"# PIO-RX"

Edge-Based UART RX (PIO + Adaptive Decode)

A robust UART receiver for unstable / drifting / low-quality serial devices, built using:

• PIO (RP2040 / RP2350) → precise edge timing capture
• CPU (C) → adaptive UART reconstruction

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🚀 Why this exists

Standard UART (including PIO UART RX) assumes:

• stable baud rate
• clean edges
• consistent timing

Cheap devices break those assumptions:

• baud drift mid-stream
• slow edge slew
• oscillator instability

Result:

• last digits corrupted
• intermittent framing errors
• "works at 9630 but not 9600" type behaviour

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💡 Solution

Instead of sampling bits at fixed timing:

👉 Measure actual edge timing and reconstruct the signal

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🧠 Architecture

UART line │ ▼ PIO (edge detector + cycle counter) │ ▼ FIFO (edge timing events) │ ▼ CPU (adaptive decode) │ ▼ Recovered UART bytes

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🔧 Features

• Handles baud drift mid-stream
• Works with slow / smeared edges
• No fixed sampling point
• Automatically adapts to real timing
• Low CPU load (event-driven)

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📦 Files

edge_uart_rx/ ├─ main.c → decode + runtime ├─ edge_delta_uart.pio → PIO edge timing engine ├─ CMakeLists.txt

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⚙️ How it works (high level)

1. PIO detects every signal transition (edge)
2. PIO measures time since last edge
3. CPU receives edge timing events
4. CPU reconstructs:
   • bit timing
   • bit values
   • UART frames

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🧪 Example use case

• Weight scale sending 5-byte packets
• 9600 baud
• cheap oscillator
• drifting timing

👉 This system recovers clean data where normal UART fails

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🔌 Setup

Hardware

• RP2040 or RP2350 (Pico / Pico 2)
• UART TX line connected to RX_PIN (default = GPIO 2)

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Build

mkdir build
cd build
cmake ..
make -j

Flash

Drag UF2 or use picotool.

🛠 Configuration

In main.c:

#define RX_PIN     2
#define UART_BAUD  9600.0
📈 Key tuning parameters
Parameter	Meaning
bit_us	Expected bit time
edge filter	Rejects noise / tiny pulses
baud adaptation	Tracks drift
idle flush	completes trailing bytes
⚠️ Limitations

Extremely noisy signals may need hardware cleanup

Very long runs with no edges rely on timing estimation

Not a drop-in replacement for high-speed UART

💥 When to use this

Use this when:

UART hardware fails intermittently

PIO UART RX loses sync

You see "last digits corrupted"

Adjusting baud manually sometimes helps

🧾 TLDR

This is a timing-based UART decoder, not a sampling-based one.

👉 It follows the signal instead of assuming it.

"# PIO-RX"