Re: low speed communication

Andrea,

Since there are no MGT's, you will need to use some IO's from IO banks.

You can use: some number of LVDS 100 ohm differential drivers to receivers (one direction only, use two sets of buses), a group of single ended IOs matched to a ribbon cable (with ground, signal, ground, dignal, this is close to 50 ohms, and can go perhaps 10 meters at the speed you need).

I suggest the single ended groups, perhaps 8 wires for data, and one forwarded clock (for nine signals), and a bus like this for each direction.

Clocking these 8 wires at even 10 MHz, would provide 80 Mb/s. At this slow a rate, you don't really need to clock forward, but I still would suggest it (along with the data, there is the clock on the ninth wire to strobe the other end). That way, you don't have to figure out how to get the clock off of one pcb, and get it to the other one (for a system synchronous solution).

Just have registers at each end, and create your own (very simple) protocol. If a "data valid" or "data waiting" signal is needed, just use another wire. 20 wire ribbon cables are pretty common (ten signal, ten grounds). Drive the transmit end with something like LVCMOS 8 mA FAST, which is very close to 50 ohms, and there should be no overshoot, or undershoot, and the Signal integrity will be good. If that is still too strong, 6 mA, and even 4 mA drivers can be selected.

The only reason a protocol is required with the MGTs (like aurora:
http://www.xilinx.com/products/design_resources/conn_central/grouping/aurora.htm
is that designers need huge bandwidth, and in order to re-synchronize multiple MGTs (channel bonding), a layer is required to manage all that stuff.

With a simple parallel interface, you pretty much only need to implement what you need.

To connect these buses to the 405PPC, you will either have to interface to the PLB bus, or create an input and output port that is connected to the 405PPC. Depending on how fancy you want this to be, it could be as simple as input and output instructions (you create the protocol in software), or as fancy as a FIFO buffer with DMA into memory mapped into the 405PPC data memory (the protocol commonly used here is referred to as "flags placed under a rock" which is a reference to one processor just waits for a memory location to change, which tells it that the data it is waiting for is all there, and ready to be read from memory...the transmitting processor sends all the data, and sets the flag to tell the receiving processor to pick up the data.

Lots of choices here, and not many "standards" as the task is very simple, and folks tend to use only what logic they need to in order to do the job.

Once you pop up to MGTs, there are many more choices, only because there are more complex interfaces: ethernet, fibre channel, PCIe, etc.....

If you don't need fibre channel, then you don't need media access controllers (MAC), etc.
http://www.xilinx.com/products/design_resources/conn_central/solution_kits/index.htm



Austin
.

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