AVR Freaks

There are many many ways to send N bits of data from one micro to another. Perhaps the easiest of all is to connect 8 wires from the port of one micro to the other. You then write the 8 bits you want to send to the PORT on one and the other just reads the whole PIN register to get the value. Of course this needs some way for the sender to say "just updated it, you need to read again". So often you have a "strobe" line which is a 9th wire and a pulse on that is an indication from sender to receiver to say "pick up new value now".

 

But not all designs can waste an entire 8 (9) lines just to transmit bytes like this. So then you might look at breaking the data up a bit. The first thing you might do is split the 8 wires into just 4 so that now, to send a whole byte you actually send 4 bits, pulse the "strobe" then send another 4 bits and pulse it again. But this still needs 5 wires. So how about splitting that again - have just 2 signal wires and one strobe/clock line. Now you present 2 bits a time and each time you also send a pulse on the strobe/clock line to say "next lot ready to read". But the ultimate is when you finally break this down into just 2 wires - one data line and one clock line. Now you just send a single bit at a time and each time you make a pulse on the other wire (clock) to say "next bit ready". When you get to this stage you have effectively invented SPI. It's a peripheral in the AVR that will do all the splitting and pulsing for you so you don't need to "bit bang" it in software. I2C is a kind of similar system but the electronics are a bit different so that now there can be more than one "listener" on the line.

 

Of course if you think that even 2 wires to send the data is still too much you could do away with the clock line all together. Instead you just make pulses on the single wire to send a bit at a time. But to do this the sender and receiver have to agree at what speed the pulses will be sent as the receiver has no other way to know when one bit ends and the next is read to be read. Also to make sure the receiver starts listening at the right moment you probably want to send an extra bit at the start as a kind of "synchronization" pulse to say "data about to start". That extra pulse is a "start bit". When you get to this stage you just invented UART.

 

BTW in all this there is always (ALWAYS!) one additional wire in addition to all the others I have mentioned and that is "ground" because to measure 0 or 1 pulses (voltage levels) you have to have a ground reference to measure them against. So even in the UART case it is the data line and a Gnd line.

 

So what is it you are trying to connect exactly? Many things you might attach to a micro already have a 8 bit data or SPI or I2C or UART interface!