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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](#platform-comparison-summary)
- [Raspberry Pi Electrical Details](#raspberry-pi-electrical-details)
- [Voltage Levels](#voltage-levels)
- [Current Limits](#current-limits)
- [Power Rails](#power-rails)
- [Internal Pull Resistors](#internal-pull-resistors)
- [Safe Driving Patterns](#safe-driving-patterns)
- [ESP32 Electrical Details](#esp32-electrical-details)
- [Voltage Levels](#voltage-levels)
- [Current Limits](#current-limits)
- [Power Consumption](#power-consumption)
- [Internal Pull Resistors](#internal-pull-resistors)
- [Drive Strength Configuration](#drive-strength-configuration)
- [Pull-up and Pull-down Resistors](#pull-up-and-pull-down-resistors)
- [When Required](#when-required)
- [Calculation Formula](#calculation-formula)
- [Strength Guidelines](#strength-guidelines)
- [Consequences of Wrong Value](#consequences-of-wrong-value)
- [Level Shifting](#level-shifting)
- [When Required](#when-required)
- [Method 1: Voltage Divider (5V → 3.3V, Unidirectional)](#method-1-voltage-divider-5v--33v-unidirectional)
- [Method 2: N-Channel MOSFET (Bidirectional)](#method-2-n-channel-mosfet-bidirectional)
- [Method 3: Dedicated Level Shifter ICs](#method-3-dedicated-level-shifter-ics)
- [Method 4: Direct Connection (3.3V → 5V Input)](#method-4-direct-connection-33v--5v-input)
- [Common Mistakes and Warnings](#common-mistakes-and-warnings)
- [NEVER Do This](#never-do-this)
- [ALWAYS Do This](#always-do-this)
- [Quick Reference Card](#quick-reference-card)
- [Formulas](#formulas)
- [Quick Values](#quick-values)
- [Current Limits Summary](#current-limits-summary)
- [Voltage Summary](#voltage-summary)
- [Common Pin Restrictions](#common-pin-restrictions)
---
## 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
---