Hi Tim
I have a ROACH board and am using a design I got from another ROACH person. The
design uses a lookup table
stored in a dram. The software interface is used to load the dram with the
data. later, the dram is read out
to stream data to MKID DAC board.
Questions:
I have my FPGA running at 128MHz. The DRAM runs at 200MHz.
What speed does the dataout port of the dram spit data? Does it just work at
128MHz and I get a 144 bit word every clock
at 128MHz? Or is the dataout running at 200MHz, where I have to send data to
FIFO at 200MHz, then readout the
FIFO at 128MHz?
The data interface to you is a pair of asynchronous FIFOs, one for
reading, one for writing. You can write in data or read it back at
128MHz. The logic internal to the SDRAM controller (reading or writing
data to or from the FIFOs on the other interface) is running at 200MHz.
The higher clock rate ensures that SDRAM overhead (refreshing of banks
etc) can be performed at the same time as your operations, even if you
are running at the full 128MHz.
Do I really care about the FIFO clock rate as long as I can get data at 128MHz?
You should care enough to ensure there is enough margin for the overhead
but everything else is taken care of for you.
>From the dram docs on the web:
" To read data out of the DRAM, hold 'RWn' high, keep the address constant for two
FPGA clock cycles, and toggle the 'cmd_valid' pin every clock. Note that a new word will
be available on the 'data_out' pin on every clock cycle. 'rd_valid' will frame valid
output data some indeterminate number of clock cycles after the read 'cmd_valid' toggles.
'cmd_ack' is high unless an attempt to write a command into the input FIFO failed, at
which point it will go low synchronously with the issuing of the failed command."
So if I wish to read out address 0, 1, 2, ....I have to keep the address
constant for 2 fpga clocks. Then I can only read out
dram at fpga_clock/2? Or, do I toggle cmd_valid, to get address 0, 1 at the
128MHz fpga clock.
Now I have to set the addr to 2 and toggle cmd_valid again. IN this case, I
still cannot readout the dram at 128MHz.
Or perhaps I toggle cmd_valid forever and get data from address 0,1,2,3... on
every fpga clock cycle?
Yes, you need to hold the address constant for two clock cycles while
keeping 'RWn' and 'cmd_valid' high. It reads data in a 'burst', two
words are stored at the same address so at some point later 'rd_valid'
will go high for two clock cycles and you will get two words. Writing is
a similar process, you write two words to the same address in a two word
'burst'.
As far as I remember, the Simulink simulation models quite accurately
modelled this behaviour.
Hope this helps.
Regards
Andrew
