Signal use from pin

[ALuPin]

Hi newsgroup,

in my last post "SYNC + FIFO" I talked about the sychronization method
of an external data stream.
Additional to the bus data I have some control signals coming from
the USB transceiver which are also synchronous to the RXCLK which
I use as the FIFO write clock (see last post).

In order to respond directly to the control signals I get from the
transceiver(for example:
USB transceiver drives one control signal high to notify the FPGA to
place the next data byte on the DATA[7..0] bus)
I have to use the control signal from the pin in my state machine
running
with the RXCLK.

What recommendations do you make when using such a signal from a pin
with regard to placement, timing constraints etc. ?

Thank you for your great help.

Rgds
André

 

[Gabor]

ALuPin wrote:

Hi newsgroup,

in my last post "SYNC + FIFO" I talked about the sychronization
method
of an external data stream.
Additional to the bus data I have some control signals coming from
the USB transceiver which are also synchronous to the RXCLK which
I use as the FIFO write clock (see last post).

In order to respond directly to the control signals I get from the
transceiver(for example:
USB transceiver drives one control signal high to notify the FPGA to
place the next data byte on the DATA[7..0] bus)
I have to use the control signal from the pin in my state machine
running
with the RXCLK.

What recommendations do you make when using such a signal from a pin
with regard to placement, timing constraints etc. ?

Thank you for your great help.

Rgds
André

If latency is not an issue, I would register the control signal at the
IOB to make sure you don't have setup timing issues. Then add another
delay stage (register) for the DATA[7..0] bus so you can use the
control signal a cycle later.

If you need to reduce latency into your FIFO, you need to create a
timing spec in the ucf file for the control signal like:
OFFSET = IN 7.2 ns BEFORE "RXCLK";
to make sure your state machine does not exceed the input setup
time available.

If you haven't assigned pins yet, I would suggest grouping the control
pin near the data pins so your state machine can be placed easily
near the control input.