Design IBIS-AMI Models to Support Clock Forwarding

Background

The IBIS 7.1 specification adds the ability pass in an external clock signal, either as a waveform or clock-times, to a data IBIS-AMI receiver GetWave model, using the clock_times pointer as defined in the IBIS specification. A new AMI Reserved Parameter, Rx_Use_Clock_Input, is used to enable this functionality.

The figure below shows a typical DDR5 coupled channel simulation setup using clock-forwarding. The clock times or waveform generated by DQS0 is passed to DQ[7:0] (DQ0, DQ1... DQ7) using the DQ Rx DLL’s clock_times pointer. The DQ Rx DLL then operates on these clock times as desired (for example triggering DFE taps, modelling the DQS clock delay tree or centering the DQ on the DQS waveform) and then passes out the same or modified clock_times as usual. This same process is repeated for DQS1 and DQ[8:15].

This example provides an introduction to clock-forwarding in SerDes Toolbox and shows how to use various Simulink® tools and MATLAB® functions to generate and test an IBIS-AMI executable that supports clock-forwarding. It does not provide a specific clock-forwarding algorithm.

Tx/Rx IBIS-AMI Model Generation in SerDes Designer App

The first part of this example sets up the target transmitter and receiver architectures using the SerDes Designer app. The SerDes system is then exported to Simulink® where clock-forwarding will be added and explored. Finally, a spec-compliant IBIS-AMI model will be generated for use in any IBIS 7.1 compliant EDA tool.

Type the following command in the MATLAB® command window to open the dq_clock_forward model in SerDes Designer:

>> serdesDesigner('dq_clock_forward')

The transmitter (Tx) uses a 4-tap FFE. The receiver (Rx) will use a DFE and CDR that supports clock-forwarding. Since clock forwarding is a time-domain only feature, a dedicated DFEClkFwd block will be added later after exporting the SerDes system to Simulink.

Configuration Setup

Transmitter Model Setup

Channel Model Setup

Receiver Model Setup

Export SerDes System to Simulink

Click on the Export button to export the configuration to Simulink for customization and generation of the AMI model executables.

Rx IBIS-AMI Model Setup in Simulink

The second part of this example takes the SerDes system exported by the SerDes Designer app and customizes it as required for clock-forwarding in Simulink.

Review Simulink Model Setup

The SerDes System imported into Simulink consists of Configuration, Stimulus, Tx, Analog Channel and Rx blocks. All the settings from the SerDes Designer app have been transferred to the Simulink model. Note that since clock-forwarding is only implemented in the Rx, the Tx block will be untouched in this example.

Rx Block Setup for Clock-Forwarding

Double-click on the Rx block to push into the Rx-subsystem. Using the Library Browser, navigate to Library->SerDes Toolbox->Datapath Blocks and drag the DFEClkFwd block onto the canvas. Alternatively, you can left-click on the canvas and type "DFEClkFwd" to quickly add the DFEClkFwd block. Connect this block to the WaveIn and WaveOut ports.

Adding Rx AMI Parameter Rx_Use_Clock_Input

When the DFEClkFwd block is added to the Rx datapath, the Rx AMI parameter Rx_Use_Clock_Input is automatically added to the Rx AMI tree. If needed, this parameter can also be manually added to the Rx AMI model by clicking on the Reserved Parameters... button and checking the box next to Rx_Use_Clock_Input.

Run Refresh Init

While clock-forwarding is a time-domain only parameter, in order to enable the DFE and CDR in Init (for Statistical simulations) you must run Refresh Init after adding the DFEClkFwd block. In the Rx sub-system, double click on the Init block, then click on the Refresh Init button.

Review DFEClkFwd Block Setup

The DFEClkFwd block contains standard SerDes Toolbox DFE and CDR system objects, a clock-forwarding CDR and a CDR output selector. When the clock-forwarding parameter Rx_Use_Clock_Input is set to "None", the standard CDR is used. When Rx_Use_Clock_Input is set to "Times" or "Wave", the clock-forwarding CDR is used to pass in clock times or a waveform from an external AMI model. This block also supports PAMn signaling.

