Backplane protocol

I have used I2C for multiple hot-swappable cards (30 cards in a single backplane, 6 different possible types). The one issue you'll run up against is most I2C devices give you 3 bits of address, requiring multiple I2C controllers / hubs (if within an FPGA) for mroe than 8 devices. This should not be a major problem. Assign the address bits from the backplane connector, and a 'zone' bit. I usually pull a line low when inserting a card.

As to ESD protection, use a proper hotswap controller (lots around, depends on your system requirements) with 3-level pin lengths, and use ESD devices designed for signal lines (Murata makes a nice range of these, although there are others).

The nice thing about I2C (or SMBus, which has the advantage in a hotswap system of implementing a timeout - I2C is spec'd to DC, so if you pull a card when a transaction is active, I2C will hang indefinitely) is everyone implements it.

Just my $0.02

Cheers

PeteS

I'm with Pete. I think SMBus is a good way to go. The advantage of SMBus and I2c over SPI and presumably one-wire, is that SMBus is inherently addressable. SPI doesn't really know anything about addresses.

As for the hot swapping, it might be worthwhile to read up on how CompactPCI does it, or how SCA connectors do it.

In compact PCI, there are three main things pertaining to hot-swap:

1) There is a static discharge strip on the card, which is connected to board ground by way of 10 Megohm (or so) high wattage resistors. The receiving chassis is designed to contact this strip as the board goes in. 2) Pins are staged so that ground connects first (i.e., the ground pins are longer than all the others), then "early power," then other power and signal nets. The cards can only have so much capacitance on the early power nets. When early power comes on, the board is supposed to pre-charge all the signal nets within some finite amount of time, and make sure that the main power switches are completely off. Then when the real power pins connect, the hot swap controller powers up the board in a controlled way, without drawing too much current. This is to avoid having one card temporarily pull the whole chassis power rail down too low and cause a system reset. Typically there is a hot-swap controller on the card, and it takes care of most of this stuff. The whole thing is pretty involved, and I'm sure I'm glossing over some of the details, but it is interesting. PICMIG is the organization that puts out the CompactPCI specification. 3) Once a board is fully powered up, it pulls a pin low or something (ENUM?) to send a signal that a hot swap event has occurred. the host would then have to go re-scan the PCI bus to see who the new kid is, and try to allocate it the PCI resources it needs. This part is more complicated than you need, I think.

Hope that helps. Sorry that a lot of the details are hazy. It's been several years since I had anything to do with CompactPCI.

--Mac

They got booted out of the parking lot at Foothill College, so moved to a parking lot at Ampex. I haven't been in a long time... all the good stuff is on ebay now.

John