Daisy chain possibility performances and limitations

[mariogarsi]

Hi evrybody, my name is Mario garsi and I´m an Italian mechanical engineer.
I´m starting using Red Pitaya in order to make an on-board data acquisition system.
And having more than six channels could be useful in many situations.
I´ve seen in the hardware that there is a daisy chain connector and, from what I understood, this should allow to mount more Red Pitaya together.
But I´ve not seen any topic about it, in order to understand set-up, configuration, performances, limitations (maximum number of red pitaya together) and so on?
Is there any documentation about it? I´ve not been able to find it out.

Many thanks in advance for your support,

Best Regards,

Mario

[Nils Roos]

Hi Mario,

The daisy chain connectors can be used to communicate between Red Pitayas, but - as far as I am aware of - there is currently no application that actually uses this channel.

Performance:
Each connector has two SSTL18 class I differential pairs (clock and DDR data) operating at 250MHz. Transfer is unidirectional, one connector is a dedicated input, the other a dedicated output. Maximum rate on each port is 500Mbps (1bit, double data rate @ 250MHz). There is no limit to the number of chainable devices but each device adds latency if you eg. wanted to propagate a trigger-signal along the chain.

Configuration:
There's not much to configure, the speed and direction are fixed, the only thing you need to do is to synchronize the receiver to the transmitter each time the physical connection is newly established - look for "training" in the HDL memory map.

In the current Red Pitaya FPGA design, the daisy chain's data paths are only accessible through the memory mapped registers (0x40500000-0x4050000c). This limits their usefulness, because you can't read / write at the full speed of 500Mbps that way.

If you have a timecritical usecase for the daisy chain, you'd first want to adapt the fpga module to connect directly to the logic you plan to make communicate over it.