Where is my $2 FPGA?
Ever since FPGA companies broke the .2 micron barrier we’ve been led to believe that gates are free, so use them – FPGAs will take over the world. Today FPGA leaders like Altera and Xilinx are leading the path to 28 nm and beyond. You have oceans of gates that are cheaper than ever, but small embedded microcontrollers can still be a quarter the cost. When compared to an Atmel AVR or Microchip dsPIC, even the smallest of the low-cost FPGA have a hard time competing against them in price and functions.
Doing a drive-by on a online distributor like Digi-Key or Mouser, you’ll find that a 40 MIPS dsPIC is packed with functions: ADC, IrDA, I2C, LIN, SPI, UART, 6 KB flash, 256 bytes RAM, PWM support, watch dog timers, motor control functions – all this for a whopping $2.
The Xilinx smallest low cost device, the Spartan 6 FPGA in the smallest package size you have – 100 pin TQFP – gets you 3800 logic cells, 4800 registers, 8 DSP48 cells, 2 clock management tiles, 2.25KB RAM/ROM and 102 I/Os. All this costs just under $11. Adding a soft processor like micro-blaze, you can eat up 1300 LUTs and 1600 registers. When you consider the soft processor in the part, your small ocean of gates gets reduced to 2500 logic cells and 3200 registers – still plenty to design most small custom logic interfaces. The FPGA designer would still need to address cost and board space of an analog-to-digital function and that ever-nagging configuration memory. Both increase board space and cost.
The Achilles heal in the DSP micro is the pin count and lack of custom logic. The dsPIC at that price comes in a 28 pin SOIC. If a larger package size is selected, like a 100 pin TQFP, the price jumps to about $5. The sample part also had 128 KB flash, 16 KB RAM, and DMA.
So which one is a better choice? As most answers go, it depends. Your decision will depend on board space, power rail voltages available, cost, mixed signal needs, MIPs need, customization needs, and pinout needs. Though a $2 FPGA might simplify the decision.