SPI Length

Okay I'll try to keep this simple:

I have a six pin ribbon cable that is 10 feet long. I am connecting 5 
devices on the bus. Each device will draw <200mA @ 5-6V from the cable. 
I will be using a hardware SPI from a AVRTiny on each board. Now my 
question is: Can I actually just use SPI to get the devices to talk 
resonably? I don't mind low bit rate spi. Something like 800 bits per 
second would even work. Is there something special I should include in 
the design? I've blown the budget already, so I'd like to keep it simple.

Thanks,

Brett

Brett Foster wrote:

> Okay I'll try to keep this simple:
> 
> I have a six pin ribbon cable that is 10 feet long. I am connecting 5 
> devices on the bus. Each device will draw <200mA @ 5-6V from the cable. 
> I will be using a hardware SPI from a AVRTiny on each board. Now my 
> question is: Can I actually just use SPI to get the devices to talk 
> resonably? I don't mind low bit rate spi. Something like 800 bits per 
> second would even work. Is there something special I should include in 
> the design? I've blown the budget already, so I'd like to keep it simple.

Multiple times 200mA from a flat ribbon cable is a lot.

Rene
-- 
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

"Brett Foster" <fosterb.no.s.p.a.m@ioctl.ca> wrote in message
news:38l6qiF5p0qipU1@individual.net...
> Okay I'll try to keep this simple:
>
> I have a six pin ribbon cable that is 10 feet long. I am connecting 5
> devices on the bus. Each device will draw <200mA @ 5-6V from the cable.
> I will be using a hardware SPI from a AVRTiny on each board. Now my
> question is: Can I actually just use SPI to get the devices to talk
> resonably? I don't mind low bit rate spi. Something like 800 bits per
> second would even work. Is there something special I should include in
> the design? I've blown the budget already, so I'd like to keep it simple.
>
> Thanks,
>
> Brett

I don't think that you have enough wires for SPI. As well as the four wire
SPI you'll need chip selects on the slaves. I2C would be more suitable as it
uses 3 wires and doesn't need chip selects when used as a bus. You should be
able to find I2C implementions for the AVR.
Peter

"Brett Foster" <fosterb.no.s.p.a.m@ioctl.ca> wrote in message 
news:38l6qiF5p0qipU1@individual.net...
> Okay I'll try to keep this simple:
>
> I have a six pin ribbon cable that is 10 feet long. I am connecting 5 
> devices on the bus. Each device will draw <200mA @ 5-6V from the cable. I 
> will be using a hardware SPI from a AVRTiny on each board. Now my question 
> is: Can I actually just use SPI to get the devices to talk resonably? I 
> don't mind low bit rate spi. Something like 800 bits per second would even 
> work. Is there something special I should include in the design? I've 
> blown the budget already, so I'd like to keep it simple.

It will probably work. SPI is used to program AVRs and it works OK over 2 m 
or so of printer cable.

Leon