Some Altera devices allow power supply rail sharing. For instance you may share the Stratix® V GX device VCC and VCCHSSI supplies which have very different specifications.
- The VCC rail is high current and may have a low Feffective frequency of 5 MHz due to the on package decoupling.
- The VCCHSSI rail has a lower current but a higher Feffective frequency of perhaps 48 MHz due to it having less on package decoupling.
If you follow the Altera® Power Distribution Network (PDN) Tool guidance for sharing supply rails, an unrealistic amount of high-frequency decoupling capacitors are required for the combined scenario of:
- Combined current of VCC VCCHSSI with an Feffective frequency of about 5 MHz
A more realistic approach is to run separate PDN analysis on each rail and place the combined number of capacitors on the shared rail. Using this approach, you should place the high frequency capacitors so they are more effective on the VCCHSSI pins that have a higher Feffective frequency requirement. The VCC supply will require mostly medium and low frequency bulk decoupling. All capacitors connected to the shared supply will contribute to the decoupling of each individual rail.
- You can acquire accurate current requirements for your project using the Altera Early Power Estimator (EPE) Tool or Quartus II Software Power Play Power Analyser (PPPA).
- You can determine actual Feffective for the rail in your design from the Altera PDN Tool
- An alternative decoupling sharing scheme is to use the Root Sum Squared (RSS) approach detailed on the Introduction tab of the Altera PDN Tool.