Specifying the RAM Activity for Jitter - 2021.1 English

Versal ACAP Hardware, IP, and Platform Development Methodology Guide (UG1387)

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2021.1 English

The USER_RAM_AVERAGE_ACTIVITY constraint specifies a value that represents the average frequency of all UltraRAM and block RAM on the device that can be switched (enabled/disabled). The value is used by the Vivado tools to model power supply noise induced by RAM switching and calculate jitter for global clocks in static timing analysis. The jitter is reported as a component of clock uncertainty in static timing analysis. If the USER_RAM_AVERAGE_ACTIVITY is not specified, a pessimistic default of 480 MHz is assumed, which can increase the difficulty of timing closure. For detailed information on calculating the USER_RAM_AVERAGE_ACTIVITY for your design, see the Xilinx Answer Record 76369.

To specify the USER_RAM_AVERAGE_ACTIVITY constraint, apply the constraint to the top-level [current_design] object in your XDC constraints:
set_property USER_RAM_AVERAGE_ACTIVITY 160 [current_design]

The following table shows how additional clock uncertainty caused by RAM switching can impact timing closure. In this example, a design has global clocks operating at 300 MHz, 400 MHz, and 500 MHz. The pessimistic default is assumed to be 480, and the USER_RAM_AVERAGE_ACTIVITY constraint is calculated to be 160. Applying the constraint decreases the clock uncertainty for all paths in each clock domain, which makes achieving design timing closure less difficult.

Table 1. Pessimistic Default Clock Uncertainty vs. Specified RAM Activity Clock Uncertainty
Clock Domain Pessimistic Default 480 Clock Uncertainty USER_RAM_AVERAGE_ACTIVITY 160 Clock Uncertainty Clock Uncertainty Decrease
300 MHz 0.133 ns 0.073 ns -0.060 ns
400 MHz 0.122 ns 0.066 ns -0.056 ns
500 MHz 0.088 ns 0.058 ns -0.030 ns