AVR-Chat

fred fred fred
X-Ameol-Version: 2.55.2048, Microsoft Windows XP Professional Service Pack 2 (Build 2600)

> You didn't say how far apart these slaves would be and what sort of 
> environment etc. If they are within a metre or two you could use TWI 
> aka I2C as this is well suited for low-speed local multi-drop. 
> Although I2C is not suited for long stretches I used to run LCD 
> displays off I2C over 10's of metres many years ago in harsh 
> environments, but this is not recommended practice :)

3 metres total, auto environment, 24V supply

I reckon I2C with a checksum would do it ... but LIN is a better 
standard, if I can do it without a h/w UART

> There is also SPI but not in the regular way which requires slave 
> selects etc. You can use it in two wire mode by multi-dropping the 
> clocks and the MOSI pins, forgetting the MISO as it becomes redundant.
> For the USI on the Tiny it looks like you need to join the DI and DO 
> together to achieve this. It's usually good practice to isolate each 
> pin via a small value resistor (I use 220R). Software leaves each 
> device in slave mode until one wants to talk, and provided there are 
> no collisions then each slave device gets the same message. When one 
> wants to reply, well it does the same thing and switches to master 
> mode temporarily. I have used this method on other micros at 
> high-speed for locally connected modules.

Hmmm... that would work... but does it end up with any advantage over 
s/w I2C? At least that defines a start and a stop condition.

> Also, the tinys do seem to work in half-duplex asynchronous mode, at 
> least according to the docs. This is the method I have used on other 
> micros where you can simply tie the txd and rxd of a uart together 
> (assuming txd can be tristated) and form a 1-wire asynchronous 
> multi-drop bus. Once again it is good practice to isolate the txd pin 
> with a small value resistor and include a high-value resistor and 
> optional filtering cap in the rxd pin for extra protection.

but I though there was only one tiny with a "true UART"???

> If you want distance then there are many options with RS485 being the 
> most common and quite doable on the tinys.

I'm not keen on adding the cost of a 485 transceiver, but it may come to 
that.

Many thanks 

David