Xilinx 7 Series PLL and MMCM Simulation

This project aims to simulate the behavior of the PLLE2_BASE as well as the PLLE2_ADV PLL and the MMCME2_BASE MMCM found on the Xilinx 7 Series FPGAs. The MMCME2_ADV MMCM is not (yet) supported. This is done in Verilog, and can for example be simulated using the Icarus Verilog simulation and synthesis tool. It follows the instantiation interface described in the documentation on page 509ff for the PLLs and 461ff for the MMCM. This way you can just drop the files listed below into your project, instantiate the PLL/MMCM like you would for real hardware and simulate it. Read on to learn how to use the module and what it can and cannot do. If you just want to know, what works and what doesn't, just have a look at the project status

Quickstart

To use this module, you need to have the following files in your project:

• plle2_base.v, plle2_adv.v or mmcme2_base.v depending on which one you want
• period_count.v
• period_check.v
• freq_gen.v
• divider.v
• phase_shift.v
• dyn_reconf.v
• pll.v

To build and simulate your project, you can use icarus verilog and vvp and view the results in GTKWave:

• iverilog plle2_base.v period_check.v period_count.v freq_gen.v divider.v phase_shift.v dyn_reconf.v pll.v <your project files> -o <your project name>,

iverilog plle2_adv.v period_check.v period_count.v freq_gen.v divider.v phase_shift.v dyn_reconf.v pll.v <your project files> -o <your project name>

or iverilog mmcme2_base.v period_check.v period_count.v freq_gen.v divider.v phase_shift.v dyn_reconf.v pll.v <your project files> -o <your project name>, depending on which one you want

• vvp <your project name>
• gtkwave dump.vcd

If you specified the name of your output file using something like $dumpfile("<your_name.vcd>"), you have to replace dump.vcd with your chosen name.

The module works by supplying an input clock, which will be transformed to an, or rather 6 (or 7 in the case of the MMCM), output clocks. In the simplest case, this output clock depends on the input clock and multiple parameters. You can set the wanted output frequency, phase shift and duty cycle. The output frequency is calculated like this: output frequency = input frequency * (multiplier / (divider * output divider)), while the output phase can be calculated (in relation to the input phase) by using this formula: output phase = feedback phase + output phase. The parts of these formulas with "output" in their name are specific to one specific output, while the others are global. There are certain limits to the values. If you hit them, the module is going to stop simulation and inform you about it. Check out the FAQ section at the end to learn more about these limits. You can also find a table there, how the allowed VCO frequency depends on FPGA model, their speed grades the used module (MMCM or PLL).

An typical instatiation of the PLLE2_BASE module might look like this:

PLLE2_BASE #(
    .CLKFBOUT_MULT(8),          // This multiplies your output clock by 8
    .CLKFBOUT_PHASE(90.0),      // This shifts the output clock by 90 degrees
    .CLKIN1_PERIOD(10.0),       // This specifies the period length of your input clock. This information is mandatory.

    // The following lines set up different dividers for every output
    .CLKOUT0_DIVIDE(128),
    .CLKOUT1_DIVIDE(64),
    .CLKOUT2_DIVIDE(32),
    .CLKOUT3_DIVIDE(16),
    .CLKOUT4_DIVIDE(128),
    .CLKOUT5_DIVIDE(128),

    // Similiarly you can set the duty cycle for every output
    .CLKOUT0_DUTY_CYCLE(0.5),
    .CLKOUT1_DUTY_CYCLE(0.5),
    .CLKOUT2_DUTY_CYCLE(0.5),
    .CLKOUT3_DUTY_CYCLE(0.5),
    .CLKOUT4_DUTY_CYCLE(0.9),
    .CLKOUT5_DUTY_CYCLE(0.1),

    // And the phase shift
    .CLKOUT0_PHASE(0.0),
    .CLKOUT1_PHASE(45.0),
    .CLKOUT2_PHASE(22.5),
    .CLKOUT3_PHASE(0.0),
    .CLKOUT4_PHASE(0.0),
    .CLKOUT5_PHASE(0.0),

    // You can also set up a divider for your input clock. This can be useful, if you have a very fast clock, which exceeds the limits of the PLL.
    .DIVCLK_DIVIDE(1),

    // At this point this instatiation differs from the real hardware. This is to allow setting a FPGA model and it's speed grade. This enables a more realistic simulation. It is, however, entirely optional. By default it is set to the most restrictive values (ARTIX -1), so it should work on every version.
    .FPGA_TYPE("ARTIX"),
    .SPEED_GRADE("-1"))
pll (
    // Bind the outputs of the PLL, for example like this.
    .CLKOUT0(output[0]),
    .CLKOUT1(output[1]),
    .CLKOUT2(output[2]),
    .CLKOUT3(output[3]),
    .CLKOUT4(output[5]),
    .CLKOUT5(output[6]),

    // These should always be set to the same wire.
    .CLKFBOUT(CLKFB),
    .CLKFBIN(CLKFB),

