VHDL coding tips and tricks: Careful RAM designing for reducing power consumption in FPGA
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Friday, August 13, 2010

Careful RAM designing for reducing power consumption in FPGA

     I will discuss some points which will be helpful for reducing the power reduction in an FPGA in this post.Mainly I am concentrating on the power dissipation caused by the RAM.These points are selected from the Xilinx white paper for Virtex-5 system power design considerations.But I will note down the points which will apply for any Xilinx FPGA.
     There are two primary types of power consumption in FPGA's: static and dynamic power. Static power is consumed due to transistor leakage. Dynamic power is consumed by toggling nodes as a function of voltage, frequency, and capacitance.The leakage current is directly proportional to the speed of the processor,operating voltage of the processor and junction(or die) temperature.So static power increases from Virtex 4 FPGA to Virtex 5 FPGA.On the other hand dynamic power reduces from Virtex 4(90 nm device) to Virtex 5(65 nm device).This is because dynamic power is directly proportional to the voltage of operation and the capacitance(this includes the transistor parasitic capacitance and metal interconnect capacitance).From Virtex 4 to Virtex 5, these two parameters decrease and so we get around 40 % reduction in dynamic power.
You can get more details from the pdf link I have shared above.

      Xilinx has given some tips in reducing the power consumption by designing RAM's intelligently.I am writing down them one by one:
1)Choose the right RAM primitive for your design.When choosing a RAM organization within the target architecture, the width, depth,and functionality must be considered. Choosing the right memory facilitates the selection of the most power-efficient resource for the end design.
2)Ensure that, the block RAM is only enabled when data is needed from it.This is because the power requirements of a block RAM is directly proportional to the amount of time it is enabled.Normally for ease of coding the enable signal is always "ON".But for power sensitive applications take some extra effort to make use of enable signal of RAM.
Another tip regarding enable signal is explained in the following example.Say you want a 2k x 8 bit RAM in your design.Then use four 512 x 8 bit RAM's for this.Now use a seperate a enable signal for each RAM.This needs some extra logic, but at any time only one RAM will be ON , so we can save around 75% of the power.
3)Ensure the WRITE_MODE of RAM is set properly.If the block RAM contents are never read during a write, the RAM power can be reduced by a significant amount with the selection of the NO_CHANGE mode rather than the default WRITE_FIRST mode.This mode can be set easily if you are using the core generator GUI to create the RAM module.

Note:- It would be wise to see the Xilinx white paper on power reduction for your particular FPGA before start the coding for your design.This will be helpful in getting useful tips which are device specific.

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