Some tips on reducing power consumption in Xilinx FPGA's

Power estimation and power reduction is an important part of any design. Especially in wireless devices, the reduction in power is a very important factor. In this article I will note down some points, on how to reduce the power consumption for xilinx based designs.

   1)  BRAM Enable signal:

Every BRAM has an enable signal which by default is high always. Most of the HDL coders never care to disable it even when the BRAM is not used. But when this enable signal is ON BRAM consumes a lot of power. It doesn't matter whether you change the address or write the data. So always have a control logic which will control the bram enable signal.

   2)  Low power option in coregen for BRAM's:

You can create a BRAM entity file using Xilinx's Core generator software. There are several options available in coregen to help you achieve what you want. For low power designs select the "Low power" option in coregen.

   3)  Decide on LUT or BRAM:

Suppose you want instantiate a memory in your design. Rather than going straight at BRAM or LUT, give it some thought. Xilinx says that for small memory blocks (less than 4 Kbits) LUT consumes less power than BRAM. Similarly for large memory blocks (more than 4 Kbits) BRAM uses less power for its operation than LUT-RAM. So from design to design, switch to LUT-RAM or BRAM depending on the size of memory block.

   4)  Global reset:

All FPGA devices have an internal global reset path. When the device is switched OFF and then ON, all the flip flops and memories are reset to their initial state. But when we define one more reset signal in the HDL code, Xilinx creates a second reset. This second reset is relatively low and hence not recommended. But if you still want to use them make sure it is synchronous, so that the number of the control signals in your design is low.

   5)  Initialization of Registers:

It is recommended that we initialize the registers in our design. Normally we do this for safe simulation purposes. But during synthesis, these initialization values will be connected to the INIT pin of the flip flops. Remember that this will work only for bits and bit vectors. It will not work for integer or natural types.

   6)  DSP slice Utilization:

Depending on how complex your FPGA is it will have some number of DSP slices. These components are highly efficient with low power consumption and high speed. All the DSP blocks in Xilinx FPGA are synchronous. So when we define asynchronous behavior for these operations, XST can't implement it using DSP slices. This will decrease the efficiency of your design.