Field Programmable Gate Array FPGA has the advantages of high integration, small size, flexible reconfiguration, and low experimental risk, and has been widely used in complex digital systems.
With the maturity and rapid development of FPGA technology, the solution of only one FPGA device, some storage devices and some electrical interface matching circuits for the design of digital circuits has become the mainstream choice. According to years of application experience, in related digital systems, the selection of FPGA devices is very important. Unreasonable selection will lead to a series of subsequent design problems, and sometimes even make the design fail; reasonable selection can not only avoid design problems, but also And it can improve the cost performance of the system, prolong the life cycle of the product, and obtain unexpected economic effects.
After in-depth research, the following selection issues are summarized: device supply channels and development tool support; device hardware resources; device electrical interface standards; device speed grades; device temperature grades; device packaging; device price .
2. Issues that should be considered in FPGA device selection
2.1 Device Supply Channels and Development Tool Support
At present, the main FPGA suppliers are Xllinx, Altera, LatTIc and Actel, of which Xllinx and Altera are the largest and provide a wide variety of devices [2, 3]. The development of FPGA is very fast, and many types of FPGA devices are no longer mainstream products. In order to improve the life cycle of products, it is best to select models from mainstream devices with sufficient supply. At present, Altera's mainstream devices include CycloneⅡ, CycloneⅢ, StaTIxⅡ, StaTIxⅡGX, StaTIxⅢ and StatixⅢGX series, among which CycloneⅡ and CycloneⅢ series are mainly used in logic design and simple digital signal processing, StatixⅡ and StatixⅢ series are mainly used in high-speed complex digital signals Processing and high-speed logic design, StatixⅡGX and StatixⅢGX series are mainly used in the field of communication; the mainstream devices of Xllinx are Spartan-3E, Spartan-3A, Virtex-4LX, Virtex-4 SX, Virtex-4 FX, Virtex-5 LX, Virtex -5SX and Virtex-5 FX series, among which Spartan-3E and Spartan-3A series are mainly used in logic design and simple digital signal processing, Vir-tex-4 LX and Virtex-5 LX series are mainly used in high-speed logic design, Virtex -4 SX and Virtex-5 SX series are mainly used in high-speed complex digital signal processing, Virtex-4 FX and Virtex-5 FX series are mainly used in embedded systems.
Both companies offer excellent development tools. Xllinx company has integrated development environment ISE, Altera company has integrated development environment Quartus Ⅱ, two integrated development environments support the design and development of all the company's devices. The integrated development environment is not only powerful and user-friendly, but also has many third-party partners to provide corresponding technical support, enabling the device to obtain higher performance. Therefore, if there are no special application requirements, it is recommended to make device selection in these two companies.
2.2 Hardware resources of the device
The requirements of logic resources and I/O resources are the most concerned issues of every designer, and they are generally considered. However, the problems that may arise from excessive consumption of I/O resources and routing resources are easily overlooked. In mainstream FPGA devices, logic resources are relatively abundant, which can generally meet application requirements. However, in a more complex digital system, excessive consumption of I/O resources may lead to two problems [2, 3]: FPGA is overloaded, and the device heats up seriously, which seriously affects the speed performance, working stability and life of the device. , The heat dissipation of the device should be considered in the design; the local wiring resources are insufficient, the running speed of the circuit is significantly reduced, and sometimes the design cannot be adapted to the device, and the design fails. According to my application experience:
(1) When doing complex digital signal processing, multipliers and dividers with relatively high number of bits consume a large amount of global routing resources;
(2) When doing logic design, the bidirectional I/O port consumes a large amount of local wiring resources;
(3) In the application of using memory resources to design filters, the consumption of local wiring resources is relatively large;
(4) In applications where there are many electrical interface standards and complex logic, the consumption of local wiring resources is relatively large. According to Altera's recommendation, it is better to reserve more than 30% of logic resources, more than 20% of I/O resources and more than 30% of routing resources in the design. Moreover, from the structure of the devices of the two companies, the programmable logic blocks of Xllinx's devices are more complicated than those of Al-tera, and they are more flexible to use. In some complex designs with many control signals, Xllinx products are suitable for selection. However, Xllinx's device routing resources are segmented, and the predictability of device delays is less. In these applications, it is best to do design simulation first. It is ideal to have a full understanding of the design and consumption of wiring resources, especially the local wiring resources that are easily overlooked, and then consider device selection. of.
In applications where there are many multiplication operations and high speed performance requirements, it is best to choose devices with more DSP resources, such as Altera's Statix II and Statix III series, Xllinx's Virtex-4 SX and Virtex-5 SX series, etc.
The memory resources in the device are mainly used for two purposes: as a high-performance filter; and to realize a small-capacity high-speed data cache. This is a relatively precious hardware resource. Generally, there are not too many memory resources in the device. The logic capacity of the device with more memory resources is also very large, and the use is relatively small. There are not many supply channels, and the price of the device is also very high. high. Therefore, when selecting a device, it is better not to pursue the integration of the design one-sidedly and select this device, and consider the design scheme of low-end device + external expansion memory.
