360 lines
13 KiB
Markdown
360 lines
13 KiB
Markdown
# Electrical Constraints Reference
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> **类别：reference** | **主题：电气约束通用规范** | **信源：** 多平台汇总
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## Table of Contents
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- [Platform Comparison Summary](#platform-comparison-summary)
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- [Raspberry Pi Electrical Details](#raspberry-pi-electrical-details)
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- [Voltage Levels](#voltage-levels)
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- [Current Limits](#current-limits)
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- [Power Rails](#power-rails)
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- [Internal Pull Resistors](#internal-pull-resistors)
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- [Safe Driving Patterns](#safe-driving-patterns)
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- [ESP32 Electrical Details](#esp32-electrical-details)
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- [Voltage Levels](#voltage-levels)
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- [Current Limits](#current-limits)
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- [Power Consumption](#power-consumption)
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- [Internal Pull Resistors](#internal-pull-resistors)
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- [Drive Strength Configuration](#drive-strength-configuration)
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- [Pull-up and Pull-down Resistors](#pull-up-and-pull-down-resistors)
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- [When Required](#when-required)
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- [Calculation Formula](#calculation-formula)
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- [Strength Guidelines](#strength-guidelines)
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- [Consequences of Wrong Value](#consequences-of-wrong-value)
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- [Level Shifting](#level-shifting)
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- [When Required](#when-required)
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- [Method 1: Voltage Divider (5V → 3.3V, Unidirectional)](#method-1-voltage-divider-5v--33v-unidirectional)
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- [Method 2: N-Channel MOSFET (Bidirectional)](#method-2-n-channel-mosfet-bidirectional)
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- [Method 3: Dedicated Level Shifter ICs](#method-3-dedicated-level-shifter-ics)
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- [Method 4: Direct Connection (3.3V → 5V Input)](#method-4-direct-connection-33v--5v-input)
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- [Common Mistakes and Warnings](#common-mistakes-and-warnings)
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- [NEVER Do This](#never-do-this)
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- [ALWAYS Do This](#always-do-this)
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- [Quick Reference Card](#quick-reference-card)
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- [Formulas](#formulas)
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- [Quick Values](#quick-values)
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- [Current Limits Summary](#current-limits-summary)
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- [Voltage Summary](#voltage-summary)
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- [Common Pin Restrictions](#common-pin-restrictions)
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---
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## Platform Comparison Summary
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- Logic voltage：Raspberry Pi=3.3V, ESP32=3.3V
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- Max per-pin current：Raspberry Pi=16mA source/sink, ESP32=40mA max (20mA recommended)
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- Aggregate GPIO current：Raspberry Pi=**50mA total**, ESP32=~1200mA total (chip limit)
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- 5V tolerant：Raspberry Pi=**NO**, ESP32=**NO**
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- Internal pull-up：Raspberry Pi=~50kΩ, ESP32=~45kΩ typical
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- Internal pull-down：Raspberry Pi=~50kΩ, ESP32=~45kΩ typical
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- Drive strength：Raspberry Pi=Fixed, ESP32=Configurable (5-40mA)
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- Input threshold (VIH)：Raspberry Pi=~1.8V, ESP32=~2.0V
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- Input threshold (VIL)：Raspberry Pi=~0.8V, ESP32=~0.8V
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---
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## Raspberry Pi Electrical Details
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### Voltage Levels
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- **All GPIO pins operate at 3.3V ONLY**
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- **5V on any GPIO pin WILL PERMANENTLY DAMAGE the SoC**
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- No built-in overvoltage protection
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- No built-in ESD protection (handle with care)
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### Current Limits
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- Per-pin source：16mA, Consequence of Exceeding=Voltage droop, pin damage
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- Per-pin sink：16mA, Consequence of Exceeding=Voltage rise, pin damage
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- **Total GPIO**：**50mA**, Consequence of Exceeding=Instability, crashes, permanent damage
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**Critical:** The 50mA limit is across ALL GPIO pins combined, not per-bank.
