使用FPGA学习数字电路设计

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"Learning FPGAs" 是一本由 Justin Rajewski 编著，O'Reilly 出版的书籍，出版于2017年。这本书面向程序员、工程师、产品设计师以及爱好者，介绍如何使用现场可编程门阵列（FPGA）设计数字电路。FPGA是一种可重构的硬件设备，使得数字电路设计变得更为便捷和经济，如今也逐渐普及到业余爱好者群体中。书中的内容从基础开始，逐步引导读者进行复杂的项目设计，适合对电学基础知识有一定了解的学习者。本书的主要知识点包括： 1. **FPGA基本概念**：FPGA是场可编程门阵列的缩写，它允许用户根据需求配置其内部逻辑结构，实现各种定制化的数字电路功能。这种技术简化了传统数字电路设计的复杂流程，降低了成本。 2. **数字电路设计**：书中涵盖了数字电路的基础知识，如逻辑门、触发器、计数器等，这些都是构建FPGA项目的基础。读者将学习如何使用这些基本元素设计并实现复杂的逻辑功能。 3. **Mojo和Lucid HDL**：书中使用Mojo开发板作为实践平台，Mojo是一个适合初学者的FPGA开发工具。同时，Lucid HDL作为一种高级硬件描述语言，用于编写和实现FPGA的设计。 4. **逐步进阶**：从简单的电路设计开始，逐渐过渡到更复杂的项目，如自定义处理器的设计。这使得学习过程更加系统和渐进，适合不同层次的读者。 5. **实践经验**：通过实际操作和项目案例，读者可以亲手实现并测试自己的设计，从而加深理解，提高技能。 6. **软件工具使用**：书中可能涵盖了FPGA设计所需的软件工具，如 Quartus II, Vivado 等，这些工具用于编译、仿真和下载FPGA设计。 7. **电子电路基础知识**：虽然读者需要具备一定的电学基础，但书中的内容也会帮助读者回顾和巩固这些知识，为FPGA学习打下坚实基础。 8. **学习路径**：本书提供了一个清晰的学习路径，适合自学或课堂教学，帮助读者从初学者成长为熟练的FPGA设计者。《Learning FPGAs》是一本实用的FPGA入门指南，通过理论与实践的结合，使读者能够快速掌握FPGA设计技能，并应用于实际项目中。对于想要进入嵌入式系统、高速数据处理、人工智能等领域的人来说，FPGA技术是不可或缺的一部分，这本书将是理想的起点。

Because of this, people have started turning to FPGAs to accelerate tasks by creating

custom hardware.

When you are working with FPGAs, you are designing hardware, not software. To

make this a little clearer, let’s look at something that could be done with either an

FPGA or a microcontroller: turning on an LED when you press a button. If you were

to do this with a microcontroller, such as an Arduino or Raspberry Pi, the code to do

this would read the button input, and if it is pressed, turn the LED on, and otherwise

turn it off. This would keep happening over and over in a loop. Read button, update

LED, read button, update LED, and so on. The problem with this is that the processor

is now spending all of its time keeping the LED up to date with the current value of

the button.

If you did the same project with an FPGA, you could simply connect the LED and the

button through the FPGA. Since they are connected, there is no loop, and there is no

code; the LED just turns on when the button is pressed. Now, if you wanted to change

it up an, say, turn on the LED when the button is not being pressed, with a microcon‐

troller you just change the test condition. However, with an FPGA, you can simply

insert a NOT gate between the LED and button.

With the microcontroller, the processor is fully busy with this trivial task. What hap‐

pens when you want it to also do something else, such as read other inputs or per‐

form calculations? The processor has to juggle the tasks, spending only finite

amounts of time on each one. As the complexity of your other code grows, the

amount of time the processor has to spend checking the button and updating the

LED shrinks, making it take longer and longer for the LED to turn off after you press

the button.

FPGAs are not like this. Instead, with an FPGA, since it is all hardware, the circuit

you designed to read the button and update the LED will operate completely inde‐

pendently of anything else you throw in your design. You could have another part of

the FPGA crunching some serious numbers, but the LED will still be just as respon‐

sive as before.

Although you can usually do the same task with a microcontroller or an FPGA, there

is usually a clear better choice. Things that require a lot of complex decision-making

or sequential operations are commonly better candidates for microcontrollers.

Things that can be done in stages or have a regular set pattern are great for FPGAs.

This is just a rule of thumb, but the takeaway is that custom hardware and a processor

fill different needs, and it’s often great to have both (and the Mojo does).

