[Discuss-gnuradio] Re: USB FIFO

[Thibaud Hottelier]

Brian Padalino wrote:
> On 3/29/07, Thibaud Hottelier <address@hidden> wrote:
> > "This way we don't
> > need a dual clock fifo that can skip, only a single clock one"
> > Yes, that's wrong, I wanted to say that we need only standard dual clock
> > fifo or single clock fifo that can skip packets but we don't need a dual
> > clock fifo that can skip.
>
> Agreed.  I suppose we can always optimize later on.  The only
> reservation I have is in regards to the latency involved with doing
> such a thing.  When considering commands that may have small payloads,
> we'd be able to send a command every 4us ( 256 / 64e6 ) since we'd
> have to traverse through every location of the 512 byte packet.  I'd
> like to be able to process those faster.

Well I thought that we would use the same implementation of a fifo for 
both tx_chan_fifo_X and tx_cmd_fifo so we can also use the skip 
functionality there.

> Do you want to use Megafunctions or write it out?  You will need to
> use an Altera LPM for the different bus widths, that's for sure.  I'd
> prefer to use a RAM instantiation in the tx_channel FIFOs and write my
> own skipping functionality - but that's just because I completely
> loathe forcing myself to stick to a specific architecture.

I would rather use Megafunctions for tx_usb_fifo and write my own for 
tx_chan_fifo_X and tx_cmd_fifo. But I think it's better to first get the 
whole thing working without skipping, and then optimize by writing out a 
skipping fifo.

> On a different note, I am also wondering what should be the course of
> action if a command is sent down and it arrives late.  Obviously the
> command was supposed to be written, but is writing it late better than
> not writing it at all?  Is this more up to the implementation?  Should
> there be a command register guideline to write when dealing with this
> sort of thing?

Yes, it looks like it could be a useful functionality that is not hard 
to implement: we can add a register in the command block controlled by 
the SPI bus that define if we drop or execute outdated commands.

> Brian