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Altera_Forum's avatar
Altera_Forum
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10 years ago

Timing Constraints for Source Sync Interface with clock coming out of FSM (not PLL)

What I want to do, is to write an SPI Master that drives both the SCLK (ouput) and MOSI (output) from the FSM and also samples MISO (input) in the FSM.

E.g. I clock the FSM process with a 50 MHz clock and generate a 25 MHz SCLK, by toggling SCLK every FSM clock.

I would drive MOSI at each rising edge of SCLK and sample MISO at each falling edge of SCLK.

The slave interface requires that.

My question is, how do I need to constrain the outputs that there is now phase shift between the outputs (SCLK and MOSI).

Or can I rely on them "leaving" the FPGA synchronously, since the come out of the same clocked process.

All examples for source synchronous interfaces I have seen have the clock coming out of a PLL and not out of an FSM.

Or is this not a good approach?

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    10 years ago

    Thanks, I didn't find that because I didn't really know what I had to look for.

    So what I take from that example is:

    1) I have my FSM clock:

    create_clock -period 20.0 -name clk_in [get_ports {clk_in}]

    For outputs:

    2) I have the output clock that I drive out of the FSM

    create_generated_clock -invert divide_by 2 -name SCLK -source [get_ports clk_in] [get_ports SCLK]

    -> divided by 2 because the frequency is divided by 2 in the FSM

    -> inverted because I drive signals at the rising edge of clk_in, but the slave samples at the falling edge of SCLK

    3) Set output delays

    set_output_delay -clock SCLK -max 5 [get_ports {MOSI}] # setup time of slave, MOSI is FPGA output

    set_output_delay -clock SCLK -min -5 [get_ports {MOSI}] # hold time of slave, MOSI is FPGA output

    For inputs:

    4) the same derived clock as above

    -> because the slave drives signals at the rising edge of SCLK, but I sample at the falling edge of SCLK

    5) Set input delays

    set_input_delay -clock SCLK -max 15 [get_ports {MISO}] # Tco of external device, MISO is FPGA input

    set_input_delay -clock SCLK -min 15 [get_ports {MISO}] # min Tco of external device, MISO is FPGA input