Teensy 4.1 Digital Capture & Playback with SD Storage

This project uses a Teensy 4.1 microcontroller to:

1. Capture a high-speed digital signal at 300 kHz for ~50 ms (~15,000 samples) on a specified input pin.
2. Store the captured samples in RAM.
3. Save the samples from RAM to the SD card so they persist across resets.
4. Load saved recordings from the SD card back into RAM for playback.
5. Playback the digital signal on an output pin at the same 300 kHz rate.
6. Wait for an input pin to go HIGH or LOW before starting a capture.
7. Loop playback as many times as desired, with a user-configurable pause between each repetition.

By default, the project is configured to:

• Use pin A0 for the digital input signal (adjust if needed).
• Use pin A9 for playback output (adjust if needed).
• Sample at 300 kHz for a 50 ms capture window (up to 15,000 samples).
• Store each capture as a raw byte array (1 byte per sample) plus a small header on the SD card (e.g., capture0.bin, capture1.bin, etc.).

Hardware Requirements

• Teensy 4.1 (for built-in SD card slot and sufficient RAM).
• An SD card inserted in the Teensy 4.1 SD slot.
• A digital signal source connected to the capture pin (default A0).
• (Optional) Additional wiring or test signals for playback on pin A9.

Installation & Setup

1. Clone or download this repository.
2. Open the .ino sketch in the Arduino IDE (or PlatformIO, etc.).
3. Select Teensy 4.1 as the target board.
4. Confirm the SD library is installed (the built-in Arduino SD library or an alternative like SdFat).
5. (Optional) Adjust CAPTURE_PIN, OUTPUT_PIN, or the SAMPLE_RATE in the code if necessary.
6. Upload the code to the Teensy 4.1.
7. Open the Serial Monitor at 115200 baud and type the commands listed below.

Command Reference

Type any of these commands into the Serial Monitor and press Enter:

• READHIGH
  Waits until the capture pin goes HIGH, then begins a 50 ms capture at 300 kHz. Prints a message when capture starts and ends.

• READLOW
  Waits until the capture pin goes LOW, then begins a 50 ms capture at 300 kHz. Prints a message when capture starts and ends.

• SAVE
  Writes the latest captured data (in RAM) to an auto-incremented file on the SD card (e.g., capture0.bin, capture1.bin, etc.). Also stores the number of samples as a small header in the file.

• PLAY
  Plays back the latest capture (in RAM) on the output pin. If no capture is in RAM, a warning is shown.

• PLAY N
  Loads the file captureN.bin from the SD card into RAM, then plays it on the output pin. For example, PLAY 3 loads capture3.bin.

• LOOP
  Toggles playback looping ON or OFF. When looping is ON, the code automatically restarts playback after it finishes, waiting for the user-defined pause interval in between.

• PAUSE <ms>
  Sets the pause (in milliseconds) to wait after a playback finishes, before restarting again (when looping is ON). For example, PAUSE 1000 creates a 1-second gap between loops.

• STOP
  Immediately stops any capture or playback in progress.

• FORMAT
  Deletes all files matching capture*.bin on the SD card (i.e., capture0.bin, capture1.bin, etc.).

Notes & Tips

• Teensy 4.1 has 512 KB of RAM, so a single 15 KB buffer is manageable. If you need to store multiple simultaneous buffers in RAM, watch for memory usage.
• High-speed captures may require more advanced methods (DMA, FlexIO) if your project demands longer capture durations or higher rates.
• If you use different pins, adjust CAPTURE_PIN, OUTPUT_PIN, and confirm the pin is suitable for digital reads/writes.
• Modify the SAMPLE_RATE or NUM_SAMPLES if you want a different capture duration or sampling speed (but ensure your code and hardware can keep up).
• This is a proof-of-concept; in production, you may need robust error handling, file management, or advanced buffering.

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Enjoy capturing and playing back signals with Teensy 4.1 and SD card storage!