Power supply noise is produced by changes in the current drawn on the power supply. Both the block RAM and UltraRAM have consistent current drawn when they are continually reading or writing data. Switching between read and write operations also requires a consistent current drawn. A change in current drawn occurs when the RAMs are enabled or disabled using the enable pins. A list of enable pins per RAM primitive are provided in the following table.
| UNISIM | Enable Pins |
|---|---|
| RAMB18E5 | ENARDEN, ENBWREN |
| RAMB36E5 | ENARDENL, ENARDENU, ENBWRENL, ENBWRENU |
|
URAM288E5 URAM288E5_BASE |
EN_A, EN_B |
Gating the clock to the RAM is equivalent to using the enable pin. Three factors determine the amount of power supply noise that is induced by the RAM being enabled and disabled.
- The number of RAMs that are simultaneously enabled or disabled. In the case where the numbers are different, use the maximum.
- The frequency of the RAM. The current drawn is a function of the RAM clock frequency. Higher frequencies produce larger deltas in the current step.
- The rate at which the RAM is enabled and disabled. Lower rates allow more time for the capacitance within the power network to recover before the next event.