The trade-offs between using fixed-point versus floating-point arithmetic in embedded designs continues to evolve. One set of trades-offs between using either type of arithmetic involves system cost, processing performance, and ease-of-use. Implementing fixed-point arithmetic is more complicated than using floating-point arithmetic on a processor with a floating-point unit. The extra complexity of determining scaling factors for fixed-point arithmetic and accommodating precision loss and overflow, has historically been offset by allowing the system to run on a cheaper processor, and depending on the application, at lower energy consumption and more accuracy than with a processor with an integrated floating-point unit.

However, the cost of on-chip floating-point units has been dropping for years and they crossed a cost threshold over the last few years as signaled by the growing number of processors that include an integrated floating-point unit (more than 20% of the processors listed in the Embedded Processing Directory device tables now include or support floating-point units). In conversations with processor vendors, they have shared with me that they have experienced more success with new floating-point devices than they anticipated, and this larger than expected success has spurred them to plan even more devices with floating-point support.

Please share your decision criteria for when to use fixed-point and/or floating-point arithmetic in your embedded designs. What are the largest drivers for your decision? When does the application volume make the cost difference between two processors drive the decision? Does the energy consumption between the two implementations ever drive a decision one way or the other? Do you use floating-point devices to help you get to market quickly and then migrate to a fixed-point implementation as you ramp up your volumes? As you can see, the characteristics of your application can drive the decision in many ways, so please share what you can about the applications with which you have experience performing this type of trade.