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### Power Rails
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- 3.3V：Source=Onboard regulator, Available Current=~50mA for peripherals, Notes=Shared with Pi's 3.3V needs
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- 5V：Source=USB/PSU direct, Available Current=1-2A minus Pi consumption, Notes=No regulation, direct pass-through
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- GND：Source=Common ground, Available Current=N/A, Notes=8 ground pins on header
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### Internal Pull Resistors
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- GPIO0-8：Default State=Pull-UP, Resistance=~50kΩ
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- GPIO9-27：Default State=Pull-DOWN, Resistance=~50kΩ
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**Note:** Internal pulls are too weak for I2C (need 4.7kΩ external).
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### Safe Driving Patterns
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- Single LED：Method=330Ω-1kΩ series resistor, Notes=3-10mA safe
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- Multiple LEDs：Method=Transistor driver, Notes=If total >50mA
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- Relay：Method=Transistor/MOSFET + flyback diode, Notes=Never direct from GPIO
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- Motor：Method=Motor driver IC (L298N, DRV8833), Notes=Never direct from GPIO
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- Buzzer (passive)：Method=Transistor driver, Notes=Inductive load
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- Buzzer (active)：Method=Direct if <16mA, Notes=Check current draw
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---
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## ESP32 Electrical Details
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### Voltage Levels
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- **All GPIO pins operate at 3.3V ONLY**
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- **5V on any GPIO pin WILL DAMAGE the chip**
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- No built-in overvoltage protection
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- Some ESD protection but don't rely on it
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### Current Limits
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- Per-pin max：40mA, Notes=Absolute maximum
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- Per-pin recommended：20mA, Notes=For reliability/longevity
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- Total chip：~1200mA, Notes=Includes WiFi, BT, CPU
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### Power Consumption
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- Active + WiFi TX：Current Draw=80-240mA, Notes=Peaks during transmission
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- Active + WiFi idle：Current Draw=20-68mA, Notes=Connected but not transmitting
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- Active, no radio：Current Draw=20-68mA, Notes=CPU running
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- Modem sleep：Current Draw=3-20mA, Notes=WiFi paused, CPU active
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- Light sleep：Current Draw=0.8mA, Notes=CPU paused, RTC running
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- Deep sleep：Current Draw=10-150µA, Notes=Only RTC + ULP available
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### Internal Pull Resistors
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- Standard GPIO：Pull-up=Yes, Pull-down=Yes, Resistance=~45kΩ
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- GPIO34-39：Pull-up=**NO**, Pull-down=**NO**, Resistance=N/A (input-only)
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**Note:** Internal pulls are too weak for I2C (need 4.7kΩ external).
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### Drive Strength Configuration
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- 5mA：Current=Weakest, Use Case=Low power, slow signals
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- 10mA：Current=Low, Use Case=General purpose
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- 20mA：Current=Default, Use Case=Most applications
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- 40mA：Current=Maximum, Use Case=Fast edges, heavy loads
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Higher drive strength = faster edges but more EMI/noise.
