最近有时间学学CPLD，很惭愧没能学好学透，但对于我预先想做的工作课题已经可以对付了。不求锦上添花，但求能解决实际问题。
我认为学习CPLD首先有必要了解一下GAL的编程，虽然他们的语言可能不同，但是基本的程序思路、结构、需要表达的方法有很多类似的
地方，马马虎虎地说：N个GAL加在一起就是CPLD,严格说是不对的，但是对于没有入门之前可以简单的这样理解。所以粗略地了解一下GAL的
编程对学习CPLD有很大帮助。我希望我能够讲清楚学习掌握一些基本编程，通过学习能够掌握手头的简单应用方法，而进一步的深入研究就
不在入门之列了，掌握的基本方法就可以按自己的想法动手试验，然后才能步步深入。
GAL编程，一般可以使用ABEL语言编程，ABEL的语法基本上是由一组逻辑表达式进行叙述，一片GAL能帮我完成什么功能呢？或者说能改
变成什么样的芯片呢？对GAL来说，由于内部的逻辑电路（门）比较少，一般只能完成一些简单的逻辑转换、简单的计数器(位数少)、锁存器
等。一般简单来讲，GAL的可用输入比输出多，如果想用他做地址译码器是比较合适的，在单片机的应用中，往往需要用几个不同的门电路组
合或加上138之类的3-8译码器组合，译出几个特定地址，这才是GAL的专长，可以在一个芯片内完成，使得你的电路简洁，而且输入输出在不
违背基本约定的前提下可以随意调换，使得布板走线容易。先看看下面的一个例子：
module M273 "这是一个类似于74LS273的锁存器程序
title '8-bit Locker
xiaoqi Data I/O Corp.'
M273 device 'P16V8R'; "这里我们使用GAL16V8进行编程
"引脚定义，注意CLK,OE的定义只能使用芯片约定的引脚，不能转移
Clk,OC,I0,I1,I2,I3,I4,I5,I6,I7 pin 1,11,2,3,4,5,6,7,8,9;
Q0,Q1,Q2,Q3,Q4,Q5,Q6,Q7 pin 19,18,17,16,15,14,13,12;
H,L,X,Z,K = 1,0, .X.,.Z.,.K.; "类似于c语言的宏定义（代数）
Input = [I7,I6,I5,I4,I3,I2,I1,I0]; "两个集合群定义
Output = [Q7,Q6,Q5,Q4,Q3,Q2,Q1,Q0];
equations " global
[Q7..Q0].clk = Clk; "输出与CLK有关
[Q7..Q0].oe = !OC; "输出受!OC才有效
WHEN !OC THEN "如果OC为低电平则
Output := Input; "受时钟触发的锁存
test_vectors ' test Load & Output'
([Clk,OC, Input ] -> Output) "写入芯片以后编程器需要校验的内容
[ K, L, ^h55 ] -> ^h55;
[ K, L, ^hAA ] -> ^hAA;
[ K, L, ^hFF ] -> ^hFF;
[ X, H, X ] -> Z;
end M273 "本模块程序结束
当我基本上设计完这个GAL后，在一个具体的电路中使用他，由于布板走线的原因，我需要将其中输出线Q0与Q1对换，这可以将上面的引
脚定义中的Q0,Q1的表达顺序对换一下就行了，当然这样的使用并不提倡，但说明他确实能给你带来不少的灵活性。
有时候我需要两组不同的互不关联的逻辑电路，我们也可以用一个GAL完成，只要引脚和内部宏单元够用，例如下面我设计了一个LED七
段码的译码器，还有IO富裕，则我可以利用多余的IO设计一个简单的地址译码电路，下面的程序是为了使一般的七段码LED数码管显示多几个
特殊字符，使用了5条输入数据线，多余的IO我们独立的制作了一个任意逻辑电路：
module BCD7
title 'LED HEX-to-seven-segment decoder
Use GAL16V8. xiaoqi Jan.5.2002'
" a
" --- HEX-to-seven-segment decoder similar to the 7449
" f| g |b
" --- 笔段定义
" e| d |c
" ---
bcd7 device 'P16V8R';
OI2,OI1,OI0,D0,D1,D2,D3,D4 pin 7,8,9,2,3,4,5,6;
a,b,c,d,e,f,g,OOC pin 13,14,15,16,17,18,19,12 istype 'com';
bcd = [D4,D3,D2,D1,D0];
led = [a,b,c,d,e,f,g];
ON,OFF = 0,1; " 共阳结构数码管
L,H,X,Z = 0,1,.X.,.Z.;
@dcset
truth_table ( bcd -> [ a , b , c , d , e , f , g ])
0 -> [ ON, ON, ON, ON, ON, ON, OFF]; "0
1 -> [OFF, ON, ON, OFF, OFF, OFF, OFF]; "1
2 -> [ ON, ON, OFF, ON, ON, OFF, ON]; "2
3 -> [ ON, ON, ON, ON, OFF, OFF, ON]; "3
4 -> [OFF, ON, ON, OFF, OFF, ON, ON]; "4
5 -> [ ON, OFF, ON, ON, OFF, ON, ON]; "5
6 -> [ ON, OFF, ON, ON, ON, ON, ON]; "6
7 -> [ ON, ON, ON, OFF, OFF, OFF, OFF]; "7
8 -> [ ON, ON, ON, ON, ON, ON, ON]; "8
9 -> [ ON, ON, ON, ON, OFF, ON, ON]; "9
10 -> [ ON, ON, ON, OFF, ON, ON, ON]; "A
11 -> [OFF, OFF, ON, ON, ON, ON, ON]; "b
12 -> [OFF, OFF, OFF, ON, ON, OFF, ON]; "c
13 -> [OFF, ON, ON, ON, ON, OFF, ON]; "d
14 -> [ ON, OFF, OFF, ON, ON, ON, ON]; "E
15 -> [ ON, OFF, OFF, OFF, ON, ON, ON]; "F
16 -> [OFF, ON, ON, OFF, ON, ON, ON]; "H
17 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
18 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
19 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