At present, phase-locked loops are integrated in mainstream FPGAs, and the use of phase-locked loops to phase-lock the clock can make the circuit obtain more stable performance. Xllinx provides a digital phase-locked loop, which has the advantage of obtaining more precise phase control, but its disadvantage is that the lower limit operating frequency is relatively high, generally above 24 MHz; Altera provides an analog phase-locked loop, whose advantage is the lower limit The operating frequency is low, generally above 16 MHz. The operating frequency of the enhanced phase-locked loop in the mainstream devices Statix II and Statix III series is only required to be above 4 MHz. The disadvantage is that the control accuracy of the clock phase is relatively poor.
In the field of communications, using optical fiber to transmit high-speed data is a more commonly used solution. A1tera's StatixⅡGX and StatixⅢGX series, Xllinx Company's Virtex-4 FX and Virtex-5 FX series all integrate high-speed serial transceivers, which are generally more expensive. At present, the high-performance dedicated serial transceiver chips provided by companies such as National and Maxim are not very expensive. Therefore, if it is only used for optical fiber data transmission, it is not necessary to use this device; if it is optical fiber data transmission + logic or algorithm comparison In complex applications, it is best to compare the two schemes and then consider whether to choose this device.
The use of FPGA devices with integrated hard-core microprocessors for embedded development represents a direction of embedded applications. Altera offers APEX series devices that integrate ARM, and Xllinx offers Virtex-4 FX and virtex-5 FX series devices that integrate Power-Pc. As the price of the device continues to drop, in many applications, the selection of this device can greatly improve the system performance and reduce the complexity of hardware design compared with the traditional FPGA+MCU application scheme without increasing the cost. At this time, it is ideal to use this device.
2.3 Electrical Interface Standards
At present, there are many electrical interface standards for digital circuits. In complex digital systems, there are often multiple electrical interface standards. At present, the electrical interface standards supported by mainstream FP-GA devices are: 1.5 V, 1.5-V, etc., which can meet most application design requirements.
However, each I/O of an FPGA device does not support all electrical interface standards. Taking Altera's FPGA as an example, only some I/Os support the SSTL-2 Class II electrical interface standard. When designing DDR, it will lead to The PCB layout is quite complicated, and the utilization of the I/O pins of the device is quite low. And almost all pins of Xllinx's FPGA support the SSTL-2 Class II electrical interface standard. It is ideal to choose Xllinx's FPGA at this time.
2.4 Device Speed Grades
Regarding the selection of device speed grades, a basic principle is: in the case of meeting application requirements, try to select devices with low speed grades. This selection principle has the following advantages:
(1) Due to the transmission line effect, devices with high speed grades are more likely to produce signal reflections, and the design should spend more effort on signal integrity; (2) Devices with high speed grades are generally used less, and their prices are often high. In addition, the supply channels of high-speed devices are generally relatively small, and the order cycle of devices is generally relatively long, which often delays the product development cycle and reduces the product launch rate.
2.5 Temperature rating of the device
In some applications, high requirements are placed on the device's ability to adapt to ambient temperature. At this time, it should be selected in industrial-grade or even military-grade or aerospace-grade devices. According to research, each type of FPGA of Altera has industrial-grade products; each type of FPGA of Xllinx has industrial-grade products, and some models of FPGAs provide military-grade and aerospace-grade products. If the design is mainly for military or aerospace applications, it is best to use Actel's devices, which are mainly aimed at these users.
2.6 Packaging of the device
At present, the packaging forms of mainstream devices are: QFP, BGA and FB-GA. The pin density of BGA and FBGA packaged devices is very high, multi-layer boards must be used in the design, the PCB wiring is quite complicated, the design cost is relatively high, and the cost of device welding It is relatively high, so it can be used as much as possible in the design. However, in applications where the density is very high, the integration is very high and the volume of the PCB board is relatively high, BGA and FBGA packaged devices should be used as much as possible. In another case, in applications with very high circuit speed, it is best to use BGA and FBGA packaged devices. These two packaged devices are beneficial to the design of high-speed circuits due to the relatively small pin lead inductance and distributed capacitance of the device.
2.7 Price of the device
The continuous improvement of device integration, the continuous increase of performance, and the continuous decline of price are the general trend of the development of FPGA devices. Therefore, it is a basic rule to select models in the new devices that are continuously introduced. Take the Virtex-5 just launched by Xllinx as an example, its performance is 30% higher than that of Virtex-4, while the relative price is 35% lower.
This paper not only summarizes the selection of FPGA devices in depth with some application examples, but also provides a lot of latest device information. These contents have great reference value for those who are engaged in FPGA application design.
Responsible editor: gt