Double-clicking on the DFEClkFwd block will bring up the default setup. The DFE is set up for four DFE taps. The Initial tap weights are set to 0, the Minimum tap values are set to [-0.2 -0.075 -0.06 -0.045] V, and the Maximum tap values are set to [0.05 0.075 0.06 0.045] V. Finally, the 2x tap weights option is selected to make the DFE tap values compliant with the JEDEC specification.

Run Simulink Model

The Simulink model is now ready to run. Press the run button to launch the simulation.

As the simulation runs, the Time Domain eye diagram gets constantly updated:

After the simulation is complete, the Init Statistical and Time Domain Analysis Results plot becomes available:

Note that since clock-forwarding only affects the Time Domain results, the Statistical results will not reflect the effects of clock-forwarding.

Controlling Clock-Forwarding Simulations

There are two AMI parameters, Rx_Use_Clock_Input and ForwardClockOffset that control the operation of clock forwarding in the Rx AMI model. In addition, there are a number of options for generating a clock or strobe waveform that can be used to test the Rx AMI model as follows.

Changing Current Value of Rx_Use_Clock_Input

The operation of the clock forwarding is controlled by the reserved AMI parameter Rx_Use_Clock_Input. When Rx_Use_Clock_Input is set to "None", the standard CDR is used. When set to "Times", the in-coming clock times are used directly. When set to "Wave", the in-coming waveform is converted to clock times using a zero-crossing detector. These clock times are used to determine when to apply the DFE taps, then are passed unchanged to the IBIS-AMI clock_times block to generate the normal clock times out for use by the EDA tool.

To change the value of Rx_Use_Clock_Input, use the AMI-Rx tab of the SerDes IBIS-AMI Manager.

Setting Clock Offset

The input AMI parameter ForwardClockOffset is included in the DFEClkFwd block. This parameter is of type Integer, with a Default of 0 and a Range of 0 to 1024. In the DFEClkFwd block, this parameter is used to delay the incoming clock times by up to 1024 samples. Using the SerDes IBIS-AMI Manager you can use this delay to adjust the location of the external clock with respect to the data waveform as desired.

For example, here is the time domain eye diagram with DFE Tap 1 set to -0.100, Rx_Use_Clock_Input set to Wave and ForwardClockOffset set to 0:

Note how the DFE taps are being applied in the center of the eye instead of at the edges of the eye. With the ForwardClockOffset delay set to 7, the DFE taps are being applied at the ideal location at the edge of the eye:

Note: Delay values less that 0 will have no effect on the resulting waveform.

Generating Clock and Strobe Waveforms

The SerDes toolbox Stimulus block is used to generate the stimulus for both the regular data path and the external clock or strobe. The regular data path uses the Stimulus tab while the external clock or strobe signal is generated with the Clock Forwarding tab.

When Rx_Use_Clock_Input is set to "None" in the IBIS-AMI Manager, the settings in this dialog are ignored.

When Rx_Use_Clock_Input is set to "Times", this dialog is used to set the name of the clock times array that resides in the Base Workspace or the Model Workspace. These times are used directly by the DFEClkFwd block. The array name is defined in the Clock Times Name field. For more information about generating this clock times array, see Creating a new clock times array.

When Rx_Use_Clock_Input is set to "Wave", this dialog can generate a clock or strobe waveform, with or without jitter, and can be passed through the same analog channel model that the data path is using. It can also be used to set the name of a waveform array that reside in the Base Workspace or the Model Workspace. This waveform will be passed through a zero-crossing detector and the resulting clock times are used directly by the DFEClkFwd block.

Note: An ideal clock waveform, defaultClockWave, and ideal clock times, defaultClockTimes, are automatically generated when clock-forwarding has been enabled. These arrays will be regenerated automatically whenever the Symbol time, Samples per symbol, or Number of symbols is changed. Any custom array that is pointed to by the model will need to be manually updated if any of these values change.

