How set timing constraint for such clock

[XQSHEN]

In my design, the spi clock to external device was divided by counter.

if use below to set timing constraint, it reports below warning:

Node: nios:nios_0|mcu_uadc:mcu_uadc_0|read_cnt[2] was determined to be a clock but was found without an associated clock assignment.
No paths exist between clock target "uadc_sclk" of clock "uadc_sclk" and its clock source. Assuming zero source clock latency.

So, what should I do to set timing constraints?

 always @(negedge clk or negedge reset_n) begin
if(reset_n == 1'b0)
write_cnt <= 8'd0;
else if(spi_current_state == SPI_STATE_WRITE)
write_cnt <= write_cnt + 8'd1;
else
write_cnt <= 8'd0;

end

always @(negedge clk or negedge reset_n) begin
if(reset_n == 1'b0)
read_cnt <= 8'd0;
else if(spi_current_state == SPI_STATE_READ)
read_cnt <= read_cnt + 8'd1;
else
read_cnt <= 8'd0;
end

assign uadc_sclk = (write_cnt[7:3] >= 5'd4 && write_cnt[7:3] <= 5'd27 && write_cnt[2:0] <= 3'd3)? 1'b1 :
(read_cnt[7:3] >= 5'd4 && read_cnt[7:3] <= 5'd27 && read_cnt[2:0] <= 3'd3)? 1'b1 : 1'b0;

[ak6dn · Accepted solution]

First, I would make uadc_sclk be a reg variable and not a wire.

Then, you have to add timing constraints for your derived clocks. For example, I have this in my .sdc file for a design:

# Input 50MHz reference clock

create_clock -period 20.0 -name CLOCK_50 [get_ports {CLOCK_50}]

# Created clocks based on PLLs (CPUCLK = 80MHz)

create_generated_clock -source {pll|altpll_component|pll|inclk[0]} -divide_by 5 -multiply_by 8 -duty_cycle 50 -name CPUCLK {pll|altpll_component|pll|clk[0]}

# Created clocks based on logic (RTCCLK = 25KHz)

create_generated_clock -source {pll|altpll_component|pll|clk[0]} -divide_by 5000 -duty_cycle 50 -name RTCCLK {dk8ea_clock:rtc|rtcclk}

where the clock RTCCLK is generated via this code:

    // system clock divider to RTC clock rate

    reg           rtcclk;       // rtc clock
    reg [31:0]    rtcdiv;       // rtc prescale clock divisor

    always @(posedge clk)
	begin
	if (reset)
	    begin
	    rtcdiv <= #TPD 32'd0;
	    rtcclk <= #TPD 1'b0;
	    end
	else
	    begin
	    rtcdiv <= #TPD (rtcdiv == PRESCALE-1'd1) ? 32'd0 : rtcdiv+1'd1;
	    if (rtcdiv == 32'd1) rtcclk <= #TPD ~rtcclk;
	    end
	end

[XQSHEN]

what make difference here?

[ak6dn]

What is your question? I do not understand what you are asking.

[SyafieqS]

Hi,

"Node: nios:nios_0|mcu_uadc:mcu_uadc_0|read_cnt[2] was determined to be a clock but was found without an associated clock assignment"

Seem like above error can be caused in two ways: 

1) A clock assignment was determined to be invalid, so its source objects no longer have a clock associated with them. 

2) When analyzing the netlist, the node was found feeding a clock port with no other clocks feeding it.

You can use the derive_clocks command to automatically find all clock nodes in the design. Also, for any clocks that were ignored, review the warning or error message associated with the command to prevent the clock from being ignored.

Once all the node with unassociated clocks are identified, you should apply you timing constraint for derived clock e.g. create_generated_clock 

Hope that helps.

[XQSHEN]

Thanks a lot!

[SyafieqS]

Let me know if there is any update or concern on this.

[XQSHEN]

No, you can close it.

[SyafieqS]

I’m glad that your question has been addressed, I now transition this thread to community support. If you have a new question, feel free to open a new thread to get the support from Intel experts. Otherwise, the community users will continue to help you on this thread. Thank you.