I will like to implement a serial communication protocol, using javascript in a mobile browser to make the browser background blink (alternating the color from black to white) according to the bits, to communicate a few bytes to an Arduino using some kind of light sensor (photo diode or resistor) to parse. Electric Imp did something with https://electricimp.com/platform/blinkup/ .
Is there any library or implementation or where to start guide?, if not, how would you implement it?
I fear one of the problems of using a mobile browser with javascript to light-encode the bits will be timing issues, since setTimeInterval is not perfect, I could split the screen, one half blinking continuously (clock) and the other half the data, but that will require two light sensors.
2 Answers 2
Doing bit stuffing can prevent timing problems when you are sending a lot of 0s or 1s in an row. Alternatively use Manchester encoding (slower).
On the browser side, I'd suggest using <Canvas>. Combine this with requestAnimationFrame which gives you a very accurate timestamp as an argument. Though you have to check for support as older browsers don't support that.
You should investigate communication protocols and encoding techniques that embed the clock signal into the same data stream as the data signal.
One common method is called Manchester Coding . This entails splitting each bit into two halves. A 1 sends light followed by dark, and a 0 sends a dark followed by a light. It's then up to you to work out which transitions are which and where in a bit they occur (is a high transition the start of a 1 or mid-way through a 0?)
Another popular method (used in things like Ethernet) is called 8b/10b. That involves sending 10 bits to represent every 8 - more efficient than manchester coding (that could be considered 8b/16b) and can be read about here: https://en.wikipedia.org/wiki/8b/10b_encoding It's somewhat harder to implement than Manchester Code, so although it's less efficient I'd recommend the simplicity of Manchester Code.