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Digital Hardware Implementation

Abstract

  1. 可编程技术
    Programmable TechnologyROM、PAL、PLA，使用ROM、PAL和PLA来实现逻辑电路
  2. ROMReadOnly Memory
    结构、编程方法、作为存储器的使用
  3. PALProgrammable Array Logic
    结构、编程方法、使用时的主要约束、输出反馈的作用、多函数实现、编程表
  4. PLAProgrammable Logic Array
    结构、编程方法、使用时的主要约束、输出反向的作用、多函数实现、编程表

Why Programmable Logic?

  • Facts:
    • It is most economical to produce an IC in large volumes
    • Many designs required only small volumes of ICs
  • Need an IC that can be:
    • Produced in large volumes
    • Handle many designs required in small volumes
  • A programmable logic part can be:
    • made in large volumes
    • programmed to implement large numbers of different low-volume designs

  • Many programmable logic devices are field-programmable, i. e., can be programmed outside of the manufacturing environment

  • Most programmable logic devices are erasable and reprogrammable.

    • Allows “updating” a device or correction of errors
    • Allows reuse the device for a different design - the ultimate in re-usability!
    • Ideal for course laboratories
  • Programmable logic devices can be used to prototype design that will be implemented for sale in regular ICs.
    • Complete Intel Pentium designs were actually prototyped with specialized systems based on large numbers of VLSI programmable devices!

Programming Technologies

Classifications

Programming Technologies are used to

  • Control connections
    把两个脚连接起来，常见的连接方法有：
    • Mask programming
      一次性编程，工厂里芯片制造时使用，厂家在生产时编程到芯片内
    • Fuse(熔丝)
      在芯片设计时，在很多芯片引脚连接之间有 fuse, 设计电路时不需要保留连接的用高电压产生高电流，熔断 fuse.
    • Antifuse
      最开始断开，击穿后电路连接好
    • Single-bit storage element
      存储一个二进制位，这个位的输出控制一个晶体管，进而控制两个引脚连接/断开
  • Build lookup tables(LUT)
    如 16 位寄存器（存真值表的值） +16-1 Mux 可构成四输入之内的逻辑模块，只需要将 16位寄存器填上不同的值就可以实现不同的逻辑函数
  • Control transistor switching
    用晶体管控制开关，类似连接控制
    浮动栅极
    • Stored charge on a floating transistor gate
      • Erasable
      • Electrically erasable
      • Flash (as in Flash Memory)
    • Storage elements

Characteristics

  • Permanent - Cannot be erased and reprogrammed
    • Mask programming
    • Fuse
    • Antifuse
  • Reprogrammable
    • Volatile(易失的) - Programming lost if chip power lost
      • Single-bit storage element
    • Non-Volatile
      • Erasable
      • Electrically erasable
      • Flash (as in Flash Memory)

Configurations

  • Read Only Memory (ROM) - a fixed array of AND gates and a programmable array of OR gates
  • Programmable Array Logic (PAL) - a programmable array of AND gates feeding a fixed array of OR gates.
  • Programmable Logic Array (PLA) - a programmable array of AND gates feeding a programmable array of OR gates.
  • Complex Programmable Logic Device (CPLD) /Field- Programmable Gate Array (FPGA) - complex enough to be called “architectures” - See VLSI Programmable Logic Devices reading supplement

基本思想类似：一个 AND 阵列加上一个一个 OR 阵列，区别在于哪个阵列是可编程的。

  • ROM: OR
  • PAL: AND
  • PLA: OR+AND

Read Only Memory

Read Only Memories (ROM) or Programmable Read Only Memories (PROM) have:

  • N input lines,
  • M output lines, and

  • \(2^N\) decoded minterms.

  • Fixed AND array with \(2^N\) outputs implementing all N-literal minterms.

  • Programmable OR Array with \(M\) outputs lines to form up to M sum of minterm expressions.
    有 M 个或门，因此最多可以实现 M 个逻辑函数（每个函数最多 N 个输入）

  • A program for a ROM or PROM is simply a multiple-output truth table

    • If a 1 entry, a connection is made to the corresponding minterm for the corresponding output
    • If a 0, no connection is made

    二维阵列：AND 阵列产生 \(2^n\) minterms, OR 阵列有 M 个输出，每个输出对应最多 \(2^n\) minterms.

  • Can be viewed as a memory with the inputs as addresses of data (output values), hence ROM or PROM names!

A 8 X 4 ROM(N = 3 input lines, M= 4 output lines)

一个或门的线表示一根总线，一个 "X" 表示与门阵列和或门阵列连接。
实现逻辑函数就是将译码器的最小项与或门连接

也可以将输入看作 addr, 输出看作 data.

Programmable Array Logic (PAL)

The PAL is the opposite of the ROM, having a programmable set of ANDs combined with fixed ORs.

  • Disadvantage
    ROM guaranteed to implement any M functions of N inputs. PAL may have too few inputs to the OR gates.
  • Advantages
    • For given internal complexity, a PAL can have larger N and M
    • Some PALs have outputs that can be complemented, adding POS functions
    • No multilevel circuit implementations in ROM (without external connections from output to input). PAL has outputs from OR terms as internal inputs to all AND terms, making implementation of multi-level circuits easier.

Example

上方 0~9 每根线，每根分别代表一个输入或者输入的反变量("*")
这里 F1 还有连到 8, 9 线，用来实现级联。 \(F1=\overline A\overline B+\overline C\)
\(F2=\overline AB\overline C+AC+A\overline B\)
\(F3=AD+BD+\overline A\overline B+\overline C\)
\(F4=AB+CD+AC+BC(\overline{F1})\)

Programmable Logic Array (PLA)

Compared to a ROM and a PAL, a PLA is the most flexible having a programmable set of ANDs combined with a programmable set of ORs.

  • Advantages
    • A PLA can have large N and M permitting implementation of equations that are impractical for a ROM (because of the number of inputs, N, required)
    • A PLA has all of its product terms connectable to all outputs, overcoming the problem of the limited inputs to the PAL Ors
    • Some PLAs have outputs that can be complemented, adding POS functions
  • Disadvantages
    • Often, the product term count limits the application of a PLA.
    • Two-level multiple-output optimization is required to reduce the number of product terms in an implementation, helping to fit it into a PLA.
    • Multi-level circuit capability available in PAL not available in PLA. PLA requires external connections to do multi-level circuits.

Example

\(F1 = AB +BC + AC\)
\(F2 = (AB + A’B’)’ = (A’ + B’) (A + B) = A’B + AB’\)
这里 F2 我们求的是反变量，因此在最后要求反，这样做可以节省重复利用之前的 AND 项。

合理利用 AND 项资源，尽可能复用，可以求反变量。

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