Computing stuff tied to the physical world


Decoding a pulse train

In AVR, Hardware, Software on Apr 27, 2010 at 00:01

The (planned) Input Plug uses an ATtiny chip to decode a 16-channel selection code using a single I/O pin.

Here is the relevant part of the schematic:

Screen Shot 2010 04 19 at 19.50.37

The DIO pin is connected to PB4 of the ATtiny. The ATtiny PB0..PB3 pins in turn are hooked up to the A..D select inputs of the analog multiplexer. So all the ATtiny needs to do is decode an incoming “pulse train” on its PB4 pin, and send out the decoded value on its PB0..PB3 output pins.

My first attempt used a simple serial protocol: a start bit and 4 data bits, clocked at a fixed rate:

Screen Shot 2010 04 19 at 19.52.59

This worked, but it was a bit flakey because the internal clock of the ATtiny is not accurate enough to ensure everything stays in sync for the entire duration of the transmission, i.e. across 5 bit periods. This is clearly explained in the following image (from here):

async1

It’s better to use a self-clocking format, for example 5 pulses of varying width, because then only the length of each individual pulse matters. Above a certain threshold = 1, below = 0. And we can reset when there are no pulses.

Here’s the a transmit test sketch, which sends a 0..15 counter every 100 ms:

Screen Shot 2010 04 19 at 19.57.36

As you can see, the pulses are 4 or 8 µs wide, with one pulse every 12 µs. Roughly.

Note that interrupts have to be disabled during each “transmission”, since these pulses are very short and need to be fairly strictly controlled.

The decoder on the ATtiny (ATtiny85 in this test) is also quite simple. It waits for a pulse start and then counts in a loop until the signal drops to zero again. Counts above a certain value are treated as “1”. Missing pulses and pulses which are way too long cause the decoder to be reset and start from scratch:

Screen Shot 2010 04 19 at 20.01.21

This can be compiled with avr-gcc, and it’s of course just a few bytes (even an ATtiny is overkill here):

Screen Shot 2010 04 19 at 20.02.44

I’m using yesterday’s Flash Board for ISP programming. Makes a good test that it also works with ATtiny MPUs.

Here’s the test setup (with the ISP programmer disconnected):

Dsc 1362

And sure enough, the LEDs display a little running 4-bit counter, driven by data sent over a single DIO pin.

Should be good to go for the Input Plug!