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I needed a way to transfer data to AVR
This blog post is about my adventures in implementing a stupidly simple way of transferring data over audio to AVR (and why not other embedded chips too), reaching speeds up to 12kbps with really tiny code and memory footprint, using the internal oscillator of Tiny AVR, with hardware parts that cost next to nothing.
How it started
Once I got an idea of a wearable message badge made out from Tiny 8-pin AVR chip and an LCD-screen (you can buy one NOW: [email protected]), I wanted to make it really easy to update the contents of it, from any device you usually carry with you (not just your laptop/desktop-computer).
Some constraints of course are set up by the Flash size (8192 bytes) and the RAM size (512 bytes) of the Attiny85 chip.
I spent hours pondering about different transfer methods USB, UART, Bluetooth and so on, but all of those had downsides:
- USB: Needs 2 pins, Firmware takes 1/4th of the Flash space.
- UART: Modern computers lack serial port and you have to have an USB adapter.
- Bluetooth: All that pairing, price and code size… argh!
- WiFi: Too expensive and complicated for a small device like this.
And on top of that, USB and UART are either simply missing or at least very hard to get to your phone or tablet.
I even made some tests in the spirit of those Timex watches and screen blinking, but that was painfully slow.
I then spent few minutes to think what kind of common output devices exist in cell phones, tablets, laptops and desktops and soon realized they all can play audio! Good old memories with Commodore 64 and Datassette came flowing back. I needed to try that.
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