Файл: Digital design with CPLD applications and VHDL (R. Dueck, 2000).pdf

Repeated Division by 16

Repeated division by 16 is a systematic decimal-to-hexadecimal conversion method that is not limited by the size of the number to be converted.

It is similar to the repeated-division-by-2 method used to convert decimal numbers to binary. Divide the decimal number by 16 and note the remainder, making sure to express it as a hex digit. Repeat the process until the quotient is zero. The last remainder is the most significant digit of the hex number.

SECTION 1.4C REVIEW PROBLEM

1.9Convert the decimal number 8137 to its hexadecimal equivalent.

Conversions Between Hexadecimal and Binary

Table 1.4 shows all 16 hexadecimal digits and their decimal and binary equivalents. Note that for every possible 4-bit binary number, there is a hexadecimal equivalent.

Binary-to-hex and hex-to-binary conversions simply consist of making a conversion between each hex digit and its binary equivalent.

SECTION 1.4D REVIEW PROBLEMS

1.10Convert the hexadecimal number 934B to binary.

1.11Convert the binary number 11001000001101001001 to hexadecimal.

1.5Digital Waveforms

K E Y T E R M

Digital waveform A series of logic 1s and 0s plotted as a function of time.

The inputs and outputs of digital circuits often are not fixed logic levels but digital waveforms, where the input and output logic levels vary with time. There are three possible types of digital waveform. Periodic waveforms repeat the same pattern of logic levels over a specified period of time. Aperiodic waveforms do not repeat. Pulse waveforms follow a HIGH-LOW-HIGH or LOW-HIGH-LOW pattern and may be periodic or aperiodic.

Periodic Waveforms

K E Y T E R M S

Periodic waveform A time-varying sequence of logic HIGHs and LOWs that repeats over a specified period of time.

Period (T) Time required for a periodic waveform to repeat. Unit: seconds (s).

Frequency (f ) Number of times per second that a periodic waveform repeats. f 1/T Unit: Hertz (Hz).

Time HIGH (th) Time during one period that a waveform is in the HIGH state.

Unit: seconds (s).

Time LOW (tl) Time during one period that a waveform is in the LOW state.

Unit: seconds (s).

Duty cycle (DC) Fraction of the total period that a digital waveform is in the

HIGH state. DC th/T (often expressed as a percentage: %DC th/T 100%).

Periodic waveforms repeat the same pattern of HIGHs and LOWs over a specified period of time. The waveform may or may not be symmetrical; that is, it may or may not be HIGH and LOW for equal amounts of time.

FIGURE 1.5

Example 1.14: Periodic Digital Waveforms

How are the waveforms similar? How do they differ?

SOLUTION

a.Time LOW: tl 3 ms Time HIGH: th 1 ms

Period: T tl th 3 ms 1 ms 4 ms Frequency: f 1/T 1/(4 ms) 0.25 kHz 250 Hz

Duty cycle: %DC (th/T) 100% (1 ms/4 ms) 100%

25%

(1 ms 1/1000 second; 1 kHz 1000 Hz.)

b.Time LOW: tl 2 ms Time HIGH: th 2 ms

Period: T tl th 2 ms 2 ms 4 ms Frequency: f 1/T 1/(4 ms) 0.25 kHz 250 Hz

Duty cycle: %DC (th/T) 100% (2 ms/ 4 ms) 100%

50%

c.Time LOW: tl 1 ms Time HIGH: th 3 ms

Period: T tl th 1 ms 3 ms 4 ms

Frequency: f 1/T 1/(4 ms) 0.25 kHz and 250 Hz Duty cycle: %DC (th/T) 100% (3 ms/ 4 ms) 100%

75%

The waveforms all have the same period but different duty cycles. A square waveform,

Aperiodic Waveforms

K E Y T E R M

Aperiodic waveform A time-varying sequence of logic HIGHs and LOWs that

does not repeat.

An aperiodic waveform does not repeat a pattern of 0s and 1s. Thus, the parameters of time HIGH, time LOW, frequency, period, and duty cycle have no meaning for an aperiodic waveform. Most waveforms of this type are one-of-a-kind specimens. (It is also worth noting that most digital waveforms are aperiodic.)

Figure 1.6 shows some examples of aperiodic waveforms.

FIGURE 1.6

Aperiodic Digital Waveforms

The time between two bits is always the same. Sketch the resulting digital waveform for each string of bits. Which waveforms are periodic and which are aperiodic?

SOLUTION Figure 1.7 shows the waveforms corresponding to the strings of bits above. The waveforms are easier to draw if you break up the bit strings into smaller groups of, say, 4 bits each. For instance:

a. 0011 1111 0110 1011 0100 0011 0000

All of the waveforms except Figure 1.7a are periodic.

FIGURE 1.7

Example 1.15: Waveforms

Pulse Waveforms

K E Y T E R M S

Pulse A momentary variation of voltage from one logic level to the opposite level and back again.

Amplitude The instantaneous voltage of a waveform. Often used to mean maximum amplitude, or peak voltage, of a pulse.

Edge The part of the pulse that represents the transition from one logic level to the other.

Rising edge The part of a pulse where the logic level is in transition from a LOW to a HIGH.

Falling edge The part of a pulse where the logic level is a transition from a HIGH to a LOW.

Leading edge The edge of a pulse that occurs earliest in time.

Trailing edge The edge of a pulse that occurs latest in time.

Pulse width (tw) Elapsed time from the 50% point of the leading edge of a pulse to the 50% point of the trailing edge.

Rise time (tr) Elapsed time from the 10% point to the 90% point of the rising edge of a pulse.

Fall time (tf ) Elapsed time from the 90% point to the 10% point of the falling edge of a pulse.

Figure 1.8 shows the forms of both an ideal and a nonideal pulse. The rising and falling edges of an ideal pulse are vertical. That is, the transitions between logic HIGH and LOW levels are instantaneous. There is no such thing as an ideal pulse in a real digital circuit. Circuit capacitance and other factors make the pulse more like the nonideal pulse in Figure 1.8b.

Pulses can be either positive-going or negative-going, as shown in Figure 1.9. In a pos- itive-going pulse, the measured logic level is normally LOW, goes HIGH for the duration

0.5

FIGURE 1.8

Ideal and Nonideal Pulses

FIGURE 1.9

Pulse Edges