DDS in System Generator: how to set up periods?

If you use DDS Compiler in System Generator, you will notice there are 4 clock periods you need to deal with (circled in red in the snapshot below). This can be confusing for first time users. Hopefully this blog can clear up some of the confusions.

1. FPGA clock period (ns): only used for FPGA synthesis and implementation. This value does NOT affect simulation in Simulink (i.e. it will NOT affect output frequency of the DDS block).
2. Simulink System Period: only used in Simulink simulation. This is the lowest sampling period (highest sampling frequency) among all blocks in the model. There is no significance in its absolute value as long as the sampling periods of all other blocks are multiple of the Simulink System Period. Usually it is set to 1 to keep everything simple. (e.g. every tick in a scope is 1 clock cycle).
3. DDS System Clock (Mhz): It's used to calculate the phase increment for DDS block (see the DDS datasheet for details). Set it to match the FPGA clock period (i.e. DDS System Clock (Mhz) = 1/FPGA clock period).
4. Explicit Sample Period: This is the sample period used for the DDS block. IMPORTANT: if you set Simulink System Period to anything other than 1, "Use explicit period" MUST be checked and "Explicit Period" set to the same value as Simulink System period. Otherwise, Simulink will use the sample period of 1 for the DDS core and the output frequency will be wrong. This is likely a bug.

 Let's use a simple model (download the model for test1 here) below to show how all these periods play together in SysGen. The clock frequency of the SineGen block is 1/100th of the Simulink System Period (or 1MHz for a 100MHz clock input).

Test1:
FPGA clock period (ns) = 10
Simulink system period = 1
DDS System Clock (MHz) = 100
Use Explicit Sample Period checked and Explicit Period = 1
DDS Output (MHz) = 1

As shown in the scope below the DDS output matches the SineGen output:

Test2:
FPGA clock period (ns) = 10
Simulink system period = 0.1
DDS System Clock (MHz) = 100
Use Explicit Sample Period checked and Explicit Period = 0.1
DDS Output (MHz) = 1

As shown in the scope below the DDS output matches the SineGen output. Note that the Simulink system period is now 1/10th of the period in Test1, so there are 10x more clock cycles in this test with the same simulation time.

Test3:
FPGA clock period (ns) = 10
Simulink system period = 0.1
DDS System Clock (MHz) = 100
Use Explicit Sample Period unchecked
DDS Output (MHz) = 1

As shown in the scope below the DDS output frequency is 1/10th of SinGen output. The incorrect DDS output frequency is caused by that when "Use Explicit Sample Period" is unchecked Simulink use a sample period of 1 for the DDS block. So it's very important to check this checkbox and set the explicit period to match the Simulink system period if it's not 1. It's worth mentioning that for sampling frequency related issues it's very helpful to turn on "Sample Time Display" (under Format menu) and then update the diagram by pressing CTRL-D. You can easily see if the sample time used for a particular is correct with sampling time displayed on all signals.