20 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
21 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
22 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
23 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
24 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
25 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
26 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
27 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
28 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
29 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
30 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
31 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
truth_table ( [OI2 , OI1, OI0] -> [ OOC ]) "多余IO制作一个门电路
[ 0, 0, 0] -> [ 1 ]; "这个逻辑关系可以随意设定
[ 1, 0, 0] -> [ 0 ];
[ 0, 1, 0] -> [ 0 ];
[ 0, 0, 1] -> [ 0 ];
[ 1, 1, 0] -> [ 0 ];
[ 1, 0, 1] -> [ 0 ];
[ 0, 1, 1] -> [ 0 ];
[ 1, 1, 1] -> [ 1 ];
"校对部分
test_vectors ( bcd -> [ a , b , c , d , e , f , g ])
0 -> [ ON, ON, ON, ON, ON, ON, OFF]; "0
1 -> [OFF, ON, ON, OFF, OFF, OFF, OFF]; "1
2 -> [ ON, ON, OFF, ON, ON, OFF, ON]; "2
3 -> [ ON, ON, ON, ON, OFF, OFF, ON]; "3
4 -> [OFF, ON, ON, OFF, OFF, ON, ON]; "4
5 -> [ ON, OFF, ON, ON, OFF, ON, ON]; "5
6 -> [ ON, OFF, ON, ON, ON, ON, ON]; "6
7 -> [ ON, ON, ON, OFF, OFF, OFF, OFF]; "7
8 -> [ ON, ON, ON, ON, ON, ON, ON]; "8
9 -> [ ON, ON, ON, ON, OFF, ON, ON]; "9
10 -> [ ON, ON, ON, OFF, ON, ON, ON]; "A
11 -> [OFF, OFF, ON, ON, ON, ON, ON]; "b
12 -> [OFF, OFF, OFF, ON, ON, OFF, ON]; "c
13 -> [OFF, ON, ON, ON, ON, OFF, ON]; "d
14 -> [ ON, OFF, OFF, ON, ON, ON, ON]; "E
15 -> [ ON, OFF, OFF, OFF, ON, ON, ON]; "F
16 -> [OFF, ON, ON, OFF, ON, ON, ON]; "H
17 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
18 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
19 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
20 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
21 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
22 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
23 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
24 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
25 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
26 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
27 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
28 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
29 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
30 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
31 -> [OFF, OFF, OFF, OFF, OFF, OFF, OFF];
"附加逻辑电路校对
test_vectors ( [OI2 , OI1, OI0] -> [ OOC ])
[ 0, 0, 0] -> [ 1 ];
[ 1, 0, 0] -> [ 0 ];
[ 0, 1, 0] -> [ 0 ];
[ 0, 0, 1] -> [ 0 ];
[ 1, 1, 0] -> [ 0 ];
[ 1, 0, 1] -> [ 0 ];
[ 0, 1, 1] -> [ 0 ];
[ 1, 1, 1] -> [ 1 ];
end BCD7
以上两个例程都是用abel4编译的，GAL编程并不很复杂，作为一般应用，编编这样的电路就已经可以对付一阵子了。如果想动手的可以
到主页上下载ABEL4软件。有关这个软件的安装使用方法，请阅读解压后的READ.ME。安装ABEL4以后，软件提供了一大堆的实例演示程序，
稍作修改就可以为你解决实际需要，篡改是学习入门的捷径，试试看吧。
最后推荐初学者读一读这份由"晓风"编写的PLD入门的好教材：pld.rar（中文PDF文档）