Another powerful feature of FPGAs is that they can read or write every I/O pin every

clock cycle. With a microcontroller, you are typically limited to reading registers of 8

or 32 bits that correspond to I/O pins. If you need to sample more pins, you need to

do it over multiple cycles, which may not be ideal. This makes it really easy to drive

4 | Chapter 1: Introduction

things that require a lot of I/O, such as LED matrices or a ton of servos. It’s also possi‐

ble to read and write I/O pins at much higher rates. The FPGA on the Mojo is capable

of I/O speeds up to 950 MHz; that’s 950 million reads or writes a second! Compare

this to a typical Arduino’s max I/O toggle rate of around 3–4 MHz (with a 16 MHz

clock), and this is achievable only if you dedicate all the processing time to toggling

the pin.

Table 1-1 summarizes some of the key distinctions.

Table 1-1. FPGA versus microcontrollers

Micro FPGA

Write software. Design hardware.

Typically executes one instruction at a time. All parts of your circuit can operate independently.

Fixed maximum clock speed. Maximum clock speed is dependent on your design.

A handful of I/O pins that can be accessed in small

groups (typically eight) at a time.

Many I/O pins that can all be accessed simultaneously.

Usually store programs in nonvolatile (persistent)

memory.

RAM based and needs to be programed after power on (the Mojo

does this automatically).

Often very power ecient with advanced sleep modes. Power usage depends on your design but typically requires more

than a microcontroller.

Fixed peripherals that limit the devices you can connect. Can interface with virtually any digital device.

So what can you do with an FPGA that you can’t with a microcontroller? In Chap‐

ter 12, we use seven microphones to detect the directions of potentially multiple

sound sources. Figure 1-3 shows the layout of the microphones on the board. This is

done by calculating the delays between all the microphones for various frequencies

and piecing them together to get an image of the sounds around the microphone

array. A crucial step is sampling all the microphones at exactly at the same time. With

an FPGA, it is fairly trivial to exert precise control over I/O pins. Once the samples

are gathered, the FPGA is capable of performing all the calculations incredibly fast.

The result is a sensor that constantly listens to its surroundings, generating direction

data at over 80 Hz (limited by the sample acquisition time and not the processing

time).

FPGAs Versus Microcontrollers: A Comparison | 5

The tools require you to be using Windows or Linux. If you’re

using Windows, Windows 7 or newer is recommended. Although

Windows 8 and above isn’t officially supported by the tools, it

should still work with some minor workarounds. Most versions of

Linux should work fine, but CentOS is known to have issues with

setting up the Mojo and is not recommended. Ubuntu and its

derivatives should work out of the box.

If you have a Mac, you can run Linux in a virtual machine or boot

Linux from a USB drive. You can do the same for Windows if you

have a license. If going this route, I recommend VirtualBox and

Ubuntu. Both are free and known to work.

Installing ISE

ISE is the name of the tool Xilinx provides to build projects for the Spartan 6 family

of FPGAs. Xilinx has a newer tool called Vivado, but that doesn’t support Spartan 6

devices.

Unfortunately, the installation process for ISE is a bit complicated. You are required

to make an account with Xilinx to verify what you are using ISE for and where you

live (export restrictions are imposed by the US government). ISE is a fairly large pro‐

gram. The download file is just over 6 GB, and the actual installation requires an

additional 16 GB or so of space. After it is installed, you will then need to license it.

The FPGA on the Mojo is covered by Xilinx’s free WebPACK license, but you still

have to get the free license.

Head over to the Xilinx Downloads page. You need to download version 14.7 of ISE.

This is the latest and final version. It should already be selected. Scroll down past the

Multi-File Download: ISE Design section to ISE Design Suite. In this section, select

the full installer for your platform (Linux or Windows). You will likely be prompted

to fill out a survey on your reasons for downloading it. Then you will be asked to sign

in. Create an account to continue with the download.

If you are on Windows, you need a way to extract the TAR file. Some TAR extractors

don’t like this archive and fail to extract the entire thing. 7-Zip is known to work and

is free. You can download it from the 7-Zip website.

With the files extracted, you need to start the installer. On Windows, double-click the

xsetup executable. On Linux, you need to run xsetup with root privileges:

sudo ./xsetup

Once in the setup, accept all the license agreements. On the page that asks which edi‐

tion to install, select ISE WebPACK, as shown in Figure 1-5.

8 | Chapter 1: Introduction

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