kΩ
  • Button to GND：Pull Type=Pull-UP, Typical Value=10kΩ
  • Button to VCC：Pull Type=Pull-DOWN, Typical Value=10kΩ
  • SPI CS line：Pull Type=Pull-UP, Typical Value=10kΩ
  • Open-drain output：Pull Type=Pull-UP, Typical Value=1-10kΩ
  • UART RX (optional)：Pull Type=Pull-UP, Typical Value=10kΩ (noise immunity)
  • Reset line：Pull Type=Pull-UP, Typical Value=10kΩ

Calculation Formula

R = (VCC - VOL) / IOL

Where:
  VCC = Supply voltage (3.3V)
  VOL = Output low voltage (~0.4V)
  IOL = Required sink current (3mA for I2C)

Example (I2C):
  R = (3.3V - 0.4V) / 3mA = 967Ω minimum
  Typical choice: 4.7kΩ (provides margin)

Strength Guidelines

  • 1kΩ：Use Case=Long wires, high capacitance, fast I2C, Notes=Strong pull, higher current
  • 4.7kΩ：Use Case=Standard I2C, 1-Wire, general purpose, Notes=Most common choice
  • 10kΩ：Use Case=Buttons, CS lines, low-power, Notes=Standard digital pull
  • 47-100kΩ：Use Case=Wake-up inputs, ultra-low power, Notes=Very weak, slow rise time

Consequences of Wrong Value

Too high (weak pull):

  • Slow signal rise times
  • Noise susceptibility
  • Communication errors at higher speeds
  • May not reach valid HIGH level

Too low (strong pull):

  • Excessive current consumption
  • Device may not be able to pull line LOW
  • Wasted power in battery applications

Level Shifting

When Required

  • 5V logic output → 3.3V GPIO input
  • 3.3V GPIO output → 5V input (if device doesn't recognize 3.3V as HIGH)
  • Bidirectional communication between 3.3V and 5V systems

Method 1: Voltage Divider (5V → 3.3V, Unidirectional)

Use for: Slow signals (<100kHz), input direction only

5V Signal ──[1kΩ]──┬──> 3.3V GPIO Input
                   │
                 [2kΩ]
                   │
                  GND

Output: 5V × (2kΩ / 3kΩ) = 3.33V
  • Simple, cheap：Cons=Input direction only
  • 2 resistors：Cons=Slow (RC time constant)
  • No active components：Cons=Loads the signal

Method 2: N-Channel MOSFET (Bidirectional)

Use for: I2C, 1-Wire, open-drain signals up to 400kHz

3.3V Side                              5V Side
    │                                      │
  [4.7kΩ]                              [4.7kΩ]
    │                                      │
    ├────────┬──────────────────┬──────────┤
    │        │                  │          │
   SDA    Source              Drain      SDA
  (3.3V)     └───── BSS138 ─────┘       (5V)
                      │
                    Gate
                      │
                    3.3V

Operation:

  • Gate tied to LOW side voltage (3.3V)

  • When LOW side pulls down, MOSFET conducts, pulling HIGH side down

  • When HIGH side pulls down, body diode conducts, pulling LOW side down

  • Pull-ups restore HIGH state on both sides

  • MOSFET：Specification=BSS138, 2N7000 (through-hole)

  • Pull-ups：Specification=4.7kΩ on each side

  • Voltage：Specification=3.3V on gate, low side; 5V on high side

Method 3: Dedicated Level Shifter ICs

  • TXB0104：Channels=4, Type=Auto-direction, Speed=100 Mbps, I2C Safe?=NO
  • TXB0108：Channels=8, Type=Auto-direction, Speed=100 Mbps, I2C Safe?=NO
  • PCA9306：Channels=2, Type=I2C-specific, Speed=1 MHz, I2C Safe?=YES
  • PCA9517：Channels=2, Type=I2C buffer, Speed=400 kHz, I2C Safe?=YES
  • 74LVC245：Channels=8, Type=Unidirectional, Speed=100 MHz, I2C Safe?=N/A (direction pin)
  • BSS138 modules：Channels=4, Type=Bidirectional, Speed=400 kHz, I2C Safe?=YES

CRITICAL WARNING: TXB-series level shifters do NOT work reliably with open-drain protocols (I2C, 1-Wire). They fight the pull-up resistors and cause communication errors. Use BSS138-based modules or PCA9306 for I2C.

