Timing problem with internal oscillator: minimum pulse width violation (tch)

Hi,

I have a system on a Cyclone 10 LP where I use the clock from the internal oscillator for some operations, and I have an external clock that I use for some other operations.

For the internal oscillator, I used the IP altera_int_osc (where I cannot program anything). The frequency is 80 MHz by default, as mentioned in the application note AN 496: Using the Internal Oscillator IP Core.

This clock is used to drive some counters, which will toggle some signals that are going out of the FPGA. Therefore, I don't have any signal driven by the internal oscillator clock that is used somewhere else in the system.

In the sdc file, I constrained the clock coming from the internal oscillator as follows:

create_clock [get_pins {altera_int_osc_inst|sd1|clkout}] -name {int_osc_clk} -period 12.5 -waveform { 0 6.25 }

where altera_int_osc_inst is the instance of the IP, clkout the name of the output of the IP, sd1 is the node name of the clock, int_osc_clk is the name of my clock signal coming from the IP, and 12.5 ns = 1 / 80 MHz.

After compilation, Quartus tells me "Timing requirements not met".

In the Timing analyzer report, under the Fmax Summary tab, I see :

Restricted Fmax: 45.49 MHz

Note: limit due to high minimum pulse width violation (tch)

In the Timing analyzer report, under the Minimum Pulse Width Summary, I see :

Slack : -8.730

End Point TNS : -18.1

As a test, I simplified my block such that it contains only a shift register with its LSB going out of the FPGA, and I still have the same problem.

Therefore, the problem in not in the VHDL design using the internal oscillator clock.

I guess it is either my sdc file that is incorrect or something else, but I cannot see what.

Any help is more than welcome.

Thanks,

Jérôme