High Performance and Low Power Fixed-point Special Function Unit for Mobile Vertex Processors

A high performance and low power fix-point Special Function Unit (SFU) for mobile vertex processors is presented in this paper. The system supports the fix-point format for OpenGL ES 1.X and implements 16 bit precision after the decimal point and faithfully rounded reciprocal, square root, reciprocal square root, logarithm, and exponential functions. The functions are approximated by using a piecewise quadratic interpolation technique. A square root 2 circuit is used in the unit, and the lookup table size is reduced by 29% with respect to previously proposed techniques, without any loss in accuracy. Based on analysis result of computer error and truncate error, the speed and area of lookup table, square unit, multiplier and fused accumulation tree reach optimal. The SFU has been implemented in a 0.18m CMOS technology. The circuit is able to operate up to 300 MHz clock frequency, with a power dissipation of 12.8 mW at 300 MHz and area only 0.112 mm2. The results show that the fixed-point SFU is ideal for mobile vertex processors computing elementary functions.