The last item that we need to bring onto an FPGA is the power. We may need multiple VDD and GND power pads to reduce supply bounce or separate VDD pads for mixed-voltage supplies. We may also need to provide power for on-chip programming (in the case of antifuse or EPROM programming technology). The package type and number of pins will determine the number of power pins, which, in turn, affects the number of SSOs you can have in a design.
As a general rule a plastic package can dissipate about 1 W, and more expensive ceramic packages can dissipate up to about 2 W. Table 6.3 shows the thermal characteristics of common packages. In a high-speed (high-power) design the ASIC power consumption may dictate your choice of packages. Actel provides a formula for calculating typical dynamic chip power consumption of their FPGAs. The formula for the ACT 2 and ACT 3 FPGAs are complex; therefore we shall use the simpler formula for the ACT 1 FPGAs as an example 1 :
(still air) 5
As an example of a power-dissipation calculation, consider an Actel 1020B-2 with a 20 MHz clock. We shall initially assume 100 percent utilization of the 547 Logic Modules and assume that each switches at an average speed of 5 MHz. We shall also initially assume that we use all of the 69 I/O Modules and that each switches at an average speed of 5 MHz. Using Eq. 6.7 , the Logic Modules dissipate
Suppose we intend to use a very thin quad flatpack ( VQFP ) with no cooling (because we are trying to save area and board height). From Table 6.3 the thermal resistance, q JA , is approximately 68 °CW –1 for an 80-pin VQFP. Thus the maximum junction temperature under industrial worst-case conditions (T A = 85 °C) will be
(with an accuracy of no better than 10 °C). Actel specifies the maximum junction temperature for its devices as T Jmax = 150 °C (T Jmax for Altera is also 150 °C, for Xilinx T Jmax = 125°C). Our calculated value is much too close to the rated maximum for comfort; therefore we need to go back and check our assumptions for power dissipation. At or near 100 percent module utilization is not unreasonable for an Actel device, but more questionable is that all nodes and I/Os switch at 5 MHz.
Our real mistake is trying to use a VQFP package with a high q JA for a high-speed design. Suppose we use an 84-pin PLCC package instead. From Table 6.3 the thermal resistance, q JA , for this alternative package is approximately 44 °CW –1 . Now the worst-case junction temperature will be a more reasonable
It is possible to estimate the power dissipation of the Actel architecture because the routing is regular and the interconnect capacitance is well controlled (it has to be since we must minimize the number of series antifuses we use). For most other architectures it is much more difficult to estimate power dissipation. The exception, as we saw in Section 5.4 “Altera MAX,” are the programmable ASICs based on programmable logic arrays with passive pull-ups where a substantial part of the power dissipation is static.
Each FPGA has its own power-on reset sequence. For example, a Xilinx FPGA configures all flip-flops (in either the CLBs or IOBs) as either SET or RESET. After chip programming is complete, the global SET/RESET signal forces all flip-flops on the chip to a known state. This is important since it may determine the initial state of a state machine, for example.