如何读懂时序图

/itangcle

CPE/EE 421

Microcomputers

WEEK #10

Interpreting the Timing Diagram

如何读懂时序图

The 68000 Read Cycle

2 Alan Clements

3 Actual behavior of a D flip - f lop

Timing Diagram of a Simple Flip - F lop

Idealized form of the timing diagram

Data hold time

Data setup time Max time for output to become valid after clock

4 An alternative form of the timing diagram

General form of the timing diagram

A memory access begins in clock

state S0 and ends in state S7

6

The most important parameter

of the clock is the duration of a cycle, t C YC.

8

Address Timing

地址时序

•We are interested in when the 68000 generates a new address for use in the current memory access

我们感兴趣的是当6800芯片能够生成一个新的地址供当前的内存访问

•The next slide shows the relationship between the new address and the state of the 68000 ’s clock

下面展示的是新的地址跟6800芯片时钟的

关系

Alan Clements

1: S0在没有初始化的时候，地址总线总是包涵新的地址。

2： S1这个状态说明一个新的地址可以供内存访问

Alan Clements

Address Timing

地址时序

• Let’s look at the sequence of events that govern the timing of the address bus 让我们看看事件发生的顺序,管理时间的地址总线 • The “old” address is removed in state S0 旧的地址已经从S0中移除

• The address bus is floated for a short time, and the CPU puts out a new address in state S1

地址要先存在一小段时间后，CPU 才能把心的地址状态写入S1中12

The old address is removed in clock state S0 and the address bus floated

t C LA V

The designer is interested in the point at which the address first becomes valid. This point is t C LA V seconds after the falling edge of S0.

Address and Address Strobe

地址跟地址选通

•We are interested in the relationship between the time at which the address is valid and the time at which the address strobe, AS*, is asserted

我们感兴趣的事在地址总线能够访问时，地址选通是否有效。

•When AS* is active-low it indicates that the

上图说明：1：当地址有效的时候把AS 低电平信号.

当地址要改变前，要把AS变成高电We now look at the timing of the clock, the address, and the address strobe

从上面的时序图可以看出到，时钟脉冲，地址信号，片选脉冲；3者的关系。

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AS* goes low in clock state S2

The Data Strobes 数据选通•The 68000 has two data strobes LDS* and UDS*. These select the lower byte or the upper byte of a word during a memory access

6800芯片有两个数据选通信号LDS（低）和UDS（高）.内存通过选择LDS和UDS来访问数据。

•To keep things simple, we will use a single data strobe, DS* 为了让事情看起来简单，我们只要选通DS就可以了。（DS的高8位是UDS低8位是LDS）

•The timing of DS* in a read cycle is the same as the address strobe, AS*

DS的读数据跟AS读地址的时序是一样的。

Alan Clements 19

Alan Clements