Method 4: Direct Connection (3.3V → 5V Input)

Many 5V devices recognize 3.3V as logic HIGH:

  • VIH (HIGH threshold)：Typical 5V TTL=2.0V, Typical 5V CMOS=3.5V
  • VIL (LOW threshold)：Typical 5V TTL=0.8V, Typical 5V CMOS=1.5V

Check datasheet for VIH. If VIH < 3.0V, direct connection usually works.

NEVER connect 5V output directly to 3.3V input — level shift or divide required.

Common Mistakes and Warnings

NEVER Do This

  • Connect 5V directly to any GPIO：Consequence=Permanent chip damage
  • Drive relay coil directly from GPIO：Consequence=Inductive kickback damages GPIO
  • Drive motor directly from GPIO：Consequence=Overcurrent, voltage spikes
  • Exceed 50mA total on RPi GPIO：Consequence=Voltage instability, damage
  • Use ESP32 GPIO6-11 (WROOM)：Consequence=Flash pins — chip crashes
  • Pull GPIO12 HIGH at boot (ESP32)：Consequence=Flash voltage brick
  • Forget pull-ups on I2C：Consequence=Communication failure
  • Forget pull-up on 1-Wire：Consequence=Bus doesn't work
  • Use TXB-series for I2C：Consequence=Unreliable communication
  • Assume GPIO is 5V tolerant：Consequence=It's not — damage results

ALWAYS Do This

  • Use current-limiting resistor for LEDs：Reason=Prevents overcurrent (220-330Ω)
  • Use flyback diode with relays/motors：Reason=Catches inductive voltage spike
  • Use level shifter for 5V ↔ 3.3V：Reason=Protects GPIO from overvoltage
  • Check total current draw：Reason=Prevent exceeding limits
  • Verify I2C addresses before wiring：Reason=Detect conflicts early
  • Add 100nF decoupling capacitor near ICs：Reason=Reduces noise, improves stability
  • Use external pull-ups for I2C (4.7kΩ)：Reason=Internal pulls too weak
  • Check ESP32 pin restrictions：Reason=Strapping, flash, input-only
  • Use transistor for loads >16mA：Reason=Protects GPIO
  • Add ESD protection for external connectors：Reason=Protects against static

Quick Reference Card

Formulas

LED Resistor:

R = (VCC - Vf) / If
R = (3.3V - 2.0V) / 10mA = 130Ω minimum
Recommended: 220-330Ω (5-10mA, plenty bright)

Voltage Divider:

Vout = Vin × (R2 / (R1 + R2))
For 5V → 3.3V: R1=1kΩ, R2=2kΩ

Pull-up Resistor:

R = (VCC - VOL) / IOL
Standard: 4.7kΩ for I2C/1-Wire, 10kΩ for buttons

Quick Values

  • LED resistor (3.3V, red/green)：220-330Ω
  • LED resistor (3.3V, blue/white)：100-150Ω
  • I2C pull-up：4.7kΩ to 3.3V
  • 1-Wire pull-up：4.7kΩ to 3.3V
  • Button pull-up/down：10kΩ
  • SPI CS pull-up：10kΩ
  • 5V → 3.3V divider：1kΩ + 2kΩ
  • Flyback diode：1N4148 or 1N4007
  • Decoupling capacitor：100nF ceramic

Current Limits Summary

  • Raspberry Pi：Per Pin=16mA, Total GPIO=50mA
  • ESP32：Per Pin=20mA recommended, Total GPIO=~1200mA chip total

Voltage Summary

  • Logic HIGH：RPi=3.3V, ESP32=3.3V
  • Logic LOW：RPi=0V, ESP32=0V
  • Max input：RPi=3.3V, ESP32=3.3V
  • 5V tolerant：RPi=NO, ESP32=NO

Common Pin Restrictions

Raspberry Pi:

  • GPIO0/1: Reserved (HAT EEPROM)
  • GPIO14/15: UART/BT conflict (Pi 3/4/Zero2W)

ESP32:

  • GPIO6-11: Flash pins — NEVER USE
  • GPIO12: Strapping — DANGER (flash voltage)
  • GPIO16-17: PSRAM (WROVER only)
  • GPIO34-39: Input only, no pulls
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