How to Visualize Results

To assist in debug or to verify proper operation of clock-forwarding, plotting the waveforms and/or clock-ticks can be very helpful. This is accomplished by using data logging to enable the use of the Data Inspector for plotting waveforms. The primary nodes of interest have already been logged for you.

The clock or strobe waveform will show up in the Data Inspector as Switch:1, the clock times into the Rx model are labeled Forward Clock Times:1, while the clock times out of the Rx model are labeled clockTime and the Rx model output waveform is labeled rxOut. To turn on additional data logging, right-click on any signal and select Log Selected Signals.

Plotting Clock and Data Waveforms

After running a simulation open the Data Inspector by clicking on the -icon in the Simulink Simulation tab. In the Data Inspector check the boxes for Switch:1 (the incoming clock or strobe waveform, Red in the figure below) and for rxOut (the Rx model out waveform, blue in the figure below). You should see that the rising/falling edges of the external clock or strobe waveform (Red) correspond with the rising/falling edges of the Rx model waveform (Blue). If they do not line up as expected, the offset can be adjusted by using the clock offset (see Setting the clock offset) or the stimulus Delay parameter (see Generating Clock and Strobe Waveforms).

Plotting Clock-Ticks In and Out

After running a simulation, in the Data Inspector check the boxes for Forward Clock Times:1 (the clock-times into the Rx model) and clockTime (the clock-times out of the Rx model). In this case, the clock times in are identical to the clock times out of the Rx model since no clock offset was used (Forward Clock Offset = 0).

Creating New Clock or Strobe Waveform Array

Generating a new clock or strobe waveform for use in the clock-forwarding tab of the Stimulus block is accomplished using Parallel Link Designer (part of Signal Integrity Toolbox), Signal Integrity Link and the MATLAB scripting interface to Signal Integrity Toolbox. Note that since you are only interested in creating a stimulus pattern, any AMI model can be used for this process. The following steps assume you are using the dq_clock_forward IBIS-AMI model generated at the end of this example.

Here is an overview of the required steps. For additional information on using Signal Integrity Link, see Signal Integrity Link. Note that if you have previously run Signal Integrity Link on this Simulink model you can begin with step 3.

% Create a SignalIntegrityProject object
% Note: Replace 'pld_stim_gen' with your Signal Integrity Toolbox project name
sip = SignalIntegrityProject('pld_stim_gen',opendesigner=false);

% Create a SignalIntegrityWaveform object
resultFiles = sip.Sheets.Simulations.Waveforms;

% Extract stimulus waveform from the TimeDomain waveform file
stimulus = resultFiles(9).table;

% Format stimulus waveform for use in SerDes Toolbox
stimWaveArray = stimulus{:,2};

% Center stimulus waveform around 0V
stimWaveArray = stimWaveArray - 0.5;

% Done
sip.exit

To use the new stimulus array, open the Clock Forwarding tab of the Stimulus dialog, make sure the current state of Rx_Use_Clock_Input is set to Wave, select the radio button for Use External Waveform or Clock Times array, then enter the name of the new stimulus array (in this case: stimWaveArray) in the Waveform Name box.

Notes:

Creating New Clock Times Array

Generating new clock ticks for use in the clock-forwarding tab of the Stimulus block is accomplished using Parallel Link Designer (part of Signal Integrity Toolbox), Signal Integrity Link and the MATLAB scripting interface to Signal Integrity Toolbox. Unlike with the new clock or strobe waveform array process above, here it is recommended to use the actual clock or strobe AMI model. The following steps assume you are using the dqs_clock_forward model included with this example. For more information on the Clock/Strobe model see Strobe Rx IBIS-AMI Model Requirements.

Here is an overview of the required steps. Note that if you have previously run Signal Integrity Link on the dqs_clock_forward Simulink model you can begin with step 3.