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---
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## Pull-up and Pull-down Resistors
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### When Required
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- I2C bus (SDA)：Pull Type=Pull-UP, Typical Value=**4.7kΩ**
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- I2C bus (SCL)：Pull Type=Pull-UP, Typical Value=**4.7kΩ**
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- 1-Wire bus (DQ)：Pull Type=Pull-UP, Typical Value=**4.7kΩ**
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- Button to GND：Pull Type=Pull-UP, Typical Value=10kΩ
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- Button to VCC：Pull Type=Pull-DOWN, Typical Value=10kΩ
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- SPI CS line：Pull Type=Pull-UP, Typical Value=10kΩ
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- Open-drain output：Pull Type=Pull-UP, Typical Value=1-10kΩ
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- UART RX (optional)：Pull Type=Pull-UP, Typical Value=10kΩ (noise immunity)
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- Reset line：Pull Type=Pull-UP, Typical Value=10kΩ
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### Calculation Formula
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```
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R = (VCC - VOL) / IOL
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Where:
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VCC = Supply voltage (3.3V)
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VOL = Output low voltage (~0.4V)
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IOL = Required sink current (3mA for I2C)
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Example (I2C):
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R = (3.3V - 0.4V) / 3mA = 967Ω minimum
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Typical choice: 4.7kΩ (provides margin)
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```
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### Strength Guidelines
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- 1kΩ：Use Case=Long wires, high capacitance, fast I2C, Notes=Strong pull, higher current
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- 4.7kΩ：Use Case=Standard I2C, 1-Wire, general purpose, Notes=Most common choice
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- 10kΩ：Use Case=Buttons, CS lines, low-power, Notes=Standard digital pull
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- 47-100kΩ：Use Case=Wake-up inputs, ultra-low power, Notes=Very weak, slow rise time
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### Consequences of Wrong Value
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**Too high (weak pull):**
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- Slow signal rise times
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- Noise susceptibility
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- Communication errors at higher speeds
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- May not reach valid HIGH level
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**Too low (strong pull):**
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- Excessive current consumption
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- Device may not be able to pull line LOW
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- Wasted power in battery applications
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---
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## Level Shifting
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### When Required
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- 5V logic output → 3.3V GPIO input
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- 3.3V GPIO output → 5V input (if device doesn't recognize 3.3V as HIGH)
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- Bidirectional communication between 3.3V and 5V systems
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### Method 1: Voltage Divider (5V → 3.3V, Unidirectional)
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**Use for:** Slow signals (<100kHz), input direction only
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```
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5V Signal ──[1kΩ]──┬──> 3.3V GPIO Input
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│
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[2kΩ]
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│
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GND
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Output: 5V × (2kΩ / 3kΩ) = 3.33V
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```
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- Simple, cheap：Cons=Input direction only
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- 2 resistors：Cons=Slow (RC time constant)
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- No active components：Cons=Loads the signal
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### Method 2: N-Channel MOSFET (Bidirectional)
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**Use for:** I2C, 1-Wire, open-drain signals up to 400kHz
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```
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3.3V Side 5V Side
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│ │
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[4.7kΩ] [4.7kΩ]
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│ │
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├────────┬──────────────────┬──────────┤
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│ │ │ │
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SDA Source Drain SDA
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(3.3V) └───── BSS138 ─────┘ (5V)
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│
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Gate
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│
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3.3V
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```
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**Operation:**
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- Gate tied to LOW side voltage (3.3V)
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- When LOW side pulls down, MOSFET conducts, pulling HIGH side down
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- When HIGH side pulls down, body diode conducts, pulling LOW side down
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- Pull-ups restore HIGH state on both sides
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- MOSFET：Specification=BSS138, 2N7000 (through-hole)
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- Pull-ups：Specification=4.7kΩ on each side
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- Voltage：Specification=3.3V on gate, low side; 5V on high side
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### Method 3: Dedicated Level Shifter ICs
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- TXB0104：Channels=4, Type=Auto-direction, Speed=100 Mbps, I2C Safe?=**NO**
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- TXB0108：Channels=8, Type=Auto-direction, Speed=100 Mbps, I2C Safe?=**NO**
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- PCA9306：Channels=2, Type=I2C-specific, Speed=1 MHz, I2C Safe?=**YES**
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- PCA9517：Channels=2, Type=I2C buffer, Speed=400 kHz, I2C Safe?=**YES**
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- 74LVC245：Channels=8, Type=Unidirectional, Speed=100 MHz, I2C Safe?=N/A (direction pin)
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- BSS138 modules：Channels=4, Type=Bidirectional, Speed=400 kHz, I2C Safe?=**YES**
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**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.
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### Method 4: Direct Connection (3.3V → 5V Input)
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Many 5V devices recognize 3.3V as logic HIGH:
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- VIH (HIGH threshold)：Typical 5V TTL=2.0V, Typical 5V CMOS=3.5V
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- VIL (LOW threshold)：Typical 5V TTL=0.8V, Typical 5V CMOS=1.5V
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**Check datasheet for VIH.** If VIH < 3.0V, direct connection usually works.