    // This informs you, if the output frequency is usable.
    .LOCKED(locked),
    // Bind your input clock.
    .CLKIN1(clk),

    // Allows you to power down or reset the PLL.
    .PWRDWN(pwrdwn),
    .RST(rst));

Example project

An example project using the PLLE2_BASE found under pll_example/pll_example.srcs/sources_1/new/. It is a simple program to show the usage of the module. It can be simulated from the tb/ or the pll_example directory using

• make pll_led_test

This runs iverilog and vvp to simulate the module. To inspect the results you can use GTKWave like this:

• gtkwave pll_led_tb.vcd

The default values chosen are meant to be seen with the naked eye on real hardware. If you run the simulation using make the values are adjusted to be easy to see in GTKWave.

There is also an example project using the PLLE2_ADV found under pll_adv_example/pll_example.srcs/sources_1/new It is similar to the other example, but uses the PLLE2_ADV module and it's dynamic reconfiguration capabilities. It can be simulated from the tb/ or the pll_adv_example directory using

• make pll_adv_example_test

This runs iverilog and vvp to simulate the module. To inspect the results you can use GTKWave:

• gtkwave pll_adv_example_tb.vcd

To learn more about the instantiation of the module, you should read Xilinx UG953 page 509ff.

Project Status

Working

• instantiation interface compatible to the one described in UG953
• setting the phase, duty cycle and divider of CLKOUT outputs (CLKOUTn_DIVIDE, CLKOUTn_DUTY_CYCLE and CLKOUTn_PHASE)
• lock detection (LOCKED)
• PWRDWN and RST signals
• setting DIVCLK_DIVIDE (divides the input clock)
• tests for RST, PWRDWN, output frequency, output phase and output duty cycle
• applying CLKFBOUT_MULT to multiply the output frequency
• applying CLKFBOUT_PHASE to set a phase shift to every output
• setting CLKINSEL and selecting one of two input clocks (PLLE2_ADV)
• basic dynamic reconfiguration functionality (PLLE2_ADV)
• CLKOUT6 (MMCME2_BASE)
• CLKOUT0_DIVIDE_F for fractional divides (MMCME2_BASE)
• CLKOUT0-3B for inverted outputs
• CLKOUT4_CASCADE for using the divider of CLKOUT6 to divide the CLKOUT4 output again (MMCME2_BASE)
• CLKFBOUT_MULT_F for fractional multipies (MMCME2_BASE)
• stopping the simulation, if illegal values are hit
• chaining PLLs should work as expected, although the CLKFBIN input is ignored

Not Working

• there is no feedback loop by design
• BANDWIDTH, REF_JITTER1, REF_JITTER2, COMPENSATION and STARTUP_WAIT settings won't work with the current design approach
• connecting CLKFBIN to any other clock than CLKFBOUT won't change the behaviour of the module
• dynamic reconfiguration only has an effect in the ClkReg1 and ClkReg2 registers as well as the DivReg register (PLLE2_ADV)
• RESERVED bits in the dynamic reconfiguration are ignored (PLLE2_ADV)

Test

You can test this project automatically using avocado or make. The testbenches themselves are written in pure verilog.

Avocado [recommended]

• install avocado: Documentation
• change into the tb/ folder
• run $ avocado run test_freq_gen.py test_high_counter.py test_period_check.py test_period_count.py test_phase_shift.py test_plle2_adv.py test_plle2_base.py test_mmcme2_base.py test_dyn_reconf.py

Make

• change into the tb/ folder
• run make. This will just run every testbench in it's default configuration.

Architecture

This diagram roughly outlines the basic architecture of the project outlining the pll.v module, which has the plle2_base.v, plle2_adv.v and mmcme2_base.v modules as wrappers determining the needed functionality.

License

This project is licensed under the ISC license.

FAQ

What limits does the PLL/MMCM have?

Use this table for parameters:

Also there is a limitation in the PLL regarding the possible frequency. They depend on the capabilities of the VCO, which itself depends on the FPGA model, it's speedgrade and if it's used by a PLL or an MMCM. It's frequency can be calculated using this formula: VCO frequency = (CLKFBOUT_MULT * 1000) / (CLKIN1_PERIOD * DIVCLK_DIVIDE). Use this table for reference:

Which PLL/MMCM should I choose?

The main differences between the two PLL versions are the support for two input clocks and dynamic reconfiguration in PLLE2_ADV. For a more in-depth overview of the differences see UGS472 page 70.

The MMCM offers an additional output (CLKOUT6), fractional divides for CLKOUT0 and CLKFBOUT (which functions as a fractional multiplier) and the possibility to use the divider of CLKOUT6 to divide CLKOUT4 again, allowing for divisors as high as 16384 (128 * 128).

Is this module synthesizable?

No, it isn't. This project is purely for simulation purposes. It is not meant to be synthesizable and contains a lot of unsynthesizable code. The examples however are synthesizable.