% Create a SignalIntegrityProject object
% Note: Replace 'pld_clock_gen' with your Signal Integrity Toolbox project name
sip = SignalIntegrityProject('pld_clock_gen',opendesigner=false);

% Create a SignalIntegrityWaveform object
resultFiles = sip.Sheets.Simulations.Waveforms;

% Extract clock times from the TimeDomain clock_ticks file
clockTimes = resultFiles(10).table;

% Format clock times for use in SerDes Toolbox
clockTimesArray = clockTimes{:,1:2};

% Done
sip.exit

To use the new clock times array, open the Clock Forwarding tab of the Stimulus dialog, make sure the current state of Rx_Use_Clock_Input is set to Times, select the radio button for Use External Waveform or Clock Times array, then enter the name of the new stimulus array (in this case: clockTimesArray) in the Clock Times Name box.

Notes:

Generate Rx IBIS-AMI Model

The final part of this example takes the customized Simulink model and generates IBIS-AMI compliant model executables, IBIS and AMI files for the clock-forwarding receiver.

Open the Block Parameter dialog box for the Configuration block and click on the Open SerDes IBIS-AMI Manager button.

On the AMI-Rx tab in the SerDes IBIS-AMI Manager, make sure that the current value of all parameters are set to the appropriate values since they may have been modified while working through this example:

Export Models

On the Export tab in the SerDes IBIS-AMI manager:

Review AMI File

The resulting Rx AMI file will look like a normal Rx AMI file with two exceptions. First, the AMI_Version is set to 7.1, while the second is the inclusion of the reserved parameter Rx_Use_Clock_Input.

Tips

With clock forwarding, any data transition that does not have an associated clock time will be ignored. For example, when using a strobe stimulus pattern, if a data transition occurs during the strobe idle period, this transition will be lost.

When using the Delay option in the stimulus or the AMI parameter ForwardClockOffset to align the clock signal with the data signal as desired, note that the inclusion of DFE in the data path can increase the edge-rate of the data signal such that the equalized waveform can cross Vref well before the un-equalized clock waveform. This can result in an apparent timing offset between data and clock as more DFE is applied, which means the clock signal may not necessarily need to offset by a full 90 degrees.

The Tx FFE block introduces a 1UI delay for every pre-tap present. If there is no FFE in the clock or strobe Tx AMI model (such as the dqs_clock_forward.slx model described below), this can cause a 1UI+ difference in the time between when the stimulus is applied and when the signal reaches the Rx data decision point. The delay will need to be accounted for when including the affects of correlated vs un-correlated jitter.

When using the zero-crossing detector to generate clock times (such as in the dqs_clock_forward.slx model described below), generally the results with Rx_Use_Clock_Input set to "Wave" will be the same as when set to "Times". However, if a conventional CDR is used to generate clock times, the resulting clock times will be output starting at time = ~0. In this case, the results with Rx_Use_Clock_Input set to "Wave" will not be equivalent to when it set to "Times".

Model Limitations

This clock forwarding AMI model requires an EDA tool that supports the IBIS 7.1 reserved parameter Rx_Use_Clock_Input.

Per the IBIS 7.1 specification, it is intended that Data and Strobe AMI models will be delivered as a matched pair. This may place additional requirements on the Strobe AMI model which are discussed in the next section.

Strobe Rx IBIS-AMI Model Requirements

Per the IBIS 7.1 specification, it is intended that Data and Strobe AMI models will be delivered as a matched pair. This means that a Strobe (or Clock) AMI model will also need to be generated for use along with this Data AMI model. The Strobe model should not use a CDR to generate clock times, but instead should only output clock times at the zero-crossings of the output waveform, which can be handled by using a MATLAB Function Block. Additional features for the Strobe AMI model include the handling of strobe pre-amble (only output clock times after the end of the pre-amble) and handling of Single-Data-Rate (SDR) signals such as a Clock that only outputs clock times on the rising edge of the output waveform.

An example Strobe/Clock AMI model is attached to this example and can be opened by typing the following command:

>> open_system('dqs_clock_forward.slx')

This model contains a pass-through differential Tx and Rx. The Rx AMI model outputs clock times only at zero-crossings and contains two controls:

Test Generated IBIS-AMI Models

The clock forwarding receiver IBIS-AMI model is now complete and ready to be tested in any industry standard AMI model simulator that supports IBIS 7.1.

References