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**NEVER** connect 5V output directly to 3.3V input — level shift or divide required.
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---
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## Common Mistakes and Warnings
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### NEVER Do This
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- Connect 5V directly to any GPIO：Consequence=**Permanent chip damage**
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- Drive relay coil directly from GPIO：Consequence=Inductive kickback damages GPIO
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- Drive motor directly from GPIO：Consequence=Overcurrent, voltage spikes
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- Exceed 50mA total on RPi GPIO：Consequence=Voltage instability, damage
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- Use ESP32 GPIO6-11 (WROOM)：Consequence=Flash pins — chip crashes
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- Pull GPIO12 HIGH at boot (ESP32)：Consequence=**Flash voltage brick**
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- Forget pull-ups on I2C：Consequence=Communication failure
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- Forget pull-up on 1-Wire：Consequence=Bus doesn't work
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- Use TXB-series for I2C：Consequence=Unreliable communication
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- Assume GPIO is 5V tolerant：Consequence=It's not — damage results
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### ALWAYS Do This
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- Use current-limiting resistor for LEDs：Reason=Prevents overcurrent (220-330Ω)
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- Use flyback diode with relays/motors：Reason=Catches inductive voltage spike
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- Use level shifter for 5V ↔ 3.3V：Reason=Protects GPIO from overvoltage
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- Check total current draw：Reason=Prevent exceeding limits
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- Verify I2C addresses before wiring：Reason=Detect conflicts early
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- Add 100nF decoupling capacitor near ICs：Reason=Reduces noise, improves stability
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- Use external pull-ups for I2C (4.7kΩ)：Reason=Internal pulls too weak
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- Check ESP32 pin restrictions：Reason=Strapping, flash, input-only
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- Use transistor for loads >16mA：Reason=Protects GPIO
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- Add ESD protection for external connectors：Reason=Protects against static
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---
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## Quick Reference Card
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### Formulas
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**LED Resistor:**
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```
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R = (VCC - Vf) / If
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R = (3.3V - 2.0V) / 10mA = 130Ω minimum
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Recommended: 220-330Ω (5-10mA, plenty bright)
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```
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**Voltage Divider:**
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```
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Vout = Vin × (R2 / (R1 + R2))
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For 5V → 3.3V: R1=1kΩ, R2=2kΩ
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```
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**Pull-up Resistor:**
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```
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R = (VCC - VOL) / IOL
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Standard: 4.7kΩ for I2C/1-Wire, 10kΩ for buttons
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```
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### Quick Values
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- LED resistor (3.3V, red/green)：220-330Ω
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- LED resistor (3.3V, blue/white)：100-150Ω
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- I2C pull-up：**4.7kΩ** to 3.3V
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- 1-Wire pull-up：**4.7kΩ** to 3.3V
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- Button pull-up/down：10kΩ
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- SPI CS pull-up：10kΩ
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- 5V → 3.3V divider：1kΩ + 2kΩ
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- Flyback diode：1N4148 or 1N4007
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- Decoupling capacitor：100nF ceramic
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### Current Limits Summary
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- Raspberry Pi：Per Pin=16mA, Total GPIO=**50mA**
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- ESP32：Per Pin=20mA recommended, Total GPIO=~1200mA chip total
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### Voltage Summary
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- Logic HIGH：RPi=3.3V, ESP32=3.3V
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- Logic LOW：RPi=0V, ESP32=0V
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- Max input：RPi=3.3V, ESP32=3.3V
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- 5V tolerant：RPi=**NO**, ESP32=**NO**
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### Common Pin Restrictions
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**Raspberry Pi:**
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- GPIO0/1: Reserved (HAT EEPROM)
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- GPIO14/15: UART/BT conflict (Pi 3/4/Zero2W)
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**ESP32:**
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- GPIO6-11: Flash pins — **NEVER USE**
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- GPIO12: Strapping — **DANGER** (flash voltage)
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- GPIO16-17: PSRAM (WROVER only)
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- GPIO34-39: Input only, no